ROEVER ENGINEERING COLLEGEDEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

Solid State Drives

Two Marks Questions and Answers

UNIT – I: FUNDAMENTALS OF ELECTRIC DRIVES

1. What are the choices of electrical drives? or What are the factors to be considered during selection of electric drives?

a. Steady state operation requirements

b. Transient operation requirements

c. Requirements related to the source

d. Capital, running and maintenance needs, life

e. Space and weight restrictions if any

f. Environment and location

g. Reliability

2. What is meant by regenerative breaking?

Regenerative Braking is the operation of the motor as a generator, while it is still connected to the supply network. Mechanical energy is converted into electrical energy, part of which is returned to the supply. Rest of the energy is lost as heat in the windings and the bearings of the electrical machine.

3. What are the components of load torques?

a. Friction torque, (Tf) results from relative motion between surfaces

b. Windage torque,(Tw) is caused by the rotating components setting up air (or other fluid) movement.

c. Load torque,(Tl) required by the application.

4. Draw the multiquadrant operation of drives.

5. Define equilibrium operating point.

Equilibrium operating point is instant in which motor torque equals to the load torque.

6. Define steady state stability.

The steady state stability is the ability of the drive to restore to its equilibrium point after a small departure from it due to a disturbance in the motor or load.

7. What are the advantages of electrical drives?

i. They have flexible control characteristics.

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

iii. Electric motors have high efficiency, low no load losses and considerable short time overloading capacity.

iv. They are adoptable to almost any operating conditions such as explosive environment, submerged, vertical mountings and so on.

v. They do not pollute the environment.

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

vii. They can be started instantly and can immediately be fully loaded.

8. What is meant by acceleration?

Drive operates in acceleration whenever an increase in its speed is required. For this speed-torque curve is changed so that the motor torque exceeds the load torque (Tm>TL).

9. What is meant by deceleration?

Motor operation in deceleration mode is required when a decrease in its speed is required. Deceleration occurs when load torque exceeds the motor torque (i.e, Tm<TL). Whenever the reducing the motor torque to zero does not provide enough deceleration, mechanical brakes may be provided. Alternatively, electrical braking may be employed.

10. What are the three modes of operation of electrical drive?

i. Steady stateii. Acceleration including starting

iii. Deceleration including stopping

11. Write down the fundamental torque equation.

The torque developed by the motor (T) is counter balanced by the load torque (Tl) and a dynamic torque, J(dωm/dt).

12. What do you mean by four quadrant operation?

Quadrant I : Forward Motoring

Quadrant II : Forward Braking

Quadrant III : Reverse Motoring

Quadrant IV: Reverse Braking

13. What are the different types of drives?

i. Group Drive

ii. Individual drive

iii. Multimotor Drive

14. Mention some of the applications of electrical drives?

i. Transportation Systems

ii. Rolling Mills

iii. Paper Mills

iv. Textile Mills

v. Machine Tools

vi. Fans and Pumps

vii. Robots

viii. Washing Machines etc.

UNIT – II: CONVERTER/CHOPPER FED DC MOTOR DRIVE

1. Draw the speed - torque characteristics of a D.C.Shunt motor.

2. Mention the methods to control the speed of a DC shunt motor.

i. Armature Voltage Control

ii. Field Flux Control

iii. Armature Resistance Control

3. What is meant by duty cycle in a chopper circuit?

It is the ratio of the ON time period to the total time period of the chopper circuit.

4. Draw the diagram of regenerative fed separately excited D.C. motor drive?

Sp

eed

Torque

B

C

E

E

La

RaD2

CV

Tr1

5. Mention the applications of chopper fed dc drives.

i. Textile Mills

ii. Cranes

iii. Machine Tools

iv. Electric Traction

v. Electric Vehicles

6. Why thyristors are not preferred now-a-days for chopper fed DC drives?

Thyristors are not preferred for chopper fed DC drives because they need high power control signals and a separate commutation circuit for triggering.

7. What is discontinuous conduction mode?

When the armature current does not flow continuously, the motor is said to operate in discontinuous conduction mode.

8. Draw the regenerative braking characteristics of a separately excited motor.

9. What is meant by critical speed in phase controlled DC drives?

The speed which separates the continuous conduction from discontinuous conduction for a given is known as critical speed.

10. What is TRC control scheme?

It is a strategy to control the average output voltage by varying (ton/T). This is implemented in two ways such as,

a. Constant Frequency System

b. Variable Frequency System

11. What are the four quadrants of dc drives?

iaia1

ia2

tTton

va

0

Quadrant I : Rectifier

Quadrant II : Inverter

Quadrant III : Rectifier

Quadrant IV : Inverter

12. What are the advantages of three phase drives over single phase drives?

a. Reduced ripple in armature current

b. Requires less filtering arrangement

c. Armature current is mostly continuous

d. Improve performance

e. Increase power rating

13. What is a line commutated inverter?

Full converter with the firing angle delay greater than 90o is called line commutated inverter. Such operation is used in regerative braking mode of a DC Motor.

14. State the advantages of dc chopper drives.

i. Reduced current ripple

ii. Reduced Machine losses

iii. Elimination of discontinuous conduction

iv. High efficiency

v. Flexible control

vi. Fast response

15. Why self commutated devices are preferred over thyristors for chopper circuits?

They need low power control signals and no need of commutation circuit.

16. What causes poor input power factor in phase controlled dc drives?

Phase controlled converter requires power for control and commutation. The harmonics do not contribute to the active power loading but they contribute to the reactive loading of the line. So that the line power factor becomes poor.

UNIT – III : INDUCTION MOTOR DRIVES:

PART – A

1. What are the different methods of speed control of induction motors?

2. What are the advantages of stator voltage control method?

3. What are the disadvantages of stator voltage control method?

4. What are the advantages of squirrel cage induction motor?

5. What are features of variable frequency control?

6. What are the three region in the speed torque characteristics of the induction motor?

7. What is meant by v/f control?

8. What is meant by slip power?

9. What is meant by slip power recovery system? List some of its disadvantages.

10.What are the different types of slip power recovery system?

11.What is meant by sub – synchronous speed operation?

12.What is meant by super – synchronous speed operation?

13.Compare conventional method of Kramer and scherbius system.

14.Compare the static Kramer and Scherbius system.

15.What is a static Kramer drive?

16.Draw the diagram of a static Kramer drive.

17.What are the disadvantages of static Kramer system?

18.Why the power factor of the slip power recovery scheme of speed control of induction motor is low?

19.Sketch the mechanical characteristics of a three phase induction motor with v/f control.

20.Compare slip power recovery scheme with rotor resistance control.

21.Mention few general features of an induction motor on a CSI.

22.Induction motor speed control with constant supply voltage and reduced supply frequency is rarely used in practice – Justify.

23.Compare static rotor control scheme with static scherbius drive.

24.What are the disadvantages of ac drives with PWM inverters?

25.Name the methods of speed control applicable on the stator side of a three phase induction motor.

PART – B

1. Explain the operation of the constant slip speed control of an induction motor drive. (8)

2. Draw and explain voltage/frequency control. (8)

3. Draw and explain the slip power recovery scheme applicable for three phase slip ring induction motor. (16)

4. Explain the advantages of variable frequency induction motor drives. State and explain the various schemes for induction motor speed control by VSI’s. (16)

5. Explain in detail about the closed loop control scheme of an induction motor? (16)

6. Derive the expression for three phase induction motor using equivalent circuit? Also draw its speed – torque characteristics. (16)

7. Explain in detail about the stator voltage control of an induction motor? (16)

8. A 400 V, 4 pole, 50 Hz , three phase star connected induction motor has r1= 0, x1= x2= 1 Ω, r2 = 0.4 Ω, Xm= 500 Ω. The induction motor is fed from (1) A constant voltage source of 231 V/phase (2) A constant current source of 28 A. For both the cases calculate the slip at which maximum torque occurs and the starting and maximum torques. (16)

9. A 3-Phase, 20 KW, 4 pole, 50 Hz, 400 V, delta connected induction motor has the following per phase parameters referred to stator. r1=0.6Ω, r2=0.4Ω, x1= x2= 1.6Ω. Its magnetizing reactance is neglected. If this motor is operated at 200 V,25 Hz with DOL starting, calculate (i) current and power factor at the instant of starting and under maximum torque conditions, compare the results with normal values (ii) starting and maximum torques and compare with normal values.(16)

10.A 3 KW , 400 V , 50 Hz, 4 pole ,1370 rpm, delta connected squirrel cage induction motor has the following parameters referred to stator .R1 = 2 Ω, X1 = X2 = 5 Ω, R2 = 5 Ω, Xm = 90 Ω. Motor speed is controlled by stator voltage control. When driving a fan load it runs at rated speed at rated voltage. Calculate (1) Motor terminal voltage, current and torque at 1200 rpm (2) Motor speed, current and torque for the terminal voltage of 300 V. (16)

11.A 3-Phase, 20 KW, 4 pole, 50 Hz, 400 V, delta connected induction motor has rotor leakage impedance of 0.5 + j2.0 Ω. Stator leakage impedance and rotational losses are assumed negligible. If this motor is energized from a source of 3-Phase, 400 V, 90 Hz, then compute (a) The motor speed at rated torque (b) The slip at which maximum torque occurs and (c) the maximum torque. (16)

UNIT – IV : SYNCHRONOUS MOTOR DRIVES

PART –A

1. What is constant margin angle control of synchronous motor drive?

2. What is self control mode of synchronous motors?

3. What is a PMSM and discuss its advantages and disadvantages?

4. What are the classifications of PMSM?

5. Define torque angle? Mention some applications of PMSM?

6. When can the synchronous motor be load commutated?

7. What are the major applications of cycloconverter fed synchronous motor?

8. What are the advantages of cycloconverter drive?

9. What is a commutator less dc motor?

10.What are the modes of speed control of a synchronous motor?

11.What are the characteristics of true synchronous mode operated synchronous motor?

12.Give the advantage of using cycloconverter for synchronous motor speed control.

13.What are the different methods of forced commutation employed in the LCI fed synchronous motors?

14.What are the disadvantages of the VSI fed synchronous motor?

15.What is the effect different type of VSI on the line power factor and the synchronous machine powerfactor?

PART – B

1. A 3 phase, 400 V , 50 Hz, 6 pole star connected round- rotor synchronous motor has ZS = 0+j2Ω. Load torque, proportional to speed squared, is 340 N-m at rated synchronous speed. The speed of the motor is lowered by keeping V/f constant and maintaining unity pf by field control of the motor. For the motor operation at 600 rpm, calculate a) supply voltage b) armature current c) excitation angle d) load angle e) the pull – out torque. Neglect rotational losses.(16)

2. Explain in detail about wound field synchronous motor? (16)

3. Describe the self control of synchronous motor fed from VSI. Discuss about the separately controlled synchronous motor fed from VSI. (16)

4. Explain in detail about the open loop v/f control and self controlled mode of the synchronous motor? (16)

5. Write brief notes about

a. Constant margin angle control. (8)

b. Synchronous motor fed from cycloconverter. (8)

6. Explain in detail about PMSM drives. (16)

UNIT – V : BLDC, STEPPER AND SWITCHED RELUCTANCE MOTOR DRIVES

PART – A

1. What are the applications of Brushless DC motors?

2. What are the advantages of Brushless DC motors over conventional DC motors?

3. What is stepper motor.

4. What are the types of stepper motors?

5. Define micro-stepping.

6. What are the applications of stepper motors?

7. What is slew range?

8. What is the relation between stepper motor and switched reluctance motors?

PART – B

1. Explain the operation of the Brushless DC motor for servo applications. (10)

2. Compare variable reluctance stepper motor with the permanent magnet stepper motor. (6)

3. What are the advantages and disadvantages of stepper motors? (8)

4. Explain the operation of variable reluctance stepper motor. (8)

5. Describe a bipolar drive for stepper motors. Which stepper motors need bipolar drive? (8)

6. Explain the operation of switched reluctance motor. (10)

7. What are the advantages of switched reluctance motor drive over other ac motor drives? (8)

8. Explain the closed loop control of switched reluctance motor drive. (8)

9. Discuss the recent developments in electrical drives. (16)