## Sunday, September 2, 2012

### Electrical machines: Part 2

[1] In a DC motor, unidirectional torque is produced with the help of
a. Brushes
b. Commutator
c. End-plates
d. Both a and b

[2] The counter emf of dc motor
a. Often exceeds the supply voltage
b. Aids the applied voltage
c. Helps in energy conversion
d. Regulates its armature voltage

[3] The Eb/V ratio of a dc motor is an indication of its
a. Efficiency
b. Speed regulation
c. Starting torque
d. Running Torque

[4] The induced emf in the armature conductors of a dc motor is
a. Sinusoidal
b. Trapezoidal
c. Rectangular
d. Alternating

[5] A dc motor can be looked upon as dc generator with the power flow
a. Reduced
b. Reversed
c. Increased
d. Modified

[6] A series motor is best suited for driving
a. Lathes
b. Cranes and hoists
c. Shears and punches
d. Machine tools

[7]The Ta/Ia graph of a dc series motor is a
a. Parabola from no load to overload
b. Straight line throughout
c. Parabola throughout
d. Parabola upto full load and a straight line at overloads

[8] When load is removed, .......motor will run at the highest speed.
a. Shunt
b. Cumulative - compound
c. Differential compound
d. Series

[9] The power factor of a squirrel cage induction motor is
a. Low at light loads only
b. Low at heavy loads only
c. Low at light and heavy loads both
d. Low at rated load only

[10] The synchronous speed of a linear induction motor does not depend on
a. Width of pole pitch
b. Number of poles
c. Supply frequency
d. Any of the above

Answer: B. See the comment for explanation..

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10:10 AM

1. Doubtful about question no.10 As Ns=120F/P so how it could be option(b)Pls. clearify *b

1. Thanks for reading and asking question...
The equation Ns=12f/P is applicable to squirrel-cage Induction Motors( or simply general Induction Motor). In this equation Ns=synchronous speed, f=supply frequncy, p=no of poles

Whereas for linear Induction motor, the linear synchronous speed Vs=2wf.
where,
Vs=linear synchronous speed(m/s).
w= width of one pole-pitch(m).
f=supply frequency(Hz)

Linear induction motor is a kind of special Induction motor having linear stator and a linear rotor(having squirrel cage winding or simply a copper plate)...

Thanks again...

2. Thanks Elangovan, for clearing this answer actually i did not focus on linear induction motor.
*b*

3. Dear Elangovan Kindly upload more electrical objective question or tell me any site having important objective question of competition.*b* (bimal)

4. In question no 4 I think option b is correct..

5. Back emf shape in dc motor is of trapazoidal shape or sinusoidal ????