Electrical Machines: Part 3

Electrical Machines: Part 3

[1] The torque-speed characteristics of an a.c. operated universal motor has a ______

characteristic and it______ be started under no-load condition.

(A) inverse, can

(B) nearly inverse, can

(C) inverse, cannot

(D) nearly inverse, cannot

Ans: C

N direct proportional to 1/ T

[2] In the heating process of the ________type a simple method of temperature control is

 possible by means of a special alloy which loses its magnetic properties at a particular

 high temperature and regains them when cooled to a temperature below this value.

(A) Indirect induction over

(B) core type induction furnace

(C) coreless induction furnace

(D) high frequency eddy current

Ans: D

Magnetic property of alloy changes with change of the temperature and Heat is produced 

due to eddy current = Square(i) * R and i proportional to square(f)

[3] In order to reduce the harmful effects of harmonics on the A.C. side of a high voltage 

D.C. transmission system ______are provided.

(A) synchronous condensers

(B) shunt capacitors

(C) shunt filters

(D) static compensators

Ans: C

Xc= 1/ωC

[4]An a.c. tachometer is just a ________with one phase excited from the carrier frequency.

(A) two-phase A.C. servomotor

(B) two-phase induction motor

(C) A.C. operated universal motor

(D) hybrid stepper motor.

Ans: D

It is a special purpose machine. It's stator coil can be energized by electronically switched 

current.

[5] The rotor frequency for a 3 phase 1000 RPM 6 pole induction motor with a slip of 0.04

 is________Hz

(A) 8

(B) 4

(C) 6

(D) 2

Ans: D

f = N P/ 120 = 1000* 6/120 = 50 Hz

Rotor frequency fr=s *  f = 0.04 * 50= 2.0 Hz

[6] The speed-torque characteristics of a DC series motor are approximately similar to those

 of the _________motor.

(A) universal

(B) synchronous

(C) DC shunt

(D) two-phase

Ans: A

Universal motor has same characteristics as DC series motor

It is known as an a.c series motor.

[7]In case of a universal motor, torque pulsation is minimized by _________.

(A) load inertia

(B) rotor inertia

(C) both rotor and load inertia

(D) none of the above

Ans: C

[8] A hysteresis motor

(A) is not a self-starting motor.

(B) is a constant speed motor.

(C) needs dc excitation.

(D) can not be run in reverse speed.

Ans: B

[9] The most suitable servomotor for low power applications is

(A) a dc series motor.

(B) a dc shunt motor.

(C) an ac two-phase induction motor.

(D) an ac series motor.

Ans: B

[10] The size of a conductor used in power cables depends on the

(A) operating voltage.

(B) power factor.

(C) current to be carried.

(D) type of insulation used.

Ans: C
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