Electrical Machines: Part 4

[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

Answer: 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

Answer:  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

Answer:  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.

Answer:  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

Answer: 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

Answer: 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

(c) None of the above

Answer: 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.

Answer: 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.

Answer: 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.

Answer:  C
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