51. _________ motor operates at high power factor.
A. Capacitor start
B. Capacitor run
C. Shaded pole
D. Reluctance
Answer: B
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.
52. 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
Answer: A
Capacitor banks are used with a 3-phase Induction motor in order to improve the power factor of the machine. The inductor motor generally operates with a .6-.8 power factor range.
53. The concept of the slip is used in ___________
A. Synchronous machine
B. Induction machine
C. DC machine
D. Transformer
Answer: B
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.
54. Which type of Induction motor is best for the e.m.f injection method?
A. SCIM
B. WRIM
C. Single-phase IM
D. Linear IM
Answer: B
The E.M.F injection method is used to control the speed of the Induction motor. It is only applicable for WRIM because the WRIM rotor consists of copper windings connected in a star connection. The external e.m.f can be connected with it using the slip rings.
55. The slots used in the Induction motor are ___________
A. Semi-closed
B. Closed
C. Vacuum
D. Open
Answer: A
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.
56. The slots used in the Synchronous machines are ___________
A. Closed
B. Semi-open
C. Vacuum
D. Open
Answer: D
The slots used in the Synchronous machine are open. Open slots have a higher air gap and minimum flux leakage. The maximum power transfer capability increase due to a decrease in the leakage reactance.
57. The unbalanced set of 3-phase voltages causes ________
A. Positive sequence current
B. Negative sequence current
C. Zero sequence current
D. Half sequence current
Answer: B
The unbalanced set of 3-phase voltages causes a negative sequence current. The magnetic field generated due to the 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.
58. The ratio of the rotor gross power and air gap power is _______
A. s
B. 1-s
C. 1+s
D. 0
Answer: B
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.
