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

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

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

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.

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

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

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 _________.
(b) Rotor inertia
(c) Both rotor and load inertia
(c) None of the above

[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.

[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.

[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.