Electric Drives Questions and Answers – Motors Rating and Heating – Loading Conditions and Classes of Duty

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This set of Electric Drives Multiple Choice Questions & Answers (MCQs) focuses on “Motors Rating and Heating – Loading Conditions and Classes of Duty”.

1. Which duty cycle has on load and off-loads period?
a) Intermittent duty
b) Short time duty
c) Continuous duty with constant load
d) Continuous duty with variable load
View Answer

Answer: a
Explanation: Intermittent duty cycle has frequent on load and off-loads period. The on-load period and off-loads periods are too short to achieve steady-state thermal condition.
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2. Which motor is preferred for Jaw crushers?
a) WRIM
b) SCIM
c) Belt slip ring IM
d) DC shunt motor
View Answer

Answer: c
Explanation: Belt slip ring IM is preferred for the operation of Jaw crushers. A high starting is provided by the Belt ring IM. Slip rings are used to add external resistance.

3. A high starting torque is offered by the Belt Conveyors.
a) True
b) False
View Answer

Answer: a
Explanation: The Belt Conveyors can provide continuous-flow transportation. The amount of material transported on a conveyor system depends upon belt speed, belt width.

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

Answer: c
Explanation: The Reactance starting method is the worst method in an Induction motor. It uses the concept of the potential divider. The losses are very high in case of Reactance starting method.

5. Induction motor is also known as __________
a) Frequency changer
b) Frequency remover
c) Time period remover
d) DC machine
View Answer

Answer: a
Explanation: The Induction motor is also known as frequency changer. The stator frequency can be changed using the slip concept. The rotor frequency of IM is slip×(stator frequency).
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6. The capacity of the Crane is expressed in terms of tonnes.
a) True
b) False
View Answer

Answer: a
Explanation: Cranes are used to carry or lift the heavy loads. The capacity of a Crane is expressed in terms of tonnes. 1 tonne is equal to 1000 Kg.

7. 1800 second rated motors are used for __________
a) Heavy duty cranes
b) Light duty cranes
c) Medium duty cranes
d) Intermittent duty cranes
View Answer

Answer: c
Explanation: 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.

8. 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
View Answer

Answer: a
Explanation: The frequency of the rotor side is low as compared to supply frequency. The rotor frequency is slip×(supply frequency)=.34×70=23.8 Hz.

9. 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
View Answer

Answer: a
Explanation: 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.
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10. 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
View Answer

Answer: a
Explanation: 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.

11. 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
View Answer

Answer: a
Explanation: Using the dynamic equation of motor J×(angular acceleration) = Motor torque – Load torque: 5×(angular acceleration) = 80-30=50, angular acceleration=10 rad/s2.

12. 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
View Answer

Answer: b
Explanation: 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.

13. The direction of the single-phase IM can be reversed by using which one of the method?
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
View Answer

Answer: a
Explanation: The direction of the single-phase IM can be reversed by changing the direction of auxiliary winding or main winding current.The net torque developed in the IM is ImIasin(Φ).
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14. 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
View Answer

Answer: a
Explanation: 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.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn