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EEE Dept./Mahalakshmi Engineering college,Trichy-621213

MAHALAKSHMI ENGINEERING COLLEGE

TIRUCHIRAPALLI – 621213

QUESTION BANK WITH ANSWER

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Sub. Code : EE2352 Semester : VI

Subject : SOLID STATE DRIVES Unit : I

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DRIVE CHARACTERISTICS 1. What is meant by electrical drives?(Dec 2007)

Systems employed for motion control are called drives and they employ any of the prime movers such as diesel or petrol engines, gas or steam turbines, hydraulic motors and

electric motors for supplying mathematical energy for motion control. Drives employing electric motion are called electric drives.

2. Draw the electric drive system.(AU-Dec2009)

3. Specify the functions of power modulator.

Power modulator performs one or more of the following four funct ions.

a. Modulates flow of power from the source to the motor in such a manner that motor

is imparted speed-torque characterist ics required by the load.

b. During transient operations, such as starting, braking and speed reversal, it

restricts source and motor currents within permissible values; excessive current drawn from

source may overload it or may cause a voltage dip.

4. Mention the different types of drives.

1) Group drive

2) Individual drive


3) Multi motor drive

5. List the different types of electrical drives.

1) Dc drives

2) Ac drives

6. What are the advantages of electric drives?(AU-May 2009)

1) They have flexible control characteristics. The steady state and dynamic

characterist ics of electrical drives can be shaped to satisfy load requirements.

2) Drives can be provided with automatic fault detection systems, programmable

logic controllers and computers can be employed to automatically ctrl the

drive operations in a desired sequence.

3) They are available in which range of torque, speed and power. 4) It can operate in all the four quadrants of speed-torque plane. Electric braking

gives smooth deceleration and increases life of the equipment compared to other

forms of braking.

5) Control gear required for speed control, starting and braking is usually simple and

easy to operate.

7. What are the functions performed by electric drives?

Various functions performed by electric drives include the following.

a. Driving fans, ventilators, compressors and pumps etc.

b. Lifting goods by hoists and cranes

c. Imparting motion to conveyors in factories, mines and warehouses and

d. Running excavators and escalators, electric locomotives, trains, cars, trolley

buses, lifts and drums winders etc.

8. What are the disadvantages of electric drives?

The disadvantages of electric drives,

a. Electric drives system is tied only up to the electrified area.

b. The condition arising under the short circuits, leakage from conductors and

breakdown of overhead conductor may lead to fatal accidents.

c. Failure in supply for a few minutes may paralyses the whole system.

9. What are the advantages of group drive over individual drive?

The advantages of group drive over individual drive are

a. Initial cost: Initial cost of group drive is less as compared to that of the

individual drive.


b. Sequence of operation: Group drive system is useful because all the operations

are stopped simultaneously.

b. Space requirement: Less space is required in group drive as compared to

individual drive.

d. Low maintenance cost: It requires litt le maintenance as compared to individua l

drive.

10. Why the group drive is not used extensively?

The initial cost of group drive is less but this system is not used extensively because of

following disadvantages.

a. Power factor: Group drive has low power factor

b. Efficiency: Group drive system when used and if all the machines are not working

together the main motor shall work at very much reduced load.

c. Reliability: In group drive if the main motor fails whole industry will come to stand still.

d. Flexibility: Such arrangement is not possible in group drive i.e., this

arrangement is not suitable for the place where flexibility is the prime factor.

e. Speed: Group drive does not provide constant speed.

f. Types of machines: Group drive is not suitable for driving heavy machines such as cranes,

lifts and hoists etc.

11. Write short notes on individual electric drives.

In individual drive, each individual machine is driven by a separate motor. This motor also imparts motion to various other parts of the machine. Examples of such machines

are single spindle drilling machines (Universal motor is used) and lathes. In a lathe, the motor rotates the spindle, moves the feed and also with the help of gears, transmits motion to

lubricating and cooling pumps. A three phase squirrel cage induction motor is used as the drive. In many such applications the electric motor forms an integral part of the machine.

12. Mention the different factors for the selection of electric drives?

1) Steady state operation requirements.

2) Transient operation requirements.

3) Requirements related to the source.

4) Capital and running cost, maintenance needs life.

5) Space and weight restrict ion.

6) Environment and location.

7) Reliability.

13. Mention the parts of electrical drives.

1) Electrical motors and load.


2) Power modulator

3) Sources

4) Control unit

5) Sensing unit 17. What is meant by regenerative braking?

Regenerative braking occurs when the motor speed exceeds the synchronous

speed. In this case the IM runs as the induction generator it converting the mechanical power

into electrical power which is delivered back to the electrical system. This method of braking

is known as regenerative braking.

18. What is meant by dynamic braking?

Dynamic braking of electric motors occurs when the energy stored in the rotating mass

is dissipated in an electrical resistance. This requires a motor to operate as a generator,

to convert the stored energy into electrical.

19. What is meant by plugging?

It is one method of braking of IM. When phase sequence of supply of the motor

running at the speed is reversed by interchanging connections of any two phases of stator with respect to supply terminals, operation shifts from motoring to plugging region.

20. What is critical speed?

It is the speed that separates continuous conduction from discontinuous conduction mode.

21. Which braking is suitable for reversing the motor?

Plugging is suitable for reversing the motor.

22. Define equivalent current method.

The motor selected should have a current rating more than or equal to the current. It is

also necessary to check the overload of the motor. This method of determining the power

rating of the motor is known as equivalent current method.

23. What are the methods of operation of electric drives?

Steady state

Acceleration including starting

Deceleration including starting

24. Define four quadrant operations.

The motor operates in two mode: motoring and braking. In motoring, it converts

electrical energy into mechanical energy which supports its motion. In braking, it works as a

generator, converting mathematical energy into electrical energy and thus opposes the


motion. Motor can provide motoring and braking operations for both forward and reverse

directions.

26. How will you select the motor rating for a specific application?

When operating for a specific application motor rating should be carefully chosen that

the insulation temperature never exceed the prescribed limit. Otherwise either it will lead to its

immediate thermal breakdown, causing short circuit and damage to winding, or it will lead to

deterioration of its quality.

UNIT-1

FUNDAMENTALS OF ELECTRICAL DRIVES

1)what are the advantages of Electrical drives?

ADVANTAGES OF ELECTRICAL DRIVES:

1. They have flexible control characteristics. The steady state and dynamic characteristics of

electrical drives can be shaped t satisfy the load requirements.

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

3. Electric motors have high efficiency, low no load losses and considerable short time

overloading capability.

4. They are adoptable to any operating conditions such as explosive and radioactive

environment, submerged in liquids, vertical mountings, and so on.

5. Do not pollute the environment.

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

7. Unlike other prime movers, there is no need to refuel or warm-up the motor. They can be

started instantly and can be fully loaded.

2)Explain the parts of Electrical Drives?

PARTS OF ELECTRICAL DRIVES:

Electrical drives have the following major parts:

1. Power modulator.

2. Electric motors

3. Source


4. Control unit

Electrical Motors

Motors commonly used in electrical drives are: DC motors – shunt, series, compound and

permanent magnet; Induction motors – squirrel cage, wound rotor and linear; Synchronous

motors - wound field and permanent magnet; BLDC motors; Stepper motors and; Switched

reluctance motor.

Power Modulators

Drives may employ more than one power modulators. It can be classified in to

1. Controlled rectifiers

2. Inverters

3. AC voltage controllers

4. DC choppers

5. Cycloconverters

Sources

In India 1- phase and 3-phase 50 Hz supplies are readily available in most of the

locations. Very low power drives are fed from 1-phase supply and rest of the supply fed from 3-

phase source. Single phase source used for economic purpose. Most of drives powered from AC

source either directly or through converter link. When fed directly from 50 Hz ac supply,

maximum speeds of induction motor and synchronous motor are limited to 3000 rpm. Low and

medium power motors are generally fed from 400 V supply: for high power ratings, motor may

be rated 3.3 kV, 6.6 kV, 11kV.

In case of aircraft and space applications, 400 Hz AC supply is generally used to achieve

high power to weight ratio for motors.

Control unit

Controls for a power modulator are provided in the control unit. Control unit for a

particular drive depends on the power modulator used.


When semiconductor converters are used, the control unit consists of firing circuits, which

employ the linear and digital integrated circuits and transistor, and a microprocessor when

sophisticated control is required. PLC is used when solid state relays are used.

CHOICE OF ELECTRICAL DRIVES

Choice of electrical drive depends on the number of factors. Some of the important

factors are:

1. Steady state operation requirements: Nature and of speed torque characteristics, speed

regulation, speed range, efficiency, duty cycle, quadrant operation.

2. Transient operation requirements: Value of acceleration and deceleration, starting,

braking and reversing performance.

3. Requirements related to the source: Type of source, and its capacity, magnitude of

voltage, voltage fluctuations, power factor and harmonics.

4. Capital and running cost.

5. Space and weight restrictions.

6. Environment and location.

7. Reliability.

STATUS OF DC AND AC DRIVES

In earlier period induction and synchronous motor drives are used for fixed speed

application. DC motor drives are used for variable speed applications.

In later day’s development of variable speed induction motor drives and squirrel cage

induction motor over DC motors, DC drives are replaced by induction motor drives fro

variable speed applications.

Even though the induction motor is cheaper than DC motor, the control unit and the

converter of the induction motor drives is costly. So that total cost of the induction

motor drive higher then the DC drives.

AC drives were not as reliable as DC.

Development in linear and digital ICs, VLSI were helpful to improve the performance

and reliability.


AC drives preferred over DC drives with number of application and result.

3)write the equation governing the motor load dynamics.

DYNAMICS OF ELECTRIC DRIVES

Fundamental Torque Equations

A motor generally used to drive the load through transmission system. When the motor

rotates load start to rotate. If the load has many parts, few parts may rotate and some of them

under go translational motion.

The motor load system is represented by convenient equivalent rotational system as shown in the

Fig 1.1

Fig 1.1 Equivalent motor-load system

Motor – load system can be described by the following fundamental equation torque equation:

T – Tl = d/dt (jωm) = j dωm/dt + ωm dj/dt

The above is applicable for variable inertia drives. For drive with constant inertia,

( j dωm/dt) = 0.Therefore

T= Tl + j dωm/dt

The torque developed by the motor is counter balanced by a load torque Tl

and a dynamic torque j (dωm/dt)


4)Explain in detail the multi quadrant dynamics in the speed-torque plane.

MULTIQUADRANT OPERATION

A motor can operates in two modes – motoring and braking.

In motoring, it converts mechanical energy and in braking it works as a generator which

will convert mechanical energy in to electrical energy.

Power developed is product of speed and torque.

Multi quadrant operation of drives


Four quadrant operation of a motor driving a hoist load

The quadrant I operation of a hoist requires the movement of the cage upward. This

corresponds to the positive motor speed.

Since developed power is positive, this is forward motoring operation.

Quadrant IV operation is obtained when a loaded cage is lowered.

Counter weight comes down due to gravity itself, it is necessary for the motor to produce

positive torque.

As power and speed are negative, drive is operating in reverse braking.

Operation in quadrant II is obtained when empty cage is moved up. Since a counter

weight is heavier than an empty cage, it is able to pull it up.

Since speed is positive and developed power negative, it is forward braking operation.

Operation in quadrant II is obtained when empty cage is lowered.


Since speed is negative and developed power is positive, this is reverse motoring

operation.

5)derive the equation for equivalent moment of inertia and load torque of loads

with translational motion.

Components of Load Torques

Load torque can be further divided in to following components:

1. Friction torque Tf: Friction will be present in the shaft and also various part of the load.

2. Windage torque Tw: When motor runs, wind generates a torque opposing the motion.

3. Torque required to do the useful mechanical work TL: Nature of the torque depends on

the particular application. It may be constant and independent of speed.

SPEED TORQUE CHARACTERISTICS OF MOTOR AND LOAD

Different types of loads exhibit the different speed torque characteristics. However most

of the industrial loads can be classified into four categories:

1. Constant torque type

2. Generator type – torque proportional to speed

3. Fan type - torque proportional to square of the speed


4. Constant power type - torque inversely proportional to speed

1. Constant torque type load – characteristics

Most of the working machines mechanical nature of work like requires constant torque

irrespective of speed.

The speed torque characteristics of this type of load is given by T = K and is shown in

fig.

Speed torque characteristics of constant torque load

2. Generator type – torque proportional to speed

Separately excited DC generators connected to a constant resistance load, eddy

current brakes and calendaring machines a speed – torque given by T = Kw


Generator type – torque proportional to speed

3. Fan type - torque proportional to square of the speed

This type in which load torque proportional to the square of the speed.

Speed torque is given by T = Kω2

Generator type load

4. Constant power type - torque inversely proportional to speed

Certain type of loads exhibit hyperbolic speed torque characteristics.

In such loads torque is inversely proportional to speed.

This type of characteristics is given by T = K/ω


6)Illustrate the conditions of electrical drive from its heating and cooling curves.

(or)

SELECTION OF MOTOR POWER RATING

For loads which operate at a constant power and speed, determination of motor power

rating is simple. Most of the loads operate at variable speed and power, and patterns of the

variations are different for different applications.

Thermal model of motor for heating and cooling

A simple thermal model of the machine is obtained by assuming machine to be

homogeneous body.

Although inaccurate, such model is good enough for selecting the motor rating

for the particular application within safe limit.

Let the machine assumed to be homogeneous body, and the cooling medium has the following

parameters at time t:

P1 = heat developed, Watts

P2 = Heat dissipated to the cooling medium, Watts

W = Weight of the active parts of the machine, Kg

A = Cooling surface, M2

h = Specific heat

d = Co-efficient of heat transfer

Ө = Mean temperature

Heat absorbed in the machine = (Heat developed inside the machine – Heat dissipated to the

surrounding cooling medium)

WhdӨ = p1dt – p2dt


P2 = ӨdA

After rearranging the terms

cd Ө/dt = p1 - D Ө

c = wh

D = dA

The first order differential equation has a solution

Ө= Өss+Ke-t/

Өss = p1/D

= C/D

Constant of integration K is obtained by at t = 0

Ө = Өss(1-e-t/) + Ө1e-t/

If time measured from the load thrown off, then

CdӨ/dt = p1’ – D’ Ө

Ө = Өss(1-e-t/) + Ө1e-t/

Ө’ss = p1 / D’

In self cooled motors, where cooling fan is mounted on motor shaft.

Fig shows the variation of motor temperature rise with time during heating and cooling.


(7)Explain the different classes of motor duty.

CLASSES OF MOTOR DUTY

There are eight classes of duty:

1. Continuous duty: It denotes the motor operation at a constant load torque for duration

long enough for the motor to reach steady – state value

2. Short time duty: In this time of drive operation is considerably less than the heating time

constant and machine allowed to cool off to ambient temperature before the motor

operate again.

3. Intermittent periodic duty: It consist of periodic duty cycle, which has running period and

rest period. During the rest period long enough time to cool off to ambient temperature.


4. Intermittent periodic duty with starting: It consist of a period of starting, a period of

operation at constant load, a braking period: with operating and rest period being too

short to attain the steady sate temperature.

5. Intermittent periodic duty with starting and braking: It consist of a period of starting, a

period of operation at constant load, a rest period and braking period: with operating and

rest period being too short to attain the steady sate temperature.


DETERMINATION OF MOTOR RATING

From the point of view calculation of motor rating various duty cycle can be broadly

classified as:

1. Continuous duty.

2. Fluctuating duty.

3. Short time and intermittent duty.

MODES OF OPERATION

An electrical drive operates in three modes:

1. Steady state

2. Acceleration including starting

3. Deceleration including stopping.

SPEED CONTROL AND DRIVE CLASSIFICATIONS

Drives where the driving motor runs at a nearly fixed speed are known as constant

speed

Drives needing stepless change in speed and multispeed drives are called variable

speed drives.

When number of motor fed from common converter, or when a load is driven by

more than one motor, the drive is called as multi motor drive.


Quality of speed control system is measured in terms of speed – regulation which is defined

as

Speed regulation = No load – Full load speed / Full load speed * 100

CLOSED LOOP CONTROL OF DRIVE

Feedback loop in electrical drives may be provided to satisfy the one or more following

requirements:

1. Protection

2. Enhancement of speed of response

3. To improve steady – state accuracy

Current limit control

Current limit control used to control the converter and motor current.

It has feedback loop with threshold logic circuit.

During the transient operation current may exceed the limit.

Feed back loop used to limit the current within set of maximum value.

Closed loop control


Closed loop control finds the application in the battery operated vehicle.

When the driver presses accelerator for the reference T, though loop is avail actual

torque follows torque reference.

Closed loop speed control

Closed loop speed control employed widely.

It has two feed back loops

Inner feedback loop control the current of the converter and motor

Outer feedback loop control the speed.


8)Derive the mathematical expression for the analysis of steady state stability

of equilibrium point.

Equilibrium speed of a motor-Load system can be obtained when motor

torque equals the load torque. Electric Drive system will operate in steady state

at this speed, provided it is the speed of the stable state equilibrium.

Concept of steady state stability has been developed to readily evaluate the

stability of an equilibrium point from the steady state speed-torque curve of the

motor and load system.


9)A motor drive two loads. One has rotation motion. Its coupled to the motor

through a reduction gear with a=0.1 and efficiency of 90%. The load has a

moment of inertia of 10kg –m2 and a torque of 10N-M . other load has

translation motion and consist of 1000kg weight to be lifted up at an uniform

speed of 1.5 m\s. coupling between this load and motor has an efficiency

85%.Motor has an inertia of 0.2kg-m2 and runs at a constant speed of 1420

rpm. Determine equivalent inertia referred to the motor shaft and power

developed by the motor.



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