# Electric Drives Questions and Answers – Induction Motors – Dynamic Braking(IM)

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Induction Motors – Dynamic Braking(IM)”.

1. Order of the circuit is the number of memory elements present in the circuit.
a) True
b) False

Explanation: The order of the circuit is the number of memory/storing elements which are non-separable present in the circuit. The examples of memory elements are capacitor, inductor.

2. In Induction motor rotor laminations are thicker than stator laminations.
a) True
b) False

Explanation: The lamination thickness basically depends upon the frequency. The rotor frequency is s×f where f is stator supply frequency. The rotor laminations are thicker than stator laminations because rotor frequency is less.

3. Choose the Induction motor with peak speed.
a) 10 Pole
b) 12 Pole
c) 14 Pole
d) 16 Pole

Explanation: Synchronous speed of Induction motor is Ns=120×f÷P. Ns∝(1÷P). The synchronous speed is inversely proportional to the pole. The maximum speed will be achieved with a minimum number of poles.

4. Which starting method is the best method in Induction motor?
a) Direct online starting
b) Autotransformer starting
c) Reactance starting
d) Star-Delta starting

Explanation: The Star-Delta starting method is one of the best methods for the starting of the Induction motor. This method is cheaper and the line current in the star is 1÷3 times of in delta connection.

5. Induction motor is analogous to __________
a) Transformer
b) DC machine
c) Synchronous motor
d) Synchronous generator

Explanation: The Induction motor is analogous to the transformer. It is also known as a rotating transformer. Both works on the principle of electromagnetic induction.

6. Which type of Induction motor is best for pole changing method?
a) SCIM
b) WRIM
c) Single-phase IM
d) Linear IM

Explanation: Pole changing method is used to control the speed of Induction motor. It is only applicable for SCIM because SCIM rotor is made up of aluminum, copper bars.

7. _________ motor operates at high power factor.
a) Capacitor start
b) Capacitor run
d) Reluctance

Explanation: In the capacitor run, the capacitor is permanently connected with auxiliary windings. The capacitor is used for reactive power compensation. It improves the power factor of the machine.

8. The use of the capacitor banks in the 3-phase Induction motor is ________
a) To increase the power factor
b) To decrease the speed
c) To increase the speed
d) To decrease the power factor

Explanation: Capacitor banks are used with 3-phase Induction motor in order to improve the power factor of the machine. The inductor motor generally operates with .6-.8 power factor range.

9. The concept of the slip is used in ___________
a) Synchronous machine
b) Induction machine
c) DC machine
d) Transformer

Explanation: The slip in the Induction motor is s = Ns-Nr÷ Ns. The slip existence is due to the relative speed between the stator magnetic field and rotor.

10. Which type of Induction motor is best for e.m.f injection method?
a) SCIM
b) WRIM
c) Single-phase IM
d) Linear IM

Explanation: E.M.F injection method is used to control the speed of Induction motor. It is only applicable for WRIM because WRIM rotor consists of copper windings connected in star connection. The external e.m.f can be connected with it using the slip rings.

11. The slots used in the Induction motor are ___________
a) Semi-closed
b) Closed
c) Vacuum
d) Open

Explanation: The slots used in the Induction motor are semi-closed. Open slots are used in the synchronous machine. Semi-closed slots have moderate air gap and lesser flux leakage.

12. The slots used in the Synchronous machines are ___________
a) Closed
b) Semi-open
c) Vacuum
d) Open

Explanation: The slots used in the Synchronous machine are open. Open slots have higher air gap and minimum flux leakage. The maximum power transfer capability increase due to a decrease in the leakage reactance.

13. The unbalanced set of 3-phase voltages causes ________
a) Positive sequence current
b) Negative sequence current
c) Zero sequence current
d) Half sequence current

Explanation: The unbalanced set of 3-phase voltages causes a negative sequence current. The magnetic field generated due to negative sequence current is in the opposite direction to the fundamental magnetic field. This induces double the frequency current in the rotor side which causes overheating of the motor.

14. The ratio of the rotor gross power and air gap power is _______
a) s
b) 1-s
c) 1+s
d) 0

Explanation: The ratio of the rotor gross power and air gap power is 1-s. The air gap power in the induction motor is Pg and the rotor gross power is (1-s)×Pg. The ratio is 1-s.

Sanfoundry Global Education & Learning Series – Electric Drives.

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