# MCQs on Electrical Machines

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01․ The shaft torque of a dc motor is less than the electromagnetic torque because of
mechanical losses.
hysteresis losses.
ohmic losses.
eddy current losses.

Electromagnetic torque = shaft torque + mechanical losses. Or, shaft torque = Electromagnetic torque - mechanical losses. ⇒ Shaft torque is less than electromagnetic torque because of mechanical losses.

02․ A 230 volt dc motor has an armature winding resistance of 0.5 ohm. Calculate the emf induced by the motor if the full load armature current is 23 ampere.
120 volt.
218.5 volt.
220.4 volt.
None of these.

The voltage equation of dc motor is given by E = Eb + Ia Ra ⇒ Eb = E - Ia Ra Or, Eb = 230 - ( 23 X 0.5). Or, Eb = 230 - 11.5 = 218.5 volt.

03․ A 4 pole wave wound dc motor having flux per pole of 9.56 X 10-3 Wb contains 460 armature conductors. Calculate the back emf produced when it is running at a speed of 1500 rpm.
220 volt.
230 volt.
240 volt.
440 volt.

Given P = 4 , φ = 9.56 X 10-3 Wb Z = 460 conductors, N = 1500 rpm A = 2, since wave winding.

04․ A 4 pole wave wound dc motor having flux per pole of 9.56 X 10-3 Wb contains 460 armature conductors with armature resistance of 0.6 ohm. Calculate the starting current flowing through the armature if the supply voltage is 240 volt.
0.5 Amp.
320 Amp.
400 Amp.
800 Amp.

The voltage equation of dc motor is E = Eb + IaRa But at staring Eb = 0 ⇒ Voltage equation of motor at starting Eb = Ia Ra ⇒ Ia = Eb / Ra ⇒ Ia = 240 / 0.6 = 400 Amp.

05․ In case of a dc motor the ratio Eb / E is indicative of
efficiency.
ratio of input to output voltage.
ratio of starting to running torque.
ratio of starting to running current.

The power equation of dc motor is given by EIa = Eb Ia + Ia2Ra. Where EIa = Input power EbIa = Output power Ia2Ra = Ohmic losses. Since efficiency = Output / Input = (EbIa) / (EIa) ∴ Efficiency = Eb / E.

06․ The output power of a dc motor is given by
EIa.
EbIa.
EbIa2.
none of these.

The power equation of dc motor is given by EIa = EbIa + Ia2Ra Where EIa = Input power EbIa = Output power Ia2Ra = Ohmic losses.

07․ When the armature current of 5 Amp is flowing through the dc motor the torque produced is 47 Nm. Keeping the speed constant if the armature current is increased to 7 amp, calculate the new torque. Assume losses to be constant.
27.73 Nm .
38.5 Nm.
65.8 Nm.
74 Nm.

The torque equation of dc motor is EbIa = Tθ ⇒ Ia is proportional to T ⇒ 5 / 7 = 47 / New torque ∴ New torque = (7 X 47) / 5 = 65.8 Nm

08․ The back emf produced in a dc motor depends on
P.
Z.
P and Z.
none of these.

Under construction.

09․ DC Motor
dc shunt motor with field weakening.
dc shunt with armature resistance control.
dc series motor.
dc compound motor.

In field weakening region field current will reduce in 3 point starter holding coil unable to hold the plunger in on position.

10․ Which motor used in Lathe machine?
D.C . series motor.
D.C. shunt motor.
D.C. cumulative compound motor.
None of the above.

For medium starting torque and nearly constant speed, D.C. shunt motor is used in lathe.

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