Answers of Electrical Machine Test No.2

Q.1  Ans: C

Explanation: 

P_g=P_m/(1-s)   so, P_m=(1-s).P_g

Q. 2  Ans: B

Explanation:

As, maximum torque, T_m=E₂²/2X₂,

Therefore, maximum torque is independent of rotor resistance r₂ but the slip and speed at which it occurs depends on rotor resistance.

Q.3  Ans: C

Explanation:

Always the armature mmf  will be stationary w.r.t  main field. It is the basic principle for generation of unidirectional torque.Otherwise,there is no flux cutting action.

Q.4 Ans: B

Explanation:

Now, if the voltage regulation is zero,R₂/X₂=-sinΦ₂/cosΦ₂=-tanΦ₂=tan(-Φ₂) so,

(- Φ₂) means leading power factor.

Q.5  Ans: D 

Q.6  Ans: C 

Explanation: 

Here V₁=110V ;  N₁/N₂=220/6

As, V₂/V₁=N₂/N₁,

Therefore, V₂=V₁*N₂/N₁=110*6/220=3.0V

Q.7   Ans: C

Explanation:

Magnetic flux lags emf induced by 90^o because flux is proportional to a rate of change, dΦ/dt.

Q.8   Ans: all options are correct

Explanation:

Because the rotating magnetic fields produced by the stator and rotor are stationary with respect to each other

Q.9   Ans: C 

Q.10  Ans: C

Explanation:

Because the field current can be varied independently.