EE6351 ELECTRICAL DRIVES AND CONTROL UNIT 4

CONVENTIONAL AND SOLID STATE SPEED CONTROL OF D.C. DRIVES PART A

1. Write down the speed equation of a dc motor.

N = (V – IaRa) / Kbφ

Where, V = applied voltage, Ia = armature current, Ra = armature resistance, φ =flux,

Kb= constant

2. What are the methods of speed control of dc motors? (Dec 2014)

Armature resistance control, Flux control, Voltage control.

3. What is meant by armature resistance control?(or) Why is armature voltage control used below rated speed? (Dec 2013) (May 2014)

A controller resistance is connected in series with armature. By varying the controller resistance R, the potential drop across the armature is varied. Therefore, the motor speed also varied. This method of speed control only applicable for speed less than no load speed.

4. What are the advantages and disadvantages of armature resistance control of dc shunt

motor? (Nov 2015)

Advantages: Simple method of speed control.

Disadvantages: This method is highly inefficient, because more power is wasted in

controller resistance. Change in speed with Change in load becomes large.

5. What is meant by flux control method?

The speed of the dc motor can be controlled by varying the field flux. This can be

increasing the speed of the motor above its rated speed, because the speed is

inversely proportional to the field flux.

6. What are the methods of speed control of dc series motor? Variable resistance in series with motor,Flux control method- Field diverter,

Armature diverter, Tapped field control, Paralleling field coils and Series parallel

control.

7. What is meant Ward Leonard system?

The speed of the dc shunt motor can be controlled by above and below rated speed

using the system. It consists of motor generator set. The armature voltage control

can be achieved by varying the field of the dc generator. The flux control can be

achieved by varying the field of the dc motor.

8. What are the advantages and disadvantages of Ward Leonard system?

Advantages:- 1. Full forward and reverse speed can be achieved.

2. A wide range of speed control is possible.

3. Power is automatically regenerated to the ac line to the mg set when speed is reduced.

4. Short time overload capacity is large.

5. The armature current is smooth.

Disadvantages:

1. High initial cost.

2. Overall efficiency low (less than 80%, because of the additional MG set).

3. Costly foundation and large amount of space is required. 4. This produces noise.

5. It requires frequent maintenance

9. What are the methods of speed control of induction motors?

Stator voltage control, frequency control, Pole changing method, Cascaded control, slip power control.

10. What are the advantages of solid state drives over conventional drives? (Dec 2011)

Wide variation of speed control is possible, less maintenance, simple, reliable, high

efficiency, low initial cost and faster response.

11. What is meant by electric-drive system?

An electric drive together with its working machines constitutes an electric drive system.

12. What are the different types of drives? Single phase dc drives, three phase dc drives, Chopper drives.

13. What are the two basic methods of speed control of dc motors? The dc motor speed controlled by (1) armature voltage control (below base speed)

(2) Flux control (above base speed).

14. What are the different types of single phase dc drives?

Single phase half controlled dc drives. Single phase full controlled dc drives.

15. What are the different types of chopper?

1. First quadrant chopper-motoring control

2. Second quadrant chopper-braking controls 3. Two quadrant chopper

4. Four quadrant chopper.

16. What are the advantages of dc chopper drives? (Dec 2014)

Dc chopper drives has the advantages of high efficiency, flexibility in control, light

weight, small size, quick response and regeneration down to very low speed.

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

1. Stator voltage control 2. Voltage/frequency control (v/f) 3. Rotor resistance control 4. Slip energy recovery control.

18. Draw the block diagram of phase controlled rectifier fed DC drives. (May 2013)

19. What is stator voltage control? Three phase induction motor speed can be controlled by varying the stator voltage.

This stator voltage can be varied by using ac voltage controllers. This method of

speed control of induction motor is called as stator voltage control. It is also called

soft start.

20. What is frequency control?

N = 120f/P

where N= speed of the motor , f = supply frequency, p= number of poles

Speed of the induction motor can be controlled by varying supply frequency as speed

is directly proportional to supply frequency.

21. What is meant by duty cycle? (Dec 2011) (May 2014)

Duty cycle is defined as the ratio between on time of chopper and total time of

chopperα= Ton/T

22. Draw the basic circuit for chopper controlled separately excited dc motor driv (Nov 2013)

23. List the Application of Chopper fed DC drives. (Nov 2015)

Cranes, traction applications, Battery operated vehicles, Subway cars.

24.Give the limitation of field control? (May2015) 1. The Speed control below normal rated speed is not possible as flux can be increased only upto its rated value. 2. As flux reduces, speed increases. But high speed affects the commutation making motor operation unstable. So there is limit to the maximum speed above normal possible by this method.

25. What are the main applications of Ward Leonard system? (May2015) Full forward and reverse speed can be achieved.

A wide range of speed control is possible.

Power is automatically regenerated to the ac line to the mg set when speed is reduced.

Short time overload capacity is large.

The armature current is smooth.

PART – B

1. Explain with neat sketch the chopper control method of speed control of DC Motors.

With this circuit diagram the various control strategies to control the speed of DC motors

can be explained.

2. Explain with neat sketches about the DC Shunt Motor speed control by using single

phase fully controlled bridge converter.

With the help of this circuit diagram the various stages of working should be

explained.

3. Discuss the Ward-Leonard speed control system with a neat circuit diagram. Also

mention its advantages and disadvantages. (Dec 2011) (May 2013) (May 2014) (Dec

2014)(Nov 2015)(May 2015)

Principle The speed equation of DC motor shows that by varying the supply voltage, the speed can be controlled.The speed of DC motor is directly proportional to the voltage applied to it. The speed equation is given by,

The Ward-Leonard method of speed control of DC motor uses voltage control, i.e., the

speed is controlled by varying the supply voltage (V). In this method the variable voltage

is obtained from ‘Motor-Generator set’.

Operation

The shunt field of the Main motor (DC shunt motor) is connected with fixed the DC

supply line. Here DC motor (M) and DC Generator (G) forms the Motor # Generator St

(i.e MG set). The armature of motor M is connected with the fixed DC supply lined

Shunt field of it is also connected with fixed DC supply. So motor M rotate with constant

speed. This motor is mechanically coupled with Generator (G). this forms MG set. The

armature of the Generator now rotates with the constant speed. The armature of

generator (G) is electrically connected with Main motor. The MG set supplies variable

voltage to the main motor.The speed control is obtained by the inserting a field

regulator (Rheostat ) in the field circuit of thegenerator (G). by adjusting the field

regulator, the flux is varied. Since the generator is rotating at constant speed, the

voltage produced by it depends on flux. When the flux is varied by adjusting the

field regulator, the variable voltage is produced by generator (G). This variable voltage

is applied to armature of main motor. So the speed of the main motor (DC shunt motor)

is varied due to variable voltage. Thus the speed is controlled by voltage control in

Ward – Leonard Method.

The reversing switch RS is used to reverse the connection of field terminals. As a result

voltage generated is also reversed. This reversed voltage when applied to the main

motor gives rotation in the opposite direction. Therefore in this method, the speed control

is possible in the both the direction

Fig. 4 Speed control by Ward Leonard Method or Voltage Control Method.

Advantages 1. It is possible to achieve lowest limit of speed variation of 5 to 7% of its base speed. 2. Smooth speed control over very wide range is possible. 3. Rapid starting and reversing of the motor without losses in the rheostats are possible. 4. By using RS switch, speed control in both direction is possible. 5. the rheostat is in the field circuit of the generator, so very small current flows through it. As a result power loss is loss. 6. regenerative braking is also possible with this method. This is achieved by suddenly decreasing the generated voltage of the generator, to a value less than the back emf of the main motor. Hence the main motor works as generator. As a result power will be supplied back to the network. 7. In this method speed of the motor is independent of the load on the motor.

Disadvantages

1. The initial cot is high because three machines are required. 2. the efficiency of the speed control is low because there is two stages of power conversion namely, mechanical to electrical (MG set) and electrical to mechanical (Main motor)

4 Explain with neat sketch the operation of a chopper fed DC Series motor drive. Also

derive the expression for average motor current.(Dec 2014)

5. Explain first quadrant chopper control of separately excited motor for continuous conduction.

(May 2013)

6. Explain in detail the single phase semi-converter speed control for DC drive for separately

excited motor.

The speed control of Dc drives by controlling the gate pulses during forward biased and

reverse bias condition should be explained with necessary waveforms.

7. A 500V series motor having armature resistance and field resistance of0.2 Ω and 0.3 Ω

respectively runs at 500 rpm when taking 70A. Assuming unsaturated field, find out its speed

when field diverter of 0.684 Ω is used constant load torque.

Answer : Eb1 = 465 V, Ia2 = 83.96 A, Eb2 = 465.703 V, speed N2 = 418 rpm

8. A 220V DC Shunt Motor takes 5A on no load and runs at 750 rpm. The resistances of

armature and field winding are 0.2 ohms and 110 ohms respectively. Calculate the speed

when the motor is loaded and taking a current of 50A. Assume the armature reaction

weakens the field by 3%.(Dec 2014)

Answer: 745 rpm

9. With neat circuit diagram, explain chopper fed four quadrant dc drive.(Dec 2013)(Nov 2015)

Class E is a four quadrant chopper. The four quadrant operation should be explained.

9. Explain the operation of single phase full converter fed separately excited dc motor

drive.(Nov2013)(May 2014)

10. Explain with neat sketch the chopper control method of speed control of DC Motors.

With this circuit diagram the various control strategies to control the speed of DC motors

can be explained.

11. A 220V DC shunt motor having a field flux of 0.8 wb, runs at a speed of 900rpm. Find

the speed of the motor, if the field flux reduced to 0.6wb by field resistance control

method. (Nov 2015).

12. A 220V, 1200rpm , 1Φ full converter fed separately excited DC motor having a armature

resistance and current of 0.25Ω and 40 A resepectively. For the delay angle of 300 , find

the speed of the motor . Consider motor constant KaΦ=0.18 N/rpm. (Nov 2015)

14 Explain the single phase half wave converter drive speed control for DC drive with waveforms.(May 2015)

SINGLE PHASE SEMI CONVERTER:

load.

Field winding is inductive in nature.So consider the load for single phase semiconverter as RL

15 A 220V, 70A dc Series motor has combined resistance of armature and field resistance of 0.12 ohm. Running on no load with field winding connected to a separate source it gave the following magnetization characteristics at 600rpm:

If(A) 10 20 30 40 50 60 70 80

Vt(v) 64 118 150 170 184 194 202 210

Motor is controlled by chopper with a source voltage

= 220V. Calculate

(i) Motor speed for a duty ratio of 0.6 and motor current

of 60A. (ii) Torque for a speed of 400 rpm and duty ratio

of 0.65. (May 2015)