Important Short Questions and Answers: Electrical Engineering and Instrumentation - DC Machines

DC MACHINES

1. Give advantages of three phase system over single phase system.

Ø Output of three phase machine is greater than single phase machine of same size.

Ø Three phase transmission system is more economical than single phase transmission system as less copper or aluminum is required.

Ø Three phase motors are normally self starting as against single phase motors.

2. Define stalling current of DC motor.

Armature current, I_a = V - E_b/R_a

At starting; E_b = 0.

Stalling current = V/R_a

3. List the essential parts of a DC generator.

Yoke, Poles, Brushes, Bearings, Shaft, Commutator, Pole shoes, commutator poles and armature windings.

4. Why yoke is required in a DC machine?

It gives a protective cover to the machine and is a mechanical support for poles.

5. Why is the core of the armature laminated?

It helps in reducing eddy current losses.

6. Give the emf equation of a DC generator.

Generated emf, E=PΦZN/60A

Where, P- No. of poles, Φ – flux per pole, Z - No. of conductors, N – Speed of the armature and A - No. of parallel paths.

7. Give the type of armature windings used in DC machines.

1                   Wave winding

ii.                 Lap winding

8. Give the conditions to build up voltage in shunt generator.

i.                   Residual magnetism should be there in the poles.

ii.                 The field winding should be connected with armature in proper way.

iii.              The shunt field resistance should be less than the critical resistance under no load conditions.

9. List the main parts of stator of DC machine.

i.                   Yoke

ii.                 Field winding

iii.              Main poles

iv.              Commutator poles

v.

10. What is the Commutator pitch of a 4 pole DC armature having 49 Commutator bars?

Commutator pitch, yc = (No. of Commutator bars (+/-) 1)/No. of pairs of poles

= 49(+/-) 1/2 = 24 or 25

11.            State Faraday’s law of Electromagnetic induction.

Whenever there is a change in the magnetic flux linked with a circuit, an emf is induced in the circuit. The magnitude of the induced emf is proportional to the rate of change of flux linkages.

12. Give an application of a differentially compounded generator.

It is mainly used in arc welding where larger voltage drop is desirable with increasing in current.

13. Why series motor cannot be started without any load?

In series motor, Φ is directly proportional to I_a under no load conditions, the armature current is extremely low and flux is also less.

By N is inversely proportional to Φ, it is clear that; low Φ will result in extremely high motor speed. Hence series motor should always be started with some load on shaft.

14. What is the significance of back emf?

In motoring mode, armature induced emf is known as the back emf to stress the fact that it opposes armature emf. It plays the rule of a regulator.

15. Give the reason for high starting current in a DC motor.

For large motors the armature resistance may be 0.01 pu or less. Thus in full voltage starting of a DC motor, the armature current can be several times (100 times of large motors) the rated value.

16.            List the methods of speed control of DC shunt motor.

i.                   Field control

ii.                 Armature control

iii.              Armature resistance control

iv.              Ward-Leonard control

v.

17.            What are the different methods of excitation of generator?

i.                   Separate excitation

ii.                 Shunt excitation

iii.              Series excitation

iv.              Compound excitation

18. Define the term armature reaction?

The interaction between the flux setup by the current carrying conductors with the main field flux is defined as armature reaction.

19. Define critical resistance of a DC shunt generator.

It is defined as the resistance of the field circuit which will cause the shunt generator just to build up its emf at specified speed.

20. Mention the applications of DC series motor.

i.                   Electric traction

ii.                 Hoists

iii.              Cranes

iv.              Battery powered vehicles

21.            What are the functions of interpoles and how are the interpole windings connected?

Commutating winding is placed on interpoles to aid commutation process by inducing emf in commutating coils to cancel reactance emf. So interpoles are located the interpolar region along MNA of main poles in just above the brushes where the coils undergo commutation.

22. What is the importance of residual emf in self excited DC generator?

The residual emf in self excited DC generator is used to develop emf in an armature.

23. A DC motor operates from a 240V supply; the armature resistance is 0.2 ohm. Determine the back emf when the armature current is 50A.

Given Data: V = 240V, R_a = 0.2 ohm, I_a = 50A.

Solution:

Back emf, E_b=V-I_a x R_a

= 240-50 x 0.2 = 230V.

24.            An 8 pole, wave wounded armature has 600 conductors and is driven at 625 rpm. If the flux per pole is 20 mWb, determine the generated emf.

Given Data: P = 8, Z = 600, N = 625 rpm, Φ =20 mWb.

Wave winding, A = 2

Solution:

Generated emf, E=PΦZN/60A

= 8 x 20 x 10^-3 x 600 x 625/60 x 2 = 500V.