CHAPTER 1

INTRODUCTION

An inverter is a circuit which converts dc to ac. The output waveform of a basic two level inverter is a square waveform, this result in high THD value. The conventional single-phase inverter topologies include half-bridge and full bridge the half-bridge inverter is configured by one capacitor arm and one power electronic arm. The dc bus voltage of the half-bridge inverter must be higher than double of the peak voltage of the output ac voltage. The output ac voltage of the half-bridge inverter is two levels. The voltage jump of each switching is the dc bus voltage of the inverter. The full-bridge inverter is configured by two power electronic arms.

The popular modulation strategies for the full-bridge inverter are bipolar modulation and unipolar modulation. The dc bus voltage of the full-bridge inverter must be higher than the peak voltage of the output ac voltage. The output ac voltage of the full-bridge inverter is two levels if the bipolar modulation is used and three levels if the unipolar modulation is used. The voltage jump of each switching is double the dc bus voltage of the inverter if the bipolar modulation is used, and it is the dc bus voltage of the inverter if the unipolar modulation is used. All power electronic switches operate in high switching frequency in both half-bridge and full bridge inverters. The switching operation will result in switching loss. The loss of power electronic switch includes the switching loss and the conduction loss. The conduction loss depends on the handling power of power electronic switch. The switching loss is proportional to the switching frequency, voltage jump of each switching, and the current of the power electronic switches. The power efficiency can be advanced if the switching loss of the dc–ac inverter is reduced.

1.1 MULTILEVEL INVERTERS

1.1.1 Introduction

Multilevel inverters comprises of power semi-conductor devices and capacitor voltage sources. These generates stepped or staircase waveforms. The on and off these devices generates voltages in steps which when added gives high voltage at the output. Thus, we get high voltage at the output with low voltage at the semiconductor devices.

Multilevel inverter technology has emerged recently as a very important alternative in the area of high-power medium-voltage energy control. Multilevel inverters include an array of power semiconductors and capacitor voltage sources, the output of which generate voltages with stepped waveforms. The commutation of the switches permits the addition of the capacitor voltages, which reach high voltage at the output, while

the power semiconductors must withstand only reduced voltages. By increasing the levels of inverters, the THD (Total Harmonic Distortion) values of inverters will be reduced, by using multilevel. We get an approximate of sinusoidal wave by increasing the levels and also the losses will be reduced.

Fig 1.1 shows the schematic diagram of one phase leg of inverter with different of levels in which the semiconductor device is represented by an ideal switch with several positions. A two level inverter generates an output voltage with two levels (values) with respect to negative terminal of the capacitor as shown in Fig. 2.1(a) while the tree level inverter generates three level voltages and so on. Inverter with voltage level greater than two comes under multilevel inverter. The three level inverter was first introduced by Nabaeet al.It is found that with the increase in the level, the steps increases and the output waveform approaches to be a near sinusoidal waveform. Thus, it reduces the THD with a disadvantage of complex control and voltage imbalance problem.

 

Fig 1.1 one phase leg of inverter with a)two levels b)three levels c)n levels

1.1.2 Features of Multilevel Inverters

There are many remarkable features for the multilevel inverters .The output voltage waveforms obtained from the multilevel inverter is staircase waveform, and hence the THD and dv/dt is lowered. Overall efficiency is increased because multilevel inverters can be switched at low frequency. As a result common mode voltage are reduced and hence the stresses on the motor bearings are reduced. The input current drawn by them has low distortion and there exists no EMI problem

1.1.3 Topologies of multilevel inverter

 

The multilevel inverters are classified into three types:

1. Diode clamped multilevel inverter (Neutral Point Clamped inverter)(DCMLI)

2. Flying Capacitor Multilevel Inverter (Capacitor Clamped Inverter)(FCMLI)

3. Cascaded bridge Multilevel Inverter(CBMLI)

 

CHAPTER 2

2.1 ELECTRIC DRIVES

2.1.1 General Introduction

Motion control is required in large number of industrial and domestic applications like transportation systems, rolling mills, paper machines, textile mills, machine tools, fans, pumps, robots, washing machines etc. Systems employed for motion control are called Drives and may employ any of the prime movers such as diesel or petrol engines, gas or steam turbines, steam engines, hydraulic motors and electric motors, for supplying mechanical energy for motion control. Drives employing electric motors are known as Electric Drives.

Fig1.2 Block diagram of an electric drive

Load is usually a machinery designed to accomplish the given task. Usually load requirements can be specified in terms of speed and torque demands. A motor having speed torque characteristics and capabilities compatible to the load requirements is chosen. Power modulator performs one or more functions:

1. Modulates flow of power from the source to the motor

2. During transient operations such as starting, braking and speed reversal, it restricts source and motor currents within permissible values.

 

 

1. Converts electrical energy of the source in the form suitable to the motor ie,if the source is dc an induction motor is to be employed then power modulator is required to convert dc to a variable frequency ac.

2. Selects the mode of operation of the motor.

Controls for power modulator are built in control unit which usually operates at much lower voltage and power levels. In addition to operating the power modulator as desired, it may also generate commands for the protection of power modulator and motor. Input command signal, which adjusts the operating point of the drive, forms an input to the control unit. Sensing of certain drive parameters, such as motor current and speed may be required either for protection or for closed loop operation.

2.1.2 Advantages of electric drives

Electric drives are widely used because of the following advantages:

1. They have flexible control characteristics. The steady state and dynamic characteristics of electric drives can be shaped to satisfy load requirements. Speed can be controlled and, if required can be controlled in wide limits

2. They are available in wide range of torque, speed and power.

3. Electric motors have high efficiency, low no load losses and considerable overloading capacity.

4. Compared to other prime movers they have longer life, lower noise, lower maintenance requirements and cleaner operation.

5. Do not pollute the environment.

6. Can operate in all the four quadrants of speed –torque plane.

7. Can be started instantly and can be immediately fully loaded.

8. They are powered by electrical energy which has a number of advantages of other forms of energy. It can be generated and transported to desired point effectively and efficiently.

TYPES OF CONTROL IN INDUCTION MOTORS

 

2.1.3 Introduction

    Induction machines can be widely used in a variety of industrial and residential applications. Right from its ease of manufacture and its robustness have made it very strong candidate for electromechanical

conversion device. Induction motor is existing from fractional horsepower ratings to megawatt levels. Generally the speed control methods of induction motor can be classified as: Scalar control and Vector control.

To control the induction motor there are different types of control strategies are used incorporating the MLIs’, they are:

1. V/F control

2. Direct torque control(DTC)

3. Field oriented control(FOC)

2.1.4 V/F CONTROL

The frequency and the voltage are the main control variables in V/F speed control and are applied to the stator windings. Open loop volts/Hz control of induction motor is simple, inexpensive and one of the popular methods. It is characterized by the absence of feedback sensors. Even though the complexity in feedback is reduced the system produces a sluggish response and hence a disadvantage.

Closed loop speed control with slip regulation adds some performance improvement to open loop volts/Hz control. Voltage induced in stator is proportional to the product of supply frequency and air gap flux. Reduction in supply frequency, without a change in the terminal voltage will saturate the motor. Reduction in supply voltage keeping frequency constant leads to insufficient torque production. The V/f ratio is kept constant which in turn maintains the magnetizing flux constant. In this method only the magnitudes of frequency and voltage are controlled hence also known as “Scalar control”.

 

Fig 2.1 voltage versus frequency control in constant v/f principle.

Fig 2.2 Closed loop slip controlled drive using v/f control

 

Motor speed is compared with the command speed and error generates a slip frequency(ωsl*) command through P-I compensator and limiter.Slip is added to the feedback speed to generate the frequency and voltage command.Slip is proportional to torque at constant air gap flux,the scheme considered as ‘torque control within a speed control loop’.Machine can accelerate/decelerate within slip limit.The scheme compensates speed drift for supply voltage and load torque variations.

(a)

(b)

Fig 2.3 Speed drift for supply voltage and load torque variations

 

As TL increases from point 1 to point 2 on the curve wr tends to decrease,but will be compensated by the increase in frequency as shown in the fig 2.3(a). Similarly,if the supply voltage drops at constant TL , as in fig 2.3(b) operating point 1 on curve a will tend to drift to point 2on curve b , the speed tend to decrease. However the speed control loop will increment the frequency , and the speed drop will be restored as shown on curve c.The flux drop can be compensated by an individual flux control loop that corrects the voltage command restoring the torque-slip sensitivity.

 

 

 

2.1.5 DIRECT TORQUE CONTROL(DTC)

In this method of speed control the motor torque and flux become direct controlled variables and hence known as Direct Torque Control. In this method it is possible to obtain a good dynamic control of the torque without any mechanical transducers on the machine shaft. Transducers are device used for sensing the parameters of the device. And hence the method is used as “Sensor less type” control technique. It switches the inverter according to the load needs. Therefore the method helps in the elimination of the fixed switching pattern. DTC response is extremely fast during the instant load

changes.

Fig 2.4 Basic DTC Scheme Block diagram

 

Fig 2.5 Flux hysterisis comparator

Fig 2.6 Torque hysterisis comparator

 

The advanced scalar control methods based on Direct torque and flux control(known as DTFC or DTC) was introduced on1985.In this type of speed control the error between the estimated torque T and the reference torque T*  is the input of a three level hysteresis comparator. The error between the estimated stator flux magnitude Ѱs and the reference stator flux magnitude Ѱ*s  is the input of a two level hysteresis comparator. Input quantities: stator flux sector and outputs of the two hysteresis comparators. Flux loop has two outputs +1 and -1 & torque loop has three outputs +1,0 and -1.From the three inputs, the voltage vector table selects an appropriate voltage vector to control the inverter switches. The inverter voltage vector table also gets the information about the location of stator flux.

 

 

 

 

The selection of appropriate voltage vector is based on the switching table given as :

 

Table 2.1:Voltage vector selection

In Direct Torque Control, the equation for Electromagnetic torque is given by:

Te =3/2p (Lm/σLsLr) ϕsϕr sin θsr

 

 

Fig.2.7. Influence of vs overϕs during a sample interval Δt

From the torque equation, we can found that the electromagnetic torque is directly proportional to

the sine of the angle between the rotor flux ϕr, and stator flux ϕs. So we can conclude that

by varying the angle between stator and rotor flux, we can control the torque

according to the load variations. If the stator flux is leading rotor flux an accelerating

torque is produced and if the stator flux lags the rotor flux a decelerating torque is

produced. Thus the control strategy in DTC is based on this torque equation.

 

 

 

 

Fig 2.8 Stator flux trajectory

Fig 2.9 control of voltage vectors to flux vector trajectory

The stator flux vector rotates in a circular orbit within the hysteresis band covering the six sectors as shown in the figure 2.8.The six active voltage vectors and two zero vectors of the inverter is controlled by the look up table as shown in the figure 2.9.If a voltage vector is applied to the inverter for time Δt  the corresponding flux is given by the relation. The flux increment vector for each voltage vector is given in the fig.2.9.The flux is initially established at zero frequency in the initial trajectory aA. With the rated flux command torque is applied and the radial vector starts rotating in the counter clockwise direction within the hysteresis band depending on the selected voltage vector. The flux is altered in the radial direction due to flux loop error whereas torque is altered by the tangential movement of flux vector. The jerky variation of stator flux and angle introduces torque ripple. The lowest speed is restricted because of the difficulty of voltage model flux estimation at low frequency.

 

 

 

Table 2.2 Flux and torque sensitvityThe table summarises the torque and flux sensitivity and direction for applying a voltage vector for the flux location shown in fig 2.9.The flux can be increased by V1,V2and V6 whereas it can be decreased by V3,V4 and V5. The zero vector short circuits the machine terminals and keep the flux and torque essentially unchanged.

 

2.1.6 FIELD ORIENTED CONTROL(FOC)

The scalar control methods discussed so far are simple to implement but have a disadvantage of sluggish response because of inherent coupling effect of the machine. This is overcome in FOC control. Basically in vector control the induction motor is controlled like a separately excited dc motor.

In FOC Control there are two synchronous reference frame currents. The reference frame currents are isd and isq.isd is oriented in the direction of rotor flux and isq in quadrature with it. They remain orthogonal in space. However neglecting the leakage inductances ѱs=ѱr=ѱm. Therefore if the torque is controlled by iqs the rotor flux is not affected, thus giving fast torque response-principle of FOC. Three phase current vectors are converted to two dimensional rotating frame.

Fig 2.10 Block diagram of basic FOC scheme

Fig 2.11Simple block of FOC Control

 

 

Fig 2.12 (a,b) Vector control principle by phasor diagram

(c) Vector control implementation

 

 

In a vector controlled drive, the machine stator current vectorIs, has two components:ids or flux component and iqs or torque component as shown in the phasor diagram. These current components are to be controlled independently,as in dc machine, so as to control the flux and torque respectively. The ids is oriented in the direction of ѱr (or ѱm if the leakage reactance is neglected) ,and iqs is orthogonally to it. Ifiqs is increased to iqs’ the corresponding chenge in Is is shown in the fig 2.12(a). Similarly if ids is decreased to ids’ the change in Is is shown in fig 2.12(b). The machine model is shown in a synchronous model at right, and the two front end conversions of phase currents in a stationary frame is also shown. The controller should make two inverse transformations.where the unit vector cos θe and sin θe in the controller should ensure the correct alignment of ids in the direction of ѱr and iqs 90 degree ahead of it. Thus the unit vector is the key element of vector control.

There are two kinds of coordinate system in FOC. One is fixed on the stator, which is a static coordinate system relative to us; the other is fixed on the rotor, which is a revolving coordinate system. Both the three -phase stator A-B-C coordinate system based on three-phase winding of three-phase stator and the two-phase stator α-β coordinate system are the static coordinate system. The two-phase stator α-β coordinate system is composed of α axis fixed on A axis and β axis that is vertical to α axis. While d-q coordinate system with d axis fixed on the rotor spool thread is revolving.

In this control system, stator current iA, iB outputted by the inverter is measured using electric current sensor, and iC  is calculate with the formula iC=-(iA+iB). Transform the electric current iA,,iB, iC  into the direct component isq, isd  in the revolving coordinate system through the Clarke and the Park transform. Then isq, isd can be used as the negative feedback quantity of the electric current loop. The deviation between the given speed and the feedback speed n is regulated through the speed PI regulator. The output is q axis reference component isqref  to

control the torque .The deviations between isqref, isdref and current feedback quantity isq, isd go through the electric current PI regulator, and the respectively output phase voltage Vsqrefand Vsdrefon the d-q revolving coordinate system.Vsqref and Vsdref are transformed into the stator phase voltage vector component Vsαref and Vsβ ref under α-β coordinate system through inverse Park transform. If the stator phase voltage vector Vsqref, Vsdref and its sector number is known, we can use the voltage space vector PWM technique to produce PWM signal to control the inverter, so as to achieve closed-loop control of the induction motor.

The FOC control is also known as the “field oriented control”, “flux oriented control” or “indirect torque control”. Using field orientation (Clarke-Park transformation), three-phase current vectors are converted to a two-dimensional rotating reference frame (d-q) from a three-dimensional stationary reference frame. The “d” component represents the

flux producing component of the stator current and the “q” component represents the torque producing component. These two decoupled components can be independently controlled by passing though separate PI controllers.

CHAPTER 3

CONCLUSION

The multilevel voltage source inverters unique structure allows to reach high voltages with low harmonics without the use of transformers. The general structure of the multilevel inverter is to synthesize sinusoidal voltage from several levels of dc voltages. For this reason multilevel inverters are a natural fit for high power applications required for large electric drives. As the number of levels increases, the synthesized output waveform has more steps which produces a staircase waveform that approaches the sinusoidal waveform. As more steps are added to the waveform the harmonic distortions get reduced until it reaches zero with an infinite number of levels .As number of levels increases the higher the voltage which can be spanned by connecting devices in series.

The most commonly used electric drive is the induction motor drive due to its reliability, low cost and robustness. The methods used for the control of induction motor incorporating the multilevel inverters are: v/f control, direct torque control (DTC) and field oriented control(FOC).Among these, due to lack in transformation techniques , less complexity and fast response the direct torque control is the most efficient method for the control of induction motor.