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, Ia = V - Eb/Ra
At
starting; Eb = 0.
Stalling
current = V/Ra
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 Ia 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, Ra = 0.2 ohm, Ia = 50A.
Solution:
Back emf,
Eb=V-Ia x Ra
= 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.
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