Electric Motor

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  • Topic: Electric motor, Induction motor, Power factor
  • Pages : 13 (3234 words )
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  • Published : April 4, 2011
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Question bank for Energy Managers & Energy Auditors

Chapter 3.2: Electric Motors
Part–I: Objective type questions and answers 1. The synchronous speed of a motor with 6 poles and operating at 50 Hz frequency is ___. a) 1500 2. b) 1000 c) 3000 d) 750

The efficiency figures for energy efficient motors (in comparison with standard efficiency motor) can be generally higher by____%. a) 1% b) 3-7% c) 10% and above d) 8-10%

3.

The power consumption, in case of centrifugal loads (like pump, fan, blower etc.), proportional to ____. a) speed b) square of speed c) cube of speed d) not applicable

4.

Which types of following motors are most efficient? a) TEFC b) SPDP c) Both d) None

5.

What determines the thermal loading on the motor? a) Duty/Load cycle c) Age of the motor b) Temperature of the winding d) Ambient conditions

6.

Unbalance in voltages at motor terminals is caused by ____. a) Supplying single phase loads disproportionately c) Both (a) & (b) b) Use of different sizes of cables d) None of the above

7.

With decrease in speed of the motor, the required capacitive kVAr: a) Increases b) Decreases c) Does not change d) None of the above

8.

Application of DC motors is generally restricted to a few load speed applications because of _____. a) cost of the motor is high c) Maintenance problems b) problems with mechanical commutation d) None of the above

9.

The speed of an AC motor depends on ____ . a) Frequency c) both (a) and (b) b) No. of poles d) None of the above

10.

The speed of the motor can be varied by ___. a) Changing supply frequency c) Using multi speed windings b) Changing no. of poles d) All the above

11.

Which of the following are ill suited for energy efficient motors application? a) Pumps b) Fans c) Punch Presses d) All the above

12.

Reduction in supply voltage by 10% will change the torque of the motor by ___. a) 38% b) 19% c) 9.5% d) no change

3.2 electric motors - revised (table format)

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Question bank for Energy Managers & Energy Auditors

13.

Five percent increase in supply frequency will change the synchronous speed by _____. a) -5% b) +5% c) -10% d) +10%

14.

The inexpensive way to improving energy efficiency of a motor which operates consistently at below 40% of rated capacity is by _____. a) Operating in Star mode c) Operating in delta mode b) Replacing with correct sized motor d) None

15.

Which of the following needs to be measured after rewinding of motor? a) No load current c) air gap b) winding resistance d) all the above

16.

Output power requirements of constant torque loads vary with _____. a) Speed b) voltage c) Current d) power factor

17.

Which of the following is an AC motor? a) Slip ring motor c) Squirrel cage Induction b) Synchronous motor d) all of the above

18.

Stray losses in an induction motor generally are ____ . a) proportional to the square of the stator current b) proportional to the square of the rotor current c) proportional to the rotor current d) inversely proportional to the square of rotor current

19.

Which of the following information available on name plate of a motor? a) HP rating c) Frame model b) RPM d) all the above

20. Machine tools are a typical example of a) Constant power load c) Variable torque load b) Constant torque load d) a & b

Part – II: Short type questions and answers 1. Name the two important parameters that attribute to efficiency of electricity use by AC induction Motors? The important parameters that attribute to efficiency of electricity use by AC induction Motors are 1. Efficiency of the motor 2. Power Factor 2. How do you define percentage unbalance in voltage? Percentage unbalance in voltage is defined as [(Vmax – Vavg)/Vavg] x 100, where Vmax and Vavg is the largest and the average of the three phase voltages respectively.

3.2 electric motors - revised (table format)

11

Question bank for Energy Managers & Energy Auditors

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