21. 1800 second-rated motors are used for __________
A. Heavy-duty cranes
B. Light duty cranes
C. Medium duty cranes
D. Intermittent duty cranes
Answer: C
Medium duty crane motor is used to develop high starting torque. They provide the output for a specified interval of time without exceeding a specified temperature.
22. Calculate the frequency of the rotor side of the IM if the value of slip is 0.34 and the supply frequency is 70 Hz.
A. 23.8 Hz
B. 22.7 Hz
C. 24.5 Hz
D. 23.1 Hz
Answer: A
The frequency of the rotor side is low as compared to the supply frequency.
The rotor frequency is slip×(supply frequency)
=.34×70=23.8 Hz.
23. Calculate the frequency of the stator side of the IM if the value of slip is 0.04 and rotor frequency is 2 Hz.
A. 50 Hz
B. 49 Hz
C. 45 Hz
D. 51 Hz
Answer: A
The frequency of the stator side is high as compared to the rotor side. The stator frequency is (rotor frequency) ÷ slip
= 2÷.04 = 50 Hz.
24. Calculate the resonant frequency if the values of the capacitor and inductor are 4 F and 4 H.
A. .25 rad/sec
B. .26 rad/sec
C. .28 rad/sec
D. .29 rad/sec
Answer: A
During resonance condition XL=Xc. The value of the resonant frequency is
1÷√LC=1÷√16=.25 rad/sec.
The voltage across the capacitor and inductor becomes equal.
25. Calculate the value of the angular acceleration of the Heavy-duty crane using the given data: J = 5 kg-m2, load torque = 30 N-m, motor torque = 80 N-m.
A. 10 rad/s2
B. 20 rad/s2
C. 30 rad/s2
D. 50 rad/s2
Answer: A
Using the dynamic equation of motor J×(angular acceleration)
= Motor torque – Load torque: 5×(angular acceleration)
= 80-30=50, angular acceleration=10 rad/s2.
26. Calculate the value of the angular acceleration of the machine using the given data: J = .541 kg-m2, load torque = 10 N-m, motor torque = 2 N-m.
A. 45 rad/s2
B. The machine will fail to start
C. 30 rad/s2
D. 51 rad/s2
Answer: B
Using the dynamic equation of motor J×(angular acceleration)
= Motor torque – Load torque: .541×(angular acceleration)
= 2-10=-8, angular acceleration=-14.78 rad/s2.
The load torque is greater than the motor torque. The motor will fail to start.
27. The direction of the single-phase IM can be reversed by using which one of the methods?
A. By interchanging the supply terminals
B. By removing the capacitor
C. By changing the direction of auxiliary winding current
D. By removing the main winding
Answer: A
The direction of the single-phase IM can be reversed by changing the direction of the auxiliary winding or main winding current. The net torque developed in the IM is ImIasin(Φ).
28. Which motor is preferred for overhead traveling cranes?
A. Intermittent periodic motor
B. Continuous duty motor
C. Slow speed duty motor
D. Short time rated motor
Answer: A
An intermittent periodic motor is preferred for overhead traveling cranes. Cranes require high starting torque for a short interval of time. Intermittent periodic motors have low synchronous speed.
29. Which one of the following motors is a 1-Φ AC motor?
A. Synchronous motor
B. Series motor
C. Shunt motor
D. Capacitor run
Answer: D
The capacitor run motor is a 1-Φ AC motor. The capacitor is used to provide a 90° phase difference between the currents of main and auxiliary winding.
30. Current rating of the IM depends upon the cross-section area of the conductors.
A. True
B. False
Answer: A
The current rating of the Induction motor depends upon the cross-section area of the conductors.
