Wednesday, September 5, 2012

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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8:30 PM

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