In: Electrical Engineering
Q1: Choose the correct answer:
1- If the rotor of the induction motor is ……, then the rotor frequency will be zero
(A) Rotating at no load.
(B) rotating at the synchronous speed.
(C) locked.
(D) All of the above.
2- The …….. winding is placed near the transformer
core.
A) primary.
B) secondary.
C) low-voltage.
D) high-voltage.
3- An ideal transformer does not have any
A) copper losses.
B) core losses.
C) leakage
flux.
D) All of the above.
Q2: True or false:
1) The synchronous speed of the induction motor decreases as the supply frequency increases ( )
2) The motoring mode of the induction machine, the slip is lower than zero. ( )
3) Normally, the transformer efficiency does not depend on load current and load power factor ( )
4) The excitation current of the transformer is a very large current compared to the load current ( )
5) the open-circuit test of the transformer, the wattmeter reading represents the core-losses in the windings ( )
6) The induction motors are used in wind power generation ( )
Detailed solution is provided below :
Answer of Q1 :
1. (B) rotating at the synchronous speed.
Reason : frotor = s*fstator
At synchronous speed, s=0. So rotor frequency is zero.
2. (C) low-voltage
Reason : LV windig requires less insulation and it has less number of turns. HV winding requires more insulation and it has more number of turns. So size of machine is less if LV winding is near to core.
3. (D) All of the above
Reason : Ideal transformer doesn't have any losses and leakage reactance.
Answer of Q2 :
(1) The synchronous speed of the induction motor decreases as the supply frequency increases (False)
Reason : Synchronous speed is directly proportional to frequency (Because Ns=120f/p).
(2) The motoring mode of the induction machine, the slip is lower than zero. (False)
Reason : For motoring mode : slip is 0 to 1
(3) Normally, the transformer efficiency does not depend on load current and load power factor (False)
Reason : Efficiency depends on load.
(4) The excitation current of the transformer is a very large current compared to the load current (False)
Reason : Excitation current is used to magnetize the core. Its value is less compared to normal load current.
(5) The open-circuit test of the transformer, the wattmeter reading represents the core-losses in the windings (False)
Reason : Wattmeter reading represents core loss in core, not in winding.
6) The induction motors are used in wind power generation (False)
Reason : In wind power generation induction generator or Doubly Fed Induction Generators (DFIG) are used, not induction motors.
Hope these answers will help you. Thank you!