Sunday, September 9, 2012

Electrical Machines: Part 5

[1] A stepper motor is
(a) A dc motor.
(b) A single-phase ac motor.
(c) A multi-phase motor.
(d) A two phase motor.

Answer: D
Stepper motor works on 1-phase-ON or 2-phase –ON modes of operation
[2] A motor which can conveniently be operated at lagging as well as leading power factors is the
(a) Squirrel cage induction motor.
(b) Wound rotor induction motor.
(c) Synchronous motor.
(d) DC shunt motor.

Answer: C
[3] The d.c. series motor should always be started with load because
(a) At no load, it will rotate at dangerously high speed.
(b) It will fail to start.
(c) It will not develop high starting torque.
(d) All are true.

Answer: A
[4] The ‘sheath’ is used in cable to
(a) Provide strength to the cable.
(b) Provide proper insulation.
(c) Prevent the moisture from entering the cable.
(d) Avoid chances of rust on strands.

Answer: A
The sheath in underground cable is provided to give mechanical strength.
[5] The drive motor used in a mixer-grinder is a
(a) DC motor.
(b) Induction motor.
(c) Synchronous motor.
(d) Universal motor.

Answer: D
The universal motor is suitable for AC & DC both supply systems.
[6] A balanced three-phase, 50 Hz voltage is applied to a 3 phase, 4 pole, induction motor. When the motor is delivering rated output, the slip is found to be 0.05. The speed of the rotor m.m.f. relative to the rotor structure is
(a) 1500 r.p.m.
(b) 1425 r.p.m.
(c) 25 r.p.m.
(d) 75 r.p.m.

Answer: D
NS = 120f /P = 120 x 50 /4 =1500rpm
N = NS ( 1-s) = 1500 (1-0.05) = 1425
Relative speed = 1500 – 1425 = 75 rpm
[7] The primary winding of a 220/6 V, 50 Hz transformer is energised from 110 V, 60 Hz supply. The secondary output voltage will be
(a) 3.6 V.
(b) 2.5 V.
(c) 3.0 V.
(d) 6.0 V.

Answer: C
[8] The current from the stator of an alternator is taken out to the external load circuit through
(a) Slip rings.
(b) Commutator segments.
(c) Solid connections.
(d) Carbon brushes.

Answer: C
[9] In a 3 – phase induction motor the maximum torque
(a) Is proportional to rotor resistance r2 .
(b) Does not depend on r2 .
(c) Is proportional to square root of r2 .
(d) Is proportional to  square of r2 .

Answer: B
[10] In a d.c. machine, the armature mmf is
(a) Stationary w.r.t. armature.
(b) Rotating w.r.t. field.
(c) Stationary w.r.t. field.
(d) Rotating w.r.t. brushes.

Answer: C
4:23 AM

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