# MCQs on Electrical Machines

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01․ Two generators G1 and G2 are to be operated in parallel to deliver a total current of 60 A. Generator one has a terminal voltage of 280 V on no load and 240 V when supplying 40 A current. Similarly the second generator has a voltage of 300 V when on no load and 240 V when supplying of 50 A of current. The output voltage of each generator will be
240 V.
238.2 V.
271.7 V.
256.4 V.

In first generator, voltage drop per ampere = ( 280 - 240 ) / 40 = 1 A. Let output current is IG1, then output voltage will be VG1 = 280 - 1 X IG1 In second generator, voltage drop per ampere = ( 300 - 240 ) / 50 = 1.2 A. Let output current is IG2, then output voltage will be VG2 = 300 - 1.2 X IG2. For parallel operation, output of each generator would be same i.e. VG1 = VG2 ⇒ - IG1 + IG2 = 20 Total current delivered, IG1 + IG2 = 60 ⇒ IG1 = 23.6 A and IG2 = 36.4 A So, the output voltage VG1 = VG2 = 256.4 V

02․ In above question, the output power delivered by generator G1 and G2 will be
9.3 kW, 9.3 kW.
6.3 kW, 9.3 kW.
9.6 kW, 12 kW.
5.7 kW, 8.7 kW.

In from hints number 19, we already get VG1 = VG2 = 256.4 V and IG1 = 23.6 A and IG2 = 36.4 Amp. So, power delivered P = VI.

03․ Armature reaction in dc motor results
decrease in speed.
increase in speed.
short circuit.
open circuit.

The effect of the magnetic field developed by the current carrying armature conductors on the distribution of flux under the main poles is called armature reaction. Because of armature reaction the flux will reduce then emf will reduce then speed will increase.

04․ In shunt dc motor the direction of rotation of the motor will reversed. When
either field terminals are reversed or armature terminals are reversed.
only armature terminals are reversed.
only field terminals are changed.
none of above.

Whenever field or armature terminals is reversed then the direction of rotation of the motor will reverses.

05․ What is the necessity of copper brushes in a dc motors?
To absorb the mechanical shocks produced during running condition.
To convert alternating supply current to direct current.
To increase the flux cutting by the rotor.
None of these.

The main function of the brushes is to relay the current from external circuit to the rotating commutator form where it flows into the armature windings. Which is not given in the options.

06․ The armature core of the rotor bars of dc motors are laminated
to reduce hysteresis loss.
to reduce mechanical losses.
to reduce eddy current loss.

The armature core of the rotor bar of a dc motor is made of several sheets of laminated steel stacked together, to reduce the path for the flow of eddy current, and thereby reducing eddy current loss in the core.

07․ A lap wound 4 pole dc motor has 460 conductors. Find the number of parallel paths between the conductors?
2.
4.
6.
data provided is insufficient.

In case of a lap wound dc motor the number of parallel paths is equal to the number of poles, irrespective of all other parameters. It can be easily remembered by remembering the word LAP ------- L A = P. So here number of poles = 4

08․ The armature resistance of the dc motor is generally of the range
0.1 ohm.
0.5 ohm.
50 ohm.
100 ohm.

The armature of the dc motor is designed in such a manner so as to keep the armature winding resistance in and around 0.5 ohm. If the resistance is increased beyond that, losses will be too high.

09․ The power flow of a dc motor, with respect to a DC generator is
modified.
reversed.
increased.
none of these.

In case of DC generator mechanical input is supplied by rotating the shaft of the machine with a prime-mover and we get electrical energy as output. Whereas in case of a dc motor, we supply electrical energy to the supply terminals and we get rotational torque or mechanical energy as output. So the power flow in case of a dc motor is just the reverse of power flow in case of a dc generator.

10․ A 4 pole dc motor with armature winding resistance of 0.6 ohm receives a supply of 230 Volt. If the back emf produced during running condition is equal to 210 volt. Calculate the current flowing through the armature at this point.
43 Amp.
44.47 Amp.
33.33 Amp.
29 Amp.

Given E = 230 volt. Eb = 210 volt. The voltage equation of a dc motor is given by E = Eb + IaRa. ⇒ Ia = (E - Eb) / ra Or Ia = ( 230 - 210 ) / 0.6 = 33.3 Amp.

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