This set of Electrical Machines Multiple Choice Questions & Answers (MCQs) focuses on “Equivalent Circuit”.
Explanation: The Rc and Xm represent the copper losses and the core losses in the stator.
Explanation: Taking the actual reactance which is slip dependent in the circuit.
Explanation: The shunt resistance will represent only the core loss of the induction motor.
4. The electrical anlogous of the mechanical load in the induction machine is given by
Explanation: Mechanical load is represented by a factor (1-s)/s of the rotor resistance.
5. The actual rotor resistance referred to the stator if the induction motor is
Explanation: The actual resistance will be be r2 only.
6. The leakage reactances are less than their values at the full load operating conditions. Why?
a) Due to large inrush current during starting
b) Due to very low saturation
c) Due to speed is not same as synchronous speed
d) All of the mentioned
Explanation: During the start, there will be large inrush currents flowing in the machine, which cause saturation and so the leakage reactance decreases at start.
7. The leakage reactance and the rotor resistance are constant throughout the operation in an induction motor.
Explanation: No, the reactance changes with the variation in the slip of the machine.
8. At standstill the equivalent of the 3-phase induction motor is similar to
a) short-circuited two winding transformer
b) open circuit two winding transformer
c) stand still synchronous motor
d) none of the mentioned
Explanation: For s=1 at standstill, so it same as the short circuit of the transformer for the induction motor.
Explanation: Torque α (speed)^2; Power = torque*speed
Explanation: Torque α (speed); Power = torque*speed
Explanation: Torque = constant ; Power = torque*speed;
Explanation: Torque α 1/(speed); Power = torque*speed;
13. In a 3-phase induction generator which is self excited, load has been increased.To keep the frequency of the generated voltage constant, speed of the induction machine should be
c) maintained less than rated synchronous speed
d) maintained more than rated synchronous speed
Explanation: Frequency should be increased to maintain the speed constant.
14. A 3-phase induction machine is running at super synchronous speed. For the machine to be self excited, it
a) draws reactive power from mains
b) draws real power from mains
c) feeds reactive power to mains
d) generates emf by gradual voltage build up
Explanation: To provide the reactive power for the machine, it will draw reactive power from a source.
15. Induction generators will deliver power at
a) leading p.f.
b) lagging p.f.
c) unity p.f.
d) zero p.f.
Explanation: Induction generator will deliver power at leading p.f.
16. A star connected capacitor bank is connected to the induction motor having capacitance C and VA rating as VAs.
If the capacitor bank is replaced by a delta bank then
a) C = C/3, VA = VAs
b) C = 3C, VA = VAs
c) C = C, VA = VAs/3
d) C = C/3, VA = VAs/3
Explanation: The VA rating will not change due the configuration, but the capacitance will change due to the delta configuration to C/3.
17. A delta connected capacitance bank is preferred over a star connected bank. This is done to ensure
a) lesser capacitance
b) easy calculation
c) easier connection
d) all of the mentioned
Explanation: Delta connected bank will have lesser capacitance than star connected and so the better power factor improvement.
18. If it is desired to approximate the efficiency of an induction motor operating at slip ‘s’, then its efficiency is given by
Explanation: Approximately efficiency is given as 1-s/1+s for an IM.
Sanfoundry Global Education & Learning Series – Electrical Machines.
To practice all areas of Electrical Machines, here is complete set of 1000+ Multiple Choice Questions and Answers.