1. The amplitude of rotating MMF wave is proportional to ___________
a) Nph and P
b) I and P
c) Nph and I
d) Nph, I and P
Explanation: FR = 3/(2Fm) = 3/(2 4√2/(ΠKw NphI/P)
where Nph = series turns per phase and I = RMS current
2. If F1 is the constant amplitude of fundamental rotating MMF wave, the space harmonics of order 6K+1 (where K=1,2,3….), the harmonic MMF wave is of _____________
a) Constant amplitude and stationary in space
b) Constant amplitude and rotates along F1
c) Varying amplitude and rotates along F1
d) Constant amplitude and rotates against F1
Explanation: Space harmonics of the order of 6K+1(7,13,19,…) have their constant amplitude equal to 1/(6K+1)F1 and rotate in the direction of fundamental component F1 at a speed of 1/(6K+1) of the fundamental synchronous speed.
3. Which of the following two windings produces/induces the working flux and working EMF respectively?
a) field winding and armature winding
b) armature winding and field winding
c) both are produced in field winding
d) both are produced in armature winding
Explanation: Field winding produces the working flux and armature winding has the working EMF, which is induced by the working flux.
4. Armature winding carries which of the following current?
a) magnetizing current only
b) load current only
c) both magnetizing current and load current
d) none of the mentioned
Explanation: The winding on the machine that carries only load current is called armature winding
5. Field winding carries which of the following current?
a) exciting current only
b) load current only
c) both exciting current and load current
d) none of the mentioned
Explanation: The winding that handles only exciting current is called field winding.
6. Which of the following statements are correct regarding exciting current?
(i) exciting current varies with the load
(ii) exciting current doesn't vary with load
(iii) exciting current produces only a working magnetic flux
(iv) exciting current is not responsible for the production of working magnetic flux
a) (i) & (iv)
b) (ii) & (iii)
c) (i) & (iii)
d) (ii) & (iv)
Explanation: The current that produces only a working magnetic flux, and does not vary with the load on the machine is called exciting current, magnetizing current or field current.
7. Current in the field winding is ___________
a) AC always
b) DC always
c) Both AC and DC
d) Either AC or DC
Explanation: Field winding is always given DC supply and hence current is also DC
8. The rating of the armature winding is ___________
a) 1/2 to 2% of the rated power of the machine
b) less than the power rating of the machine
c) greater than the power rating of the machine
d) equal to the power rating of the machine
Explanation: The armature winding handles all the power that is being converted or transformed.
9.The power rating of the field winding is ___________
a) equal to the power rating of the machine
b) 1/2 to 2% of the rated power of the machine
c) greater than the power rating of the machine
d) less than the power rating of the machine
Explanation: The power input to the DC field winding is dissipated as i2R loss in the field winding (once the required field current is established).
10. What is the reason behind armature structure for both DC and AC machines being laminated?
a) reduce i2R losses
b) reduce the leakage flux
c) reduce the eddy current losses
d) for better operating power factor
Explanation: The armature windings of both the DC and AC machines have to deal with alternating current only; this is the reason why the armature structure of all rotating machines are laminated in order to reduce the eddy current losses.