UNIVERSITY OF CAPE TOWN DEPARTMENT OF ELECTRICAL ENGINEERING

RONDERBOSCH, CAPE TOWN, SOUTH AFRICA

COMPARISON OF AN INVERTER DRIVEN

INDUCTION MOTOR FOR A PV WATER PUMP

WITH A DC MOTOR DRIVEN SYSTEM

PREPARED FOR:

Department of Electrical Engineering

University of Cape Town

PREPARED BY:

Sihaam Saban (SBNSIH001)

Fourth Year Student

Department Electrical Engineering

University of Cape Town

22 October 2007

Thesis prepared in partial fulfilment of the requirements for the Degree of

Bachelor of Science (BSc) in Electrical Engineering at the University of

Cape Town

DECLARATION

I declare that this thesis is my own unaided work. I affirm that all information gathered from

various sources in this document has been referenced accordingly where necessary.

This thesis is prepared in partial fulfillment of the requirements for the Degree of Bachelor of

Science (Bsc) in Electrical Engineering at the University of Cape Town.

________________ _____________________

S.Saban Date

I

ACKNOWLEDGEMENTS I would like to express my sincere thanks to:

My creator, for granting and blessing me with the ability to further my knowledge in all

aspects of life.

My family, specially my parents as well as siblings for their helpful support throughout.

Dr. Ben Sebitosi, my supervisor, for his invaluable support, encouragement and

guidance throughout my thesis project.

Mr. Chris Wozniak and Mr. Phillip, the lab supervisors, for their assistance and their

patience in helping me throughout my thesis research.

Prof. G McLaren for his exceptional advice on my thesis project.

Lastly, to all my friends and colleagues, especially, Heskin, Richard, Emma, Wesley,

and many more for their help, encouragement and moral support.

Thanks a lot.

Allah’s Name I begin, the Beneficent, the Merciful

II

TERMS OF REFERENCE

Dr Ben Sebitosi, a senior member of the Department of Electrical Engineering at the

University of Cape Town, proposed this thesis topic. This project involves the comparison of

an inverter driven induction motor for a PV water pump system to a DC motor driven system.

Specific instructions given were:

• To investigate the comparison between a DC motor driven system for solar water

pumping and an Inverter Induction Motor driven system.

• To investigate how to optimize the performance of these systems in order to maximize

water delivery, whereby,

• The most efficient system should be able to pump water despite variations in the

irradiance level.

• Comment on findings made.

• Make suitable Recommendation and Conclusions based on findings.

III

ABSTRACT

With the growing shortage of water in the metropolis, it is of major concern that our most

valuable asset become available for all South Africans, specifically those located in remote

areas.

According to our Provincial Government, water consumption and demand for water in the

Western Cape region is growing annually at a rate of 3-4% for households and 1.5-2% for

agriculture [1]. Many people living in remote areas do not have access to the national electric

grid and therefore water pumping in theses areas are often difficult.

This thesis aims to address the issue of using renewable energy to provide these remote areas

with clean drinking water and electricity. The best possible water pumping system is thus

studied and the method of selecting the best motor system for photovoltaic water pumping is

being analysed, such that it is able to maximize the amount of water delivery during low levels

of solar irradiation.

The purpose of this investigation is to revise the performance of both the AC and DC motor

water pumping system and to observe each system response under various weather conditions.

Each system consists of a positive displacement pump, solar panels and a 3-phase inverter used

with the induction motor to convert DC voltage from the panel to AC voltage.

The Induction motor has been expected to perform significantly better than the DC motor. The

power rating of the Induction motor is 0.37kW. To run a high rated motor of 0.37kW, at least 5

panels are needed. Due to cost and time ordering panels, a variable frequency drive that is

connected to the mains was used for testing purposes. This drive allowed the author to

distinguish the performance characteristics of the Induction motor-pump setup.

The big disparity in the sizes of the DC and Induction motor caused major challenges when

comparing the relative system performances, even though the pumps were identical. Because

the only matched system was the pumps, comparisons on the performance of the pumps were

made.

IV

From various tests conducted, for the DC motor, we observed that the motor stalls at 4.5V;

1.02A. This means that at low levels of solar irradiation the DC motor pumping system

becomes inefficient. The minimum flow rate for the DC system was 0.25 litres/min at a power

rating of 4.59W and pump speed ≈ 40 rpm. For the AC system the minimum flow rate was

0.001 litres/min at a power rating of 1W and pump speed of 0.5 rpm.

From these results it was observed that the Induction motor system therefore operates at lower

speeds and input power, compared to the DC motor system. Lower speeds and input power

means that the induction motor operates better at low levels of insolation, hence maximizes the

amount of water delivery during cloud cover.

A conclusive result was thus made on the implementation of the Induction motor-pump

system. The Induction motor-pump system offered significant advantages over the DC motor

system. Even though the ratings for the DC and AC motor differed considerably, the pumps of

each system helped the author determine various performance characteristics based on each

system. The various tests conducted allowed the author to conclude and make

recommendations based on further improvements and possible solutions to a better working

system.

Because, the smallest available AC motor was 10 times the size of the DC motor, the author

would therefore recommend that future test be done using motors of comparable sizes.

V

TABLE OF CONTENTS PAGE

ACKNOWLEDGEMENTS…………………...……………………………….….....I

TERMS OF REFERENCE…………..…………………………………………......II

ABSTRACT………………………………………………………………….……..III

TABLE OF CONTENTS……………………………….……………………....…..V

LIST OF ILLUSTRATIONS……………………….………………...………....VIII

LIST OF TABLES…………………………………………………………………..X

NOMENCLATURE…………………….…………………………………...……..XI

1 INTRODUCTION……………………………..…………………………….1

1.1 Background…………….…………………………………………..….1

1.2 Overview of solar photovoltaic water pumping systems……….….….3

1.3 Thesis Objectives………………………………….……………….….3

1.4 Thesis Limitations……………………………………………………..3

1.5 Thesis Outline………………………………..………………………...4

2 LITERATURE REVIEW……………………………………………………5

2.1 Solar Water Pumping……………………………..…………………...5

2.2 Problems associated with Solar Water Pumping……………….…......9

2.3 Techniques used to maximize the performance of a Solar

Water Pumping system………………………………………..………10

2.4 Efficiency of the DC Motor system…………………………..……….11

2.5 Efficiency of the AC Motor system.……………...……….……......…12

3 MODELLING THE PV SOURCE………………………………………….15

4 MODELLING THE POSITIVE DISPLACEMENT PUMP……………..16

VI

5 DC MOTOR SYSTEM EXPERIMENTATION AND DATA

ANALYSIS…………………………………………………………………..18

5.1 Basic operation of a DC Motor……………………...……………….18

5.2 Circuit parameters and characteristics of a DC motor…………….…..19

5.3 DC motor system description…………………………………..……...20

5.4 Motor performance analysis……………………………………….…..21

5.4.1 Test conducted without compensating for internal

impedance……………………………………………………22

5.4.2 Test conducted compensating for internal impedance.………22

6 AC MOTOR SYSTEM EXPERIMENTATION AND DATA

ANALYSIS………………………………………………………………....26

6.1 Basic Operation of an Induction Motor…………………………......26

6.2 Circuit parameters and characteristics of an induction motor…….…26

6.3 AC motor system description………………………………………..30

6.4 Motor performance analysis…………………………………………32

6.4.1 Resistive Load test at varying frequency and varying

voltage……………………………………………………….32

6.4.2 Induction motor test at varying frequency and varying

voltage.....................................................................................34

6.4.3 Test Simulations and Results………………………………..36

6.4.4 Factors affecting system performance during testing……….43

6.4.5 Possible improvements and solutions…………………….....44

7 DESIGN OF SYSTEM COMPONENTS FOR DC/AC INVERTER…..45

7.1 Basic operation of DC-AC inverter…………………………………45

7.2 Inverter design………………………………………………………45

7.3 MOSFET driver circuit……………………………………………...46

7.4 Interfacing with Motor Controller…………………………………..47

8 CONCLUSIONS…………………………………………….……….…….49

VII

9 RECOMMENDATIONS…………………………………………….…….51

10 REFERENCES……………………………………………………………..52

11 APPENDICES………………………………………………………………55

VIII

LIST OF ILLUSTRATIONS

Figure 1 - Yearly sum of global irradiation in South Africa

Figure 2 - DC Motor Driven System

Figure 3 - AC Motor Driven System

Figure 4 - Typical Layout of Photovoltaic Water Pumping System

Figure 5 - Levels of insolation in a day

Figure 6 - PV water pumping system using an inverter induction motor system

Figure 7 - Measured results for ‘inverter-motor-pump set’ comparing efficiency in function

of flow rate with the frequency as a parameter

Figure 8 - I-V characteristics of the motor-pump assembly and a PV array with six panels

Figure 9 - Pump flow rate vs. electrical motor power input

Figure 10 - PV module equivalent circuit

Figure 11 - Positive Displacement diaphragm pump

Figure 12 - Positive Displacement Characteristic Curve

Figure 13 - DC Motor

Figure 14 - Permanent Magnet DC Motor

Figure 15 - Torque-speed characteristics of a permanent magnet DC motor, where Vt4 is the

rated voltage

Figure 16 - Lab simulation of DC pumping system

Figure 17 - Graph of input power required (W) vs. Pressure Head (Pa)

Figure 18 - Cross-section of an Induction Motor

Figure 19 - IEEE Equivalent Circuit for an Induction Motor

Figure 20 - Thevenin equivalent circuit

Figure 21 - Typical torque-speed characteristic of an Induction motor

Figure 22 - Lab simulation of AC pumping system

Figure 23 - Shaft of pump aligned with shaft of induction motor and connected using a heat

shrink tubing

Figure 24 - Y– Connected load

Figure 25 - Illustration of Lab simulation of resistive load

Figure 26 - Relationship between varying voltages (V) with varying frequency (Hz) for

resistive load

IX

Figure 27 - Induction Motor characteristics and capabilities

Figure 28 - Photograph of (a) induction motor water pumping system and (b) variable

frequency drive

Figure 29 - Voltage (V) vs. Frequency (Hz) for Induction Motor

Figure 30 - Illustration of Two Wattmeter Method

Figure 31 - Illustration of white strip used on pump shaft to measure speed

Figure 32 - Pump Power (W) vs. Flow Rate (litres/sec) at full load

Figure 33 - Comparison between Pump Power (W) vs. Speed (rpm) for AC and DC system

Figure 34 - DC-AC inverter connected to a 3-phase AC motor

Figure 35 - Waveform generated by inverter of a PWM variable frequency drive compared

with that of a true AC sine wave

Figure 36 - MOSFET Driver Circuit

Figure 37 - Start/Stop Switch from motor controller

Figure 38 - Photograph of Motor Controller Circuit

Figure 39 - Square wave switching scheme from one inverter leg

X

LIST OF TABLES

Table 1 - Electrical Parameters of solar module SM55

Table 2 - DC Motor Voltage and Current readings

Table 3 - Relationship between varying voltage and varying frequency for resistive load

Table 4 - Induction Motor test showing values of varying voltage with varying frequency

Table 5 - Induction Motor test showing real Power measured as the frequency is adjusted

Table 6 - Induction motor test showing real power losses (at no load) and speed at which

pump shaft rotates

XI

NOMENCLATURE

Ia armature current (A)

Iph photo-current (A)

Is diode saturation current (A)

Ra armature resistance (Ω)

Rs series resistance (Ω)

Rp parallel (or shunt) resistance (Ω)

R1 stator resistance (Ω)

R2’ rotor resistance referred to stator (Ω)

X’2 rotor reactance referred to stator (Ω)

Xm magnetizing reactance (Ω)

Vth thevenin equivalent voltage (V)

Vm motor voltage (V)

A thermal voltage of one solar cell (V)

ε electromotive force (V)

τ motor torque (N.m)

Tmech mechanical torque (N.m)

ω motor shaft angular velocity (rads/s)

φ permanent magnet flux (Wb)

κ constant flux coefficient

ns synchronous speed (rpm)

s slip

f supply frequency (Hz)

efficiency

1

1 INTRODUCTION

Following the Johannesburg World Summit on Sustainable Development, it has been a

key issue for South Africa to focus on the needs of people for many generations to

come. The vast shortage and provision of clean drinking water has created a serious

concern in meeting the social and health needs of the population.

1.1 Background

Issues have been raised to make sustainable use of renewable resources for the supply

of freshwater and generation of electricity. This will allow us to minimize the wastage

of scarce mineral resources as well as reduce the amount of global pollution.

As a Capetonian, I have come to realize, traveling through South Africa, that many

areas and suburbs particularly low quality areas outside Cape Town, do not have access

to clean drinking water. Many children particularly areas in the Cape flats such as

Khayelitsha, Langa and other informal settlements die because of impurities in water.

Remote areas outside Cape Town are often not connected to the electrical grid as they

are situated many miles away from power stations. This has initiated my interest in

using renewable resources such as wind and solar energy to provide pasteurized water

and generate and supply electricity in these remote areas, to help improve the standard

of living of our poor communities.

Developments in using renewable energy for alternative energy distribution came to the

fore, because of the worldwide energy crises. Wind and solar energy seemed the

obvious sources of renewable energy, for the inhabitants of South Africa, as they are

resources that are most readily available in many remote areas in South Africa.

If any technology is considered for a remote application, it should be redesigned

according to the socio-economic structure of that particular area [2]. Taking into

account the socio-economic structure, a wind farm would not be feasible in an area

2

where wind is not always available, but water is. The most viable option would be to

use solar energy for pumping clean drinking water and generating electricity.

The advantage of renewable resources is that they are replenished every day. Using

renewable energy resources reduces the amount of environmental pollution and harmful

waste through its utilization.

Owing to the amount of global irradiation in South Africa, as shown in figure 1, using

the sun as a renewable source to allow us to generate electricity or pump

uncontaminated water would be most practical.

Figure 1: Yearly sum of global irradiation in South Africa [3]

Using photovoltaic as a power source for pumping water, more attention has been paid

to design and optimize the utilization of PV systems to achieve the most reliable and

economical operation of solar energy [4]. On a clear day the sun’s radiation on the earth

can be 300 watts per square meter depending on the location [5].

3

The technology of using Photovoltaic (PV) is to convert radiant energy from the sun to

direct current (DC) electricity [5]. These pumping systems are often located in remote

areas where there is a high demand for clean drinking water and power demand is

moderately small.

1.2 Overview of solar photovoltaic water pumping systems

The current photovoltaic water pumping system uses a DC motor to drive the water

pump. Various studies have shown that the DC motor system, although cost-efficient

and easy to use, has not been reliable. The motor uses mechanized commutators and

brushes, which need regular maintenance and are also prone to failure [6]. It has also

been noted that during low levels of irradiation, the motor stops pumping. The DC

motor is thus not reliable, as it requires a high threshold to keep the motor running in

order to pump water.

1.3 Thesis Objectives

The author is therefore undertaking this study to investigate a low-cost and more robust

system, by comparing whether an AC motor system is more efficient in maximizing

water delivery for the water pump system as compared to using a DC motor system.

Analyzing this matter, it will allow the author to help uplift the community in Cape

Town and other remote areas to use the PV water pump system (PVWPS).

1.4 Thesis Limitations

Tests for comparison between the actual DC and AC motor could not be conducted due

to the vast size difference between both motors. The only tests that could be conducted

were the comparison between the performance of the pumps of each system, since the

same pump is used for both the AC and DC system. Actual real time tests using panels

could not be conducted due to limited time available and shortage in the amount of

panels required driving the induction motor.

4

1.5 Thesis Outline

Chapter 1 introduces solar water pumping and briefly introduces PV pumping systems

that has already been used. Also gives a brief description on thesis limitations and

objectives.

Chapter 2 discusses in more detail solar water pumping (SWP), problems associated

SWP, efficiency of DC and AC motor systems and techniques used to maximize the

performance of a SWP system.

Chapter 3 models the photovoltaic source.

Chapter 4 models how the positive displacement pump operates.

Chapter 5 outlines how the DC motor system works as well as deals with experiments

and results, which was conducted in the UCT machines lab.

Chapter 6 outlines how the AC motor system works as well as deals with experiments

and results, which was conducted in the UCT machines lab.

Chapter 7 covers the general operation and use of the DC/AC inverter for the AC motor

driven system.

Chapter 8 and 9 consists of conclusions that were based on the findings and

recommendations for future improvements made.

The thesis concludes with a set of References and Appendices containing all additional

information.

5

2 LITERATURE REVIEW

This review discusses recent literature and focuses on the following issues:

• Optimising the performance of the system to maximise water delivery.

• Comparing the efficiency of the DC motor against the Induction motor.

• Selecting the most suitable motor so that the photovoltaic array or solar panel is

utilized in a way that it maximises the amount of water during low irradiance

levels.

2.1 Solar Water Pumping

The technology of solar PV water pumping has been advancing steadily in recent years,

for the efforts to overcome the problem of developing countries not having access to

safe drinking water [5, 7].

The sun has provided us with radiant energy, which is used to pump water in places

where there is a scarcity of freshwater. This has brought about the concept of solar

water pumping in remote areas. These areas are also often not connected to the national

electric grid, thus the use of solar water pumping has become economically

advantageous where the typical head lies between 1m and 100m [5].

According to Kenna and Gillet [7], there are two methods by which solar energy can be

converted to mechanical energy required for pumping. These methods are:

• Direct conversion of solar radiation to electricity using PV cells and then

conversion of electrical energy to mechanical energy using a motor/pump

system or

• The conversion of solar energy to heat, which can then be used to drive a heat

engine [7].

6

The main components that make up the photovoltaic pumping system are:

• The PV array which converts solar energy to DC electricity

• The motor and pump subsystem comprising the components which convert the

electrical output of the PV array into hydraulic power, and

• The storage and distribution system that delivers the water to its point of use

[7].

The volume of pumped water is dependant on five major factors [5]:

• The amount of solar irradiation, which is the measure of the suns available

energy.

• The photovoltaic array area.

• The conversion efficiency of the photovoltaic array.

• The ambient temperature.

• The pump motor for the hydraulic system characteristics.

Two different systems configurations are currently been studied:

• System 1: PV array is directly coupled to a DC motor and a pump.

• System 2: PV array coupled to an inverter which is then coupled to a 3 phase

AC motor and a pump

The directly couple systems, where the PV array is directly coupled to a DC motor-

pump system, is shown in figure 2 [5].

Figure 2: DC Motor Driven System

PV Array

DC Motor

Pump

Tank

7

The PV array coupled to an inverter, which is then coupled to a 3-phase AC motor, and a

pump system is shown in figure 3:

Figure 3: AC Motor Driven System

When using an AC motor, the solar photovoltaic array needs to be connected to an

inverter that is then connected to the AC motor. The inverter converts the DC voltage

coming from the solar panel into AC voltage, which drives the induction motor.

These systems have revolutionised, where many people in remote areas with no access

to clean water, could now have access to clean drinking water. The ability of providing

pasteurised water has freed labourers, mainly women and children from the time

consuming task of carrying water from distant springs [8].

With the availability of groundwater and sunlight, solar photovoltaic powered water

pumping are more cost effective in remote areas and for small applications. Figure 4

below shows the complete set-up of the photovoltaic pumping system.

PV Array

Inverter AC Motor

Pump

Tank

8

Figure 4: Typical Layout of Photovoltaic Water Pumping System

For many years the old conventional pumps used were the DC motor water pump,

which is directly attached to a solar panel as discussed before. Today several types of

motors can be used, such as AC, DC, permanent magnet, brushed, brushless,

synchronous and asynchronous, variable reluctance, and many more [9]. In South

Africa the DC motor system, is most commonly used.

As technology progressed, many different solar pumps also became available for

various applications. They are specifically the solar submersibles, floating water pumps

and the solar surface pumps.

Submersible pumps draw water from shallow wells, springs, ponds, rivers or tanks and

floating pumps draws water from reservoirs with height ability [9]. There are also

different pumps according to their pumping principle:

• Centrifugal pump- where liquid is sucked by the centrifugal force created by the

impeller and the casing directs the liquid to the outlet as impeller rotates. The

liquid leaves with a higher velocity and pressure than it had when it entered [9].

• Positive Displacement Pump – causes a liquid to move by trapping a fixed

amount of fluid and then forcing (displacing) that trapped volume into the

discharge pipe.

9

These pumps are often connected or built into the motor.

2.2 Problems associated with Solar Water Pumping

One of the main problems associated with solar water pumping is the level of insolation

received on a day that fluctuates because of weather conditions. Figure 5 [10] below

shows the levels on insolation in a day when it is clear, cloudy and overcast,

respectively.

Figure 5: Levels of insolation in a day [10]

On an overcast day the maximum amount of insolation received during noon can be as

low as 60% when compared to a clear day.

Because of the high threshold required by the traditional DC PM driven pump system,

the motor stops with cloud cover or pollution and does not pump water.

According to Fiaschi, Graniglia and Manfrida [11] the power and head characteristics

of a centrifugal pump are not well matched to solar PV water supply systems. When the

solar irradiance levels are low, the pump is unable to provide the head necessary for

10

extracting water. When irradiation is high, some of the solar power is wasted, if costly

energy storage devices such as batteries are added [11].

Eskander and Zaki [12] investigated a system in which an induction motor was

controlled to operate at maximum efficiency. The operating characteristic of the array

was then controlled to match the voltage and current required by the motor. This

system was reported to be more efficient in providing a large quantity of water being

pumped at low levels of insolation [12].

2.3 Techniques used to maximize the performance of a Solar Water pumping

system

Various techniques have been studied to improve the performance of a solar water

pumping system. One of the main aspects of a good design for a photovoltaic driven

water supply system is the choice of pump, the flow rate of water and the daily

radiation peak power.

According to Fiaschi, Graniglia and Manfrida [11], the importance of maximising the

use of the available solar power is by “following” the daily radiation curve, so as to

cover the maximum possible time extension, thus allowing the pump to work in the

early morning and late afternoon hours by varying the number of active stages (in a

modular pump) and rotational speed [11].

It has also been discovered that using an AC motor pump combined with a variable-

speed DC-AC inverter is optimised by working at low speeds and full on during hours

of daylight as well as during hours of low irradiation.

11

According to Fiaschi, Graniglia and Manfrida [11], the reason for this discovery is the

consequence of the characteristic head-flow rate curves of centrifugal pumps and by the

similitude laws at variable shaft speed. This has lead them to conclude that coupling a

centrifugal pump with variable rotational speed, resulted in a more effective use of

daily solar available power curve [11].

According to Munir, Al-Karaghouli and Al-Douri [13], the inverter has a voltage and

frequency regulation of the output voltage that supplies the water pump [13]. (See

figure 6 below) As the insolation levels increase, the inverter frequency and voltage

increases proportionately to maintain the supply of water during cloud cover or

pollution.

Figure 6: PV water pumping system using an inverter induction motor system [13]

2.4 Efficiency of the DC Motor system

Much research has been done to conclude whether either AC or DC motors should be

integrated to design the most viable photovoltaic water pumping system.

In the DC motor system, the PV array generates sufficient electrical power from solar

irradiation to operate the DC motor [14]. The solar irradiation then converts the

electrical energy into mechanical energy to drive the pump [14]. The mechanical

energy is in turn converted into hydraulic energy in order to draw water up a well [14].

12

DC motors are simple to use and very efficient. They are expensive maintenance wise,

but relatively cheaper and easier to control than an AC motor [15].

A solar energy system should seek to provide an optimal combination of efficient

performance, low initial and running costs, robustness and durability [16]. DC motors,

even though attractive, they are not suitable for high-power applications. According to

Firatoglu and Yesilata [14], the use of a brushless DC motor is found to be more

suitable for photovoltaic pumping systems. The brushless DC motor has a rotating

permanent magnet and stationary armature winding instead of the conventional brush-

commutator assembly [14]. Because of electronic commutation, the brushless

permanent magnet DC motor has a high efficiency [14]. However, due to electronic

commutation this increases the running cost and decreases reliability.

2.5 Efficiency of the AC Motor system

On the other hand, AC motor systems are convenient because household electrical

appliances such as lights, televisions and power tools can run off it. It is simply easy to

plug in an AC pump to provide a domestic water supply [17].

The AC motor can however be expensive, as it requires an inverter, which lowers the

overall system efficiency rate [17]. Though, when a better output performance is

required during low levels of irradiation, the AC motor exceeds its performance

capabilities compared to the DC motor. The DC motor stops completely during cloud

cover or environmental pollution.

Given the continuously changing solar irradiance levels, the most practical solution is

choosing a motor that is highly efficient in providing the maximum amount of water

during solar water pumping, specifically during times when solar irradiance is low.

13

Davies [18] states that most effective motor and pump combination should be designed

for rural areas, which have little technical support and may not be easily accessible. He

noticed significant benefits in using the AC motor system, in terms of cost and

reliability. The efficiency of the AC motor system for a 300W (peak power output of

the solar panel) system was found to be 67%, which is not much less than the efficiency

of the DC motor system. The cost of the induction motor is significantly less than that

of the DC motor, as it is more reliable [18].

A solar powered induction motor-driven water pump system was installed in a desert

well in Jordan to provide pasteurised water to Bedouins living in the well area [19].

From Laboratory results, as shown in the figure 7 below, Daud and Mahmoud [19]

have shown that increasing the frequency caused the flow rate to increase significantly,

which is proportional to the input power.

Figure 7: Measured results for ‘inverter-motor-pump set’ comparing efficiency in

function of flow rate with the frequency as a parameter [19].

Suehrcke, Appelbaum and Reshef [20] performed various experimental tests on the

performance characteristics of motors and pumps. The motor-pump assembly was then

mathematically modelled and thus confirmed that the photovoltaic array yields the

relationship between solar radiation and pump flow rate [20].

14

The I-V characteristics in figure 8 [20] obtained during the experiments of the motor-

pump assembly were compared to the relationship between the flow rate and the motor

electrical input power in figure 9 [20].

Figure 8: I-V characteristics of the motor-pump assembly and a PV array with

six panels [20]

Figure 9: Pump flow rate vs. electrical motor power input [20]

They concluded that the conversion efficiency between the electrical power supplied to

the motor and the solar radiation is approximately constant and the motor input power

can be scaled to represent the solar radiation [20].

15

3 MODELLING THE PV SOURCE

The mathematical model of determining the series resistance of the solar module varies

as the ambient temperature and irradiation level changes.

The photovoltaic module used, is a Siemens SM-55 PV panel. It consists of 36 series

connected monocrystalline silicon solar cells. The equivalent circuit of one PV cell

consists of a diode, a current source, a series resistor and a shunt (or parallel) resistor.

Figure 10 [21] below is an equivalent circuit of the photovoltaic cell.

Figure 10: PV module equivalent circuit [21]

The relationship between the current (I) and the voltage (V) are derived as follows:

exp 1s sph s

p

V IR V IRI I I

A R + + = − − −

(1)

The current source generates the photo-current Iph (A), which is generated from the

amount of solar irradiation received. The diode represents a p-n junction of a solar cell

[21]. The amount of leakage current is determined by the parallel resistance Rp ( Ω ) and

the voltage loss is expressed by a series resistance Rs ( Ω ). Is (A) is the diode saturation

current and A represents the thermal voltage (V). The parallel resistance Rp ( Ω ) is

independent of intensity and can therefore be neglected. This however only applies in

the case of a crystalline silicon cell. In this case the equation can thus be reduced to:

exp 1sph s

V IRI I I

A + = − −

(2)

16

4 MODELLING THE POSITIVE DISPLACEMENT PUMP

The pump used in the photovoltaic water pumping system is a Flojet Quad II

diaphragm positive displacement pump. Both the DC and AC pumping system

incorporates the positive displacement pump.

The DC Motor (“Quad II Diaphragm 4406 Series Automatic Water System Pump With

Internal Bypass Valve”) was obtained from Flojet SA. This motor includes the positive

displacement pump attached to it. The Induction motor, however, was not sold or

readily available with a pump attached. Because of this, an extra DC motor was

purchased. The pump of the extra DC motor was then removed and placed onto the

induction motor.

As shown in the figure 11 [22] below, the pump will function when a diaphragm is

forced into reciprocating motion by mechanical linkage, compressed air, or fluid from a

pulsating, external source. The pump construction eliminates any contact between the

liquid being pumped and the source of energy. This eliminates the possibility of

leakage. Figure 11 [22] below is an indication of how the diaphragm pump operates.

Figure 11 – Positive Displacement diaphragm pump [22]

17

The positive displacement pump delivers a definite volume of liquid for each cycle of

pump operation. The only factor that affects the flow rate in an ideal positive

displacement pump is the speed at which it operates [22]. Figure 12 [22] below shows

the characteristic curve of the positive displacement pump.

Figure 12 – Positive Displacement Characteristic Curve [22]

The dashed line in the graph above shows the actual positive displacement pump

performance. This line reflects that as the discharge pressure of the pump increases,

some amount of liquid will leak from the discharge of the pump back to the pump

suction, reducing the effective flow rate of the pump. The rate at which the liquid leaks

from the pump discharge to its suction is called slippage [22].

18

5 DC MOTOR SYSTEM EXPERIMENTATION AND DATA

ANALYSIS

5.1 Basic Operation of a DC Motor

In a DC motor, the field winding is placed on the stator and the armature winding on

the rotor. A DC current is passed through the field winding to produce flux in the

machine. Voltage in the armature winding is alternating. A mechanical commutator and

a brush assembly function as a rectifier or inverter, making the armature terminal

voltage unidirectional. Figure 13 [23] below shows a DC motor and indicates how the

direction armature movement is attained [23].

Figure 13: DC Motor [23]

DC motors are used almost everywhere, as they are cheaper to produce, small in size

and powerful. There are various types of motors classified according to their excitation

field [4]: separately excite (permanent magnet), series and shunt motors. The

convenience of using a DC motor is, it does not require any converters [4], because the

solar panel supplies DC power.

19

5.2 Circuit parameters and characteristics of a DC motor

A permanent magnet DC motor (PMDCM) was used in the application of the solar PV

water pumping system. Figure 14 [4] below is a circuit diagram indicating the

permanent magnet DC motor, with, armature current (Ia), armature resistance (Ra) and

the motor voltage (Vm).

Figure 14: Permanent Magnet DC Motor [4]

The motor voltage Vm (V) is determined by the equation,

aam RIV += ε (3)

where Ia (A) is the motor armature current and Ra ( Ω ) is the motor armature resistance.

The electromotive force (ε ) is given by,

mε κφω= (4)

where φ (Wb) is the permanent magnetic flux, ω (rads/s) is the motor shaft angular

velocity and κ is the constant flux coefficient.

DC motors produce high torque at low speeds. The motor torque τ (Nm) is given by,

aIτ κφ= (5)

The speed of the motor can be calculated by substituting equation 3.2.2 into equation 3.2.1, hence, the speed of the motor is given by:

m a a

m

V I Rωκφ−= (6)

20

The speed-torque characteristics of the DC motor

Figure 15: Torque-speed characteristics of a permanent magnet DC motor, where Vt4 is the rated voltage [24]

Pressure Head can be expressed as, P

Hgρ

= (7)

5.3 DC motor system description The operating point of a DC photovoltaic pumping system depends on the current-

voltage (I-V) characteristics of both the motor –pump assembly and the PV array [14].

Tests were conducted in order to obtain the necessary I-V characteristic curve. The

system below was set up in the laboratory. The solar panel was replaced with a power

supply for an indoor performance test.

21

Equipment used during testing was:

• Power supply

• Water/Flow Metre (see Appendix D for manufacturer specifications)

• Pressure Gauge

• Quad II Diaphragm 4406 Series Automatic Water System Pump With Internal

Bypass Valve DC Motor (see Appendix B for manufacturer specifications)

• Positive Displacement pump

• Water tank

The closed loop circuit configuration was set up as shown in the figure 16 below:

Figure 16: Lab simulation of DC pumping system

5.4 Motor performance analysis

For a good load regulation, the internal impedance of any power supply is very low,

close to zero. The solar panel, however, has a high internal resistance (called series

resistance) present and their output power varies significantly with environmental

factors.

The DC motor system was then first tested without compensating for internal

impedance, and then tested compensating for internal impedance.

22

5.4.1 Test conducted without compensating for internal impedance

The Motor was run from rated voltage (i.e. 12V DC) and adjusted to lower

voltages, to observe at which voltage the motor stops running or stops pumping

water. A low voltage in this case would indicate a low level of solar irradiation;

where as rated voltage indicates a high level of solar irradiation.

The tests have shown that the motor keeps running at low voltages and the

motor stops pumping water at 0.08V at 0.6A. Reason being, as discussed

before, the power supply has zero internal impedance, and thus compensation

for internal impedance that is present in a solar panel has not been taken into

consideration.

5.4.2 Test conducted compensating for internal impedance

A test was then conducted compensating for internal impedance. The series

resistance was then calculated using values from the solar module SM55

datasheet.

Table 1 – Electrical Parameters of solar module SM55 (See Appendix for full

data sheet)

23

The short circuit current, as shown above in table 1, is ISC = 3.45A. The series

resistance was calculated as:

ds

sc

VR

I= , Where Vd is the voltage drop across the diode. Assuming the voltage

drop across each diode is 0.6V. Therefore, ds

sc

VR

I= =

0.60.17

3.45= Ω (series

resistance for one cell)

The total series resistance through 18 cells or half of the panel, i.e. 18 cells is

0.17 * 18 = 3.06

The power rating for the series resistance is thus, P = I2 R, where I is the rated

current of the motor (which is 2 Amps).

Hence, P = I2 R = (2)2(3.06) = 12.24 W

To get the best power-rating resistor, close to what was calculated, a 1 , 10W

and a 2.2 , 10W resistor was connected in series. Due to tolerance in resistors,

the total impedance across the two resistors was 3.6 , 20W.

The system was then tested and the motor was run from rated voltage, 12V, and

then adjusted to lower voltages to observe at which voltage the motor stops

running or stops pumping water.

From the test, we observed that the motor stalls at 4.5V, 1.02A. The table of

the recorded corresponding voltage and current readings are shown in table 2

below.

24

Table 2: DC Motor Voltage and Current readings (Valve Completely Open)

This shows and proves that at low levels or irradiation or cloud cover the DC

motor stops. Therefore during early hours in the morning and late afternoon

hours, as well as overcast days when irradiation levels are low, the DC system

will not be operational.

Figure 17 below represents the input power required by the pump as a function

of the pressure head. The pressure head is due to mechanical forces acting on

the fluid and is the vertical lift in height at which a pump can no longer exert

enough pressure to move the water. The head was simulated with a valve and

measured with a pressure gauge.

This does not exactly simulate a pumping system, as the head is dependant on

the pressure, which in turn varies with the pump speed. Therefore, in order to

simulate a particular head at different speeds, the valve needed continual

adjustment where the voltage was kept constant [18].

Voltage (V) Current (A) Flow Rate

(l/min) Input Power to

Pump (W) 12 1.71 5.5 20.52 11 1.6 4.5 17.6 10 1.47 4 14.7 9 1.32 3.5 11.88 8 1.22 3 9.76 7 1.08 2.8 7.56 6 0.95 2 5.7 5 0.92 1 4.6

4.5 1.02 0.25 4.59

DC Motor Stalls at 4.5V, 1.02A with minimum flow rate of 0.25 litres/min

25

Figure 17: Graph of input power required (W) vs. Pressure Head (Pa)

From the figure we can observe that as the input power required by the pump

increases, the amount of pressure head increases proportionately. (See

Appendix for full experimental results)

Input Power Required (W) vs Pressure Head (Pa)

0

10

20

30

40

0 50000 100000 150000 200000

Pressure Head (Pa)

Pow

er (W

) 12V10V8V6V4V

26

6 AC MOTOR SYSTEM EXPERIMENTATION AND DATA

ANALYSIS

6.1 Basic Operation of an Induction Motor

The induction motor is the most robust machine and the most broadly used machine in

industry. It consists of a stator and a rotor, which is mounted on bearings and separated

from the stator by an air gap. The stator and rotor winding both carry alternating

currents. The alternating current (AC) is supplied to the stator winding directly and to

the rotor winding by induction [25].

Figure 18 [23] below shows a cross-section of an induction machine.

Figure 18: Cross-section of an Induction Motor [23]

6.2 Circuit parameters and characteristics of an induction motor

The stator windings are connected to a three-phase AC voltage supply, which produces

an induced voltage in the rotor windings producing a rotor current. The rotor will rotate

in the direction of the stator-rotating magnetic field in the air gap [25].

27

The synchronous speed ns at which the stators rotating field rotate is greater than the

steady state speed n. The synchronous speed ns is given by,

120s

fn

p= or

4syn

fp

πω = (7)

where f is the supply frequency and p is the number of poles [19]. The difference

between the rotor speed n or ω and the synchronous speed ns or synω of the rotating

field is called the slip s and is given by [25],

syn s

s

n ns

n

ω ωω

− −= = (8)

The frequency induced in the rotor can be expressed as,

2f sf= (9)

where 2f is the frequency of voltage and current in the rotor, f is the supply frequency

connected to the stator and s is the slip.

For a three-phase machine the power input to the stator is,

1 1 13 cosinP V I θ= (10)

and the power loss in the stator is, 2

1 1 13P I R= (11)

The efficiency of the induction motor is,

out

in

PP

η = (12)

The efficiency is dependant on the slip and therefore if all losses are neglected except

those in the resistance of the rotor circuit, then the ideal efficiency becomes,

1outideal

in

Ps

Pη = = −

(13)

The ideal efficiency represents the ratio of the power output to the air gap power. This

indicates that for maximum efficiency the induction motor should operate at

28

synchronous speed. This is the reason why the slip is low for normal operation of the

induction motor.

The IEEE per phase equivalent circuit of a three-phase induction motor is shown in

figure 19 below.

Figure 19: IEEE Equivalent Circuit for an Induction Motor

For simplicity, V1, R1, X1 and Xm can be replaced by the thevenin equivalent circuit

values Vth, Rth, and Xth as shown in figure 20 below.

Figure 20: Thevenin equivalent circuit

Where, ( )

1 122 2

1 1

mth

m

XV V

R X X

= + +

(14)

The thevenin impedance is ( )( )

1 1

1 1

mth th th

m

jX R jXZ R jX

R j X X

+= = +

+ + (15)

29

where,

( )2

122

1 1

mth

m

R XR

R X X=

+ + (16)

and ( )

( )

21 1 1

221 1

m mth

m

X R X X XX

R X X

+ + =+ +

(17)

If, ( )221 1 mR X X+ then,

2

11

mth

m

XR R

X X +

and since 1 mX X therefore, 1thX X . The typical torque-speed characteristic of an induction motor is shown in the graph below.

Figure 21: Typical torque-speed characteristic of an Induction motor [26] From the equivalent circuits we can thus determine the performance characteristics of

the induction motor. The mechanical torque Tmech per phase of the induction motor can

be obtained as a function of the slip.

30

2 '2

'2 ' 22

2

1

( ) ( )

thmech

synth th

V RT

R sR X Xs

ω=

+ + + (18)

For the total mechanical torque of the three-phase induction machine, equation (18)

should be multiplied by 3.

6.3 AC motor system description

The starting of the induction motor must be started from rest and both the motor and the

load must be accelerated up to full speed. The closed loop circuit configuration for the

AC motor system is shown in figure 22.

Figure 22: Lab simulation of AC pumping system

31

Equipment used during testing was:

• Power Supply

• Water/Flow Metre

• Pressure Gauge

• 0.37kW, 6-pole cast-iron three-phase induction squirrel cage electric motor

• Positive Displacement pump

• 3-phase DC-AC Inverter

• Water tank

The author’s objective was to obtain the lowest power rated induction motor. A local

company, Alstom SA, provided the author a 0.37kW, 6-pole cast-iron three-phase

induction squirrel cage electric motor at a reasonably cost efficient price. The lowest

power rated motor, a 0.25kW motor was unavailable at the time.

The Induction motor, however, was not sold or readily available with a pump attached.

Because of this, an extra DC motor was purchased. The pump of the extra DC motor

was then removed and placed onto the induction motor. This had an additional

advantage of having identical pumps for each of the systems being compared. The

pump was connected to the induction motor as shown in the figure 23 below:

Figure 23: Shaft of pump aligned with shaft of induction motor

and connected using a heat shrink tubing

Heat shrink tubing

32

6.4 Motor performance analysis

An indoor performance test was conducted on the AC driven water-pumping

system. For testing purposes a variable frequency drive (VFD) was used in

place of the solar panel and 3-phase DC-AC inverter. This VFD was first

connected to a resistive load to enable the author to see the corresponding I-V

characteristics. The same test was then conducted with the Induction motor

system. Calculations, comparisons and conclusions were then made based on

the results obtained.

6.4.1 Resistive Load test at varying frequency and varying voltage

The resistive load was connected as a Y-connected load as shown in figure 24.

A digital ammeter and voltmeter was connected up as shown in the figure 24, to

measure the necessary readings required.

Figure 24: Y– connected load

Neutral

V

A

33

Figure 25: Illustration of Lab simulation of resistive load

The frequency was then varied between 0 – 50Hz and the corresponding voltage,

current readings were recorded. Table 3 shows the voltage, current and power readings

with varying frequency.

Table 3: Relationship between varying voltage and varying frequency for resistive load

VLN (V) I (A) f (Hz) P (W) 3*Power (W) 10 0.5 2 5 15 25 0.7 5 17.5 52.5 50 1 10 50 150 70 1.3 15 91 273

100 1.5 20 150 450 120 1.75 25 210 630 145 1.92 30 278.4 835.2 170 2.1 35 357 1071 190 2.25 40 427.5 1282.5 210 2.4 45 504 1512 230 2.55 50 586.5 1759.5

Voltmeter and Ammeter

Resistive Load

Variable Frequency

Drive

34

A graph was then plotted to show the relationship between varying voltages with

varying frequency for the resistive load.

Figure 26: Relationship between varying voltages (V) with varying frequency

(Hz) for resistive load

From the graph we can see that as the frequency increases, the voltage increases

linearly. This allowed the author to obtain the required voltage/frequency (V/f) ratio.

6.4.2 Induction motor test at varying frequency and varying voltage [27]

Figure 27 (a) below shows the family of torque-speed curves at variable voltage and

frequency and (b) shows the stator voltage (Vs), the magnetising current (Im) and the

rotor current (Ir) which is referred to the stator. The magnetising current combined with

the rotor current gives the stator current. At no load the magnetising current is equal to

the stator current.

The constant torque region in (a) consists of many linear sections. The peak amplitude

of the torque curve in the constant power and high-speed region reduces exponentially.

35

The maximum operating torque (Tem) reduces along the constant power hyperbola (Tem

* r = constant value) in the constant power region. In the high speed region the

maximum permissible torque reduces exponentially.

Figure 27: Induction Motor characteristics and capabilities [24]

A frequency beyond the rated frequency (f3) would require a higher voltage than rated

voltage or allowing the motor to draw current in excess of its rated current. These

conditions are not desirable and would ultimately lead to breakdown. Motor ratings are

usually exceeded by overloading the motor.

The stator voltage is non-zero at zero rotor speed (b). The voltage drop across the

resistance in the stator windings becomes larger relative to the applied voltage as the

voltage across the motor terminal is reduced. The boost voltage therefore compensates

for this voltage drop.

36

Any motor speed can be generated at a particular torque, which is determined by the

load as shown in figure 27, provided that the motor drive is able to apply the exact

voltage and frequency. If the load on the motor output causes the motor to operate near

or at the rated torque, which is in excess of the frequency range, concern has to be

taken not to overload the motor. This normally occurs in the region beyond the rated

frequency.

In PV water pumping applications the motors are usually oversized to allow for

efficiency improvements by the motor designer. Therefore, there is no danger in

overloading the motor since the motor is consistently operated in the partial load region

of about 60% of the rated torque or less.

6.4.3 Test Simulations and Results

The induction motor is rated at 400V, star-connected. The star connected load means

that the start-up current is minimized. Varying the frequency applied to the motor

controls the speed. Application of this speed control method also required the VFD.

If the voltage drop across R1 and X1 (Figure 9) is small compared to the terminal

voltage V1, then the motor flux is directly proportional to the voltage/frequency ratio

[25],

p V/f

The terminal voltage is thus varied in proportion to the frequency. This type of control

is known as constant volts per hertz [25].

The induction motor system was then set-up as shown in the figure below and tested to

see the variation in voltage with change in frequency.

37

(a) (b)

Figure 28: Photograph of (a) induction motor water pumping system and (b)

variable frequency drive

Table 4 below shows values of voltage as the frequency varied. Input power to the

pump was calculated as well as the flow rate in litres/second.

VLN

(V) Current

(A) f (Hz) Flow Rate (litres/min)

Flow Rate (litres/sec) Pin (W) = 3VI

2.887 0.1 0.65 0.001 0.00002 0.866 9.238 0.3 1 0.075 0.00125 8.314

13.279 0.4 1.5 0.175 0.00292 15.935 16.166 0.5 2 0.25 0.00417 24.249 20.207 0.55 2.5 0.35 0.00583 33.342 25.981 0.58 3 0.4 0.00667 45.207 28.868 0.6 3.5 0.45 0.00750 51.962 31.754 0.62 4 0.5 0.00833 59.063 34.641 0.65 4.5 0.57 0.00950 67.550 37.528 0.7 5 0.65 0.01083 78.808

Table 4: Induction Motor test showing values of varying voltage with varying frequency

Minimum Flow Rate of Induction Motor at 2.887VLN, 0.1A and 0.001 litres/min

38

Figure 29 below shows the relationship between varying voltages (V) with varying

frequency (Hz). As discussed before, varying the frequency f can control the speed of

the motor or the synchronous speed, keeping the flux in the air gap constant and

varying Vs in a linear proportion to f. It is clearly seen from figure 29 below that there is

a linear relationship between voltage and frequency.

Figure 29: Voltage (V) vs. Frequency (Hz) for Induction Motor

The amount of power consumed by the pump was then compared against the flow rate

and the frequency. The calculated input power illustrated in table 4 makes up the real

and reactive power. Therefore, a re-test was done measuring the real power using the

two-wattmeter method (by using two analog meters) as show in figure 30 below.

39

Figure 30: Illustration of Two Wattmeter Method

The table below shows the measured power obtained from the two wattmeters. Each

wattmeter had a scaling factor of 10. The total power for the load at each frequency is

P1 + P2.

Table 5: Induction Motor test showing real Power measured as the frequency is

adjusted

Frequency (Hz)

Power 1 (x10) (W)

Power 2 (x10)(W)

Flow Rate (litres/min)

Flow Rate (litres/sec)

Total Pin (W)

0.65 0.1 0 0.001 0.00002 1 1 0.3 0 0.075 0.00125 3 2 1 0 0.25 0.00417 10 5 3 0 0.55 0.00917 30

10 6 0 1.5 0.02500 60 15 9 1 2.5 0.04167 100 20 11 2 3 0.05000 130 25 13 3 4 0.06667 160 30 16 5 5 0.08333 210 35 18 6 5.5 0.09167 240 40 20 7 6 0.10000 270 45 22 9 7 0.11667 310 50 24 10 8 0.13333 340

40

The pump was then disconnected in order to measure the losses in the pump and the

corresponding speed was measured by placing a white strip across the shaft of the

pump (shown in figure 31) to measure the speed using a tachometer.

Figure 31: Illustration of white strip used on pump shaft to measure speed

The losses were then calculated as shown in the table below. An interesting observation

was then made. The losses in the pump turned out to be equivalent to the power

calculated when the pump was connected to the motor.

This means that the pump is almost hundred efficient and does not consume many

losses. Possible cause of this result may be that the power rating of the Induction motor

does not match the power rating of the pump. The positive displacement pump has

much lower power rating to that of the induction motor.

White strip used to determine speed using a tachometer

41

Table 6: Induction motor test showing real power losses (at no load) and speed at

which pump shaft rotates

Because the losses turned out to be similar when the pump was connected, different

characteristic figures were obtained for each set of data. This set of results is thus

equivalent to a no load test. The total pump power was obtained as a function of the

flow rate and speed at full load and the total power losses was obtained as a function of

the speed of rotation of pump at no load.

Figure 32: Pump Power (W) vs. Flow Rate (litres/sec) at full load

Frequency (Hz)

Power 1 (x10)(W)

Power 2 (x10)(W)

Power Loss (W) Speed (rpm)

0.65 0.1 0 1 0.5 1 0.3 0 3 10.1 2 1 0 10 44.1 5 3 0 30 129.8

10 6 0 60 221.2 15 9 1 100 294.8 20 11 2 130 400.5 25 13 3 160 500.9 30 15 5 200 597 35 17 6 230 693.4 40 19 8 270 799.7 45 21 9 300 895 50 23 11 340 995.5

Pump Power (W) vs. Flow Rate (litres/sec)

0

50

100

150

200

250

300

350

400

0.00000 0.05000 0.10000 0.15000

Flow Rate (litres/sec)

Pum

p P

ower

(W)

AC (Full Load)DC (Full Load)

42

From figure 32 above the author can deduce that the AC system requires less power to

pump water as compared to the DC system at minimum flow rate.

Since both motors made use of the same pump, technically they would operate at the

same synchronous speed. The pump power was then obtained as a function of the speed

as shown in figure 33 below.

Figure 33: Comparison between Pump Power (W) vs. Speed (rpm) for AC and DC

system

The corresponding speed matching to the minimum flow rate was obtained for each

system. The minimum flow rate for the DC system was 0.25 litres/min at a power

rating of 4.59W and synchronous speed ≈ 40 rpm. For the AC system the minimum

flow rate was 0.001 litres/min at a power rating of 1W and synchronous speed 0.5 rpm.

From this analogy the author deduced that the Induction motor system operates at lower

speeds and input power, compared to the DC motor system. Lower speeds and input

power means that the induction motor operates better at low levels of insolation, hence

maximizes the amount of water delivery during cloud cover.

Pump Power (W) vs. Speed (rpm)

050

100150200250300350400

0 500 1000 1500

Speed (rpm)

Pum

p P

ower

(W)

AC

DC

43

6.4.4 Factors affecting system performance during testing

An induction motor essentially operates at the frequency of the supply connected to it.

The rated frequency of the induction motor is 50Hz. In order for the motor to turn, the

frequency should be varied as the speed of the motor varies. This means that, as you

reduce the voltage of the motor the frequency needs to be reduced in order to retain the

same flux density. If the voltage is reduced and the frequency does not change, the flux

density will increase and the iron will saturate.

The inverter that was built should have had a constant voltage/flux ratio from the rated

frequency down to a low frequency. Thus at low frequencies, with a constant

voltage/flux ratio, the flux will reduce due to the ratio between the reactive component

and the resistive component of the stator. From the equivalent circuit of the induction

motor we want the current flowing through the magnetizing coil to be constant at all

frequencies, as this will allow us to maintain a constant flux.

When the AC motor system was set-up in the laboratory, the motor refused to operate,

as it should. An input DC voltage of 12V was supplied across the DC bus of the

inverter, using a power supply. Technically speaking, the output voltage from the

inverter should have been 230V AC per phase. However, the output voltage coming out

was 11V AC. The output voltage waveform from the inverter was a rectified signal,

only producing the positive half cycle. This is because the DC bus runs from 0V to

+12V, not supplying a negative supply rail for the inverter output.

The motor controller used to design the inverter did not allow the frequency to vary

with varying voltage. Due to this, with an input DC voltage of 12V DC, and an

induction motor rated at 400V AC, it was unable to rotate. The frequency of the

induction motor was fixed at 50 Hz. The DC bus voltage needed to be increased to

match the ratings of the Induction motor in order to operate as it should.

44

6.4.5 Possible improvements and solutions

Because the induction motor is a pseudo synchronous machine, it behaves as a speed

source. As discussed before, the running speed is set by the frequency applied to it and

is independent of the load torque provided the motor is not overloaded [26].

Due to time constraints, here are possible solutions to how the author would have gone

about solving the problem:

1. Increasing the voltage across the DC bus to match the system requirements

of running a high power rated induction motor of 400V AC.

- Problem with this is that many solar panels will be required to

increase the voltage at the DC bus to 400V.

- This affects cost and ordering of the panels would take long.

- This solution also does not allow automatic variation of frequency as

the voltage changes.

2. Adding a transformer to step the voltage up to the required voltage and use a

voltage inverter that converts the input voltage from the panel to produce a

negative voltage.

- Problem is, still does not allow automatic variation of frequency as

the voltage changes.

3. A possible working system will contain and take into account the following

procedures:

- To run a 0.37kW motor, at least 5-7 panels should be used, where

the system capacity should be extended to operate up to the full

rating of the motor.

- A variable voltage/frequency inverter, that automatically allows the

frequency to change as the input voltage changes.

- An equivalent size DC motor should be used such that the complete

AC and DC motor system-pump configuration is matched.

45

7 DC–AC INVERTER

7.1 Basic operation of DC-AC inverter

Inverters play an important role between a DC power and the AC power, which is

usually needed by standard household electrical systems. The inverter used in the

photovoltaic water pumping system is thus used to convert the DC voltage from the

solar array panel into AC voltage to drive an induction motor.

7.2 Inverter Design

Low impedance MOSFETs were used in the design of the inverter, rated at

60V (drain-source), R (drain-source) 0.008Ω. The inverter uses three sets of high speed

switching MOSFETS to create DC pulses that emulate all three phases of the sine

wave. An advantage of Mosfets, it has extremely low power consumption.

The three-phase inverter circuit consists of three legs, one for each phase as shown in

the figure below. The large filter cap across the DC bus is connected in parallel and acts

as an energy reservoir. This helps prevent fluctuations on the bus from disturbing motor

waveforms, i.e. the capacitor filters the AC ripple voltage before it enter the inverter

section. Fluctuations are often caused by voltage surges on the AC mains or high

frequency ripple resulting from rectification of the AC line.

Figure 34: DC-AC inverter connected to a 3-phase AC motor

Large Filter Cap

46

The MOSFET’s are driven by a pulse width modulated (PWM) signal. The PWM is

used to control the output voltage (or speed) and frequency with varying input voltage

Vdc. The expected output waveform generated by the inverter is shown in figure 35[28]:

Figure 35: Waveform generated by inverter of a PWM variable frequency drive

compared with that of a true AC sine wave [28]

7.3 MOSFET driver circuit

A high-speed power MOSFET and IGBT driver circuit was constructed with use of the

IR2113 IC, which has independent high and low side referenced output channels. See

Appendices for full circuit diagram with component values.

Figure 36: MOSFET Driver Circuit

15 V

6 PWM Inputs from Motor Controller

5V

6 PWM Outputs to 3-Phase Inverter

IR 2113 IC (Low and High Driver chip)

47

7.4 Interfacing with Motor Controller

A monolithic motor controller chip (MC3PHAC) was used for a low-cost, variable

speed, three-phase AC motor control system. The MC3PHAC is commonly used for

various applications such as pumps and fans.

This device consists of 6 outputs PMW’S which have been modulated with variable

voltage to control the three-phase AC motor. It also includes a start/stop motion motor

control feature. The start/stop pin is constantly high and when the start pin on the

device is set low, the motor starts up. This prevents accidental motor startup in the

event of the MC3PHAC being powered up, where the switch was left in the start

position. In order to accommodate for this feature, a switch was made (as shown in the

diagram below) for easy startup and stop.

Figure 37– Start/Stop Switch from motor controller

START/STOP SWITCH

48

Figure 38: Photograph of Motor Controller Circuit (See Appendices for full circuit

diagram)

Each switch on one inverter leg was monitored for one half cycle. The output voltage

waveform is shown in figure 39 below.

Figure 39: Square wave switching scheme from one inverter leg

Monolithic Motor Controller

IC

6 PWM Inputs to MOSFET driver

circuit

Start/Stop Switch

5V Regulator

49

8 CONCLUSIONS

Based on various tests conducted on the performance of both the AC and DC system,

the following conclusions can be drawn regarding both the DC motor and the induction

motor and inverter.

Due to the vast difference in the size of the DC motor compared to the Induction motor,

the motors of each system never formed a matched system; therefore the pump

performance of each system was compared as both systems used the positive

displacement pump.

The performance results obtained were:

For the DC motor, we observed that the motor stalls at 4.5V, 1.02A.

This means that at low levels of solar irradiation the DC motor pumping system

becomes inefficient.

The minimum flow rate for the DC system was 0.25 litres/min at a power rating

of 4.59W and synchronous speed ≈ 40 rpm.

The AC system the minimum flow rate was 0.001 litres/min at a power rating of

1W and synchronous speed 0.5 rpm.

The Induction motor system therefore operates at lower speeds and input power,

compared to the DC motor system at minimum flow rate.

Lower speeds and input power means that the induction motor operates better at

low levels of insolation, hence maximizes the amount of water delivery during

cloud cover.

Problems encountered during experiments:

Because the motors were difficult to compare since the AC Induction motor was

a much higher rated motor (or oversized) than the DC motor, each motor had a

different set of losses.

50

Power losses should have been measured at shaft of motor; however the DC

shaft was inaccessible. Power losses in the DC motor were thus assumed

negligible.

5-7 solar panels were required to drive an Induction motor rated at 0.37kW. Due

to time constraints and cost of panel’s, real life testing of the AC motor system

was not possible.

The hardware built, i.e. the 3-phase inverter, never allowed a varying frequency

with varying voltage for the AC system. The reason being, the motor controller

used operated at a fixed frequency of 50Hz.

Because of the big discrepancy between the two motors, a conclusive result could not

be made. However, a conclusive result was made based on tests conducted according to

the pump performance.

From theses tests the author concluded that the Induction motor-pump system is the

best-suited system for solar water pumping because:

The induction motor is maintenance free and relatively cheap.

Accommodates for wider speed control range.

Has the ability to pump water at low power levels, hence, at low levels of

irradiation the induction motor performs exceptionally well, and,

Subsequently, maximizes the amount of water delivery.

51

9 RECOMMENDATIONS

Based on the findings and conclusions of this thesis, the following recommendations

are made:

The complete motor-pump configuration should be matched (i.e. Use a higher

rated DC motor to match the AC motor.) between the two systems such that

suitable comparisons can be made between both systems.

The power rating of the induction motor should match the power rating of the

pump being used.

If, however, a lower rated induction motor cannot be found an equivalent rated

pump should be bought to match the ratings of the motor.

Use a sun tracker with the Induction motor system for a better optimal

performance of maximizing water delivery.

Generally for larger photovoltaic water pumping systems, the AC motor system

is a more viable option, because of the size of the motor available and cost. DC

motors systems are best suited for low power applications.

52

10 REFERENCES

1. Opening Speech by Western Cape Premier, Marthinus Van Schalkwyk, addressing

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10.1016/j.rser.2006.10.020

3. Huld T., Šúri M., Dunlop E., Albuisson M, Wald L (2005). Integration of

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53

12. Mona N Eskander and Aziza M. Zaki, “A Maximum Efficiency- Photovoltaic-

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drinking water in a remote residential complex”. Desalination 209 (2007) 58-63

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couple PV pumping systems”. Solar Energy 77 (2004) 81-93

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PV array size on PV water pumping system performance”. Solar Energy doi:

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17. M.D.J. Wainwright, “The Potential For Photovoltaic Water Delivery Systems in

South Africa”, Technical Report for Master of Business Administration Degree,

December 1981, (Pages 108-130)

18. J.L. Davies, “The Design and Optimization of a System using an Induction Motor

driven pump, powered by solar panels”, 30 April 1992 (Pages 14,)

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operating on a desert well, simulation and field tests”. Renewable Energy 30 (2005)

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Motor/Centrifugal pump assembly in a photovoltaic energy system”, Pergamon PII:

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Conservation and Management 47 (2006) 1159-1178

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24. Mohan, Undeland and Robbins, “Power Electronics”, Media Enhanced Third

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54

26. L M Photonics Ltd, “Induction Motor Control Theory”, [ONLINE]

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28. Joe Evans, Ph.D, “Variable Frequency 101”, June 2003

55

APPENDICES

Appendix A SM55 Solar Panel Specification Sheet Appendix B Flojet DC Motor Pump System Specification Sheet Appendix C Alstom Squirrel Cage AC Induction Motor Specification Sheet Appendix D Flow Meter Specification Sheet Appendix E The DC Motor Pumping Current and Voltage Test Results Appendix F Circuit diagram for Low and High MOSFET Driver Appendix G Circuit Diagram for Monolithic Intelligent Motor Controller (MC3PHAC) Appendix H Full diagram of DC – AC Inverter

56

Appendix A

SM55 Solar Panel Specification Sheet

57

Appendix B

Flojet DC Motor Pump System Specification Sheet

58

Appendix C Alstom Squirrel Cage AC Induction Motor Specification

Sheet

59

Appendix D

Flow Meter Specification Sheet

60

Appendix E

The DC Motor Pumping Current and Voltage Test Results

61

DC SYSTEM USING 3.6 OHM, 20 WATT RESISTOR

12V

Current Pressure (bar) Pressure (Pa) Input Power (W) 1.71 0.2 20000 20.52 1.82 0.4 40000 21.84 2.05 0.6 60000 24.6 2.27 0.8 80000 27.24 2.48 1 100000 29.76 2.67 1.2 120000 32.04 2.84 1.4 140000 34.08 2.92 1.6 160000 35.04

10V

Current Pressure (bar) Pressure (Pa) Input Power (W) 1.48 0.2 20000 14.8 1.6 0.4 40000 16 1.8 0.6 60000 18 2.05 0.8 80000 20.5 2.3 1 100000 23 2.45 1.2 120000 24.5 2.47 1.4 140000 24.7

8V

Current Pressure (bar) Pressure (Pa) Input Power (W) 1.23 0.2 20000 9.84 1.35 0.4 40000 10.8 1.58 0.6 60000 12.64 1.8 0.8 80000 14.4 1.84 1 100000 14.72 1.92 1.2 120000 15.36

6V

Current Pressure (bar) Pressure (Pa) Input Power (W) 1.05 0.2 20000 6.3 1.15 0.4 40000 6.9 1.31 0.6 60000 7.86 1.41 0.8 80000 8.46 1.42 1 100000 8.52

4V

Current Pressure (bar) Pressure (Pa) Input Power (W) 0.96 0.2 20000 3.84 0.96 0.4 40000 3.84

62

Appendix F

Circuit diagram for Low and High MOSFET Driver

63

Circuit diagram for Low and High MOSFET Driver

64

Appendix G

Circuit Diagram for Monolithic Intelligent Motor Controller (MC3PHAC)

65

Circuit Diagram for Monolithic Intelligent Motor Controller (MC3PHAC)

66

Appendix H

Full diagram of DC – AC Inverter

67

DC – AC Inverter

Side View of 3-phase Inverter with Filter Cap across DC Bus