MCQs on Electrical Machines

Page 7 of 114. Go to page
01․ Ideal value of voltage regulation of a device is?
a positive finite value
a finite negative value

Voltage regulation should be as minimum as possible. Its ideal value should be zero. It means there is no change in no-load voltage to full-load voltage.

02․ In which of the following generators negative voltage regulation can occur?
shunt generator
series generators
over compound generator
both B and C

Series generators and over compound generators have rising voltage characteristics. In both cases terminal voltage rises when load increases due to series field characteristics. Therefore these two have negative voltage regulation and these are not suitable for ordinary power supplies.

03․ Essential conditions for two DC generators are connecting for parallel operation is/are
terminal voltage should be same.
polarities should be same.
rating of generators should be same.
both A and B.

The most essential condition for connecting two DC generators in parallel is that they must have same terminal at the bus.

04․ A shunt generator is running at 1000 rpm. If flux is reduced by half, then what is the new speed?

From this equation if all other things remains constant speed is inversely proportional to flux. So if flux is reduced by half then speed will be increased to double.

05․ A DC 4 pole lap wound generator is running at 1000 rpm having 1200 conductors and flux density is 10 mwb. find the generated emf?

Eb = Generated e.m.f. Φ = Flux per pole Z = Total no of conductors P = No of poles N = Revolution in r.p.m. A = No of parallel paths = No of poles for Lap Winding = 2 for wave winding

06․ Maximum power will be developed when back emf is
equal to supply voltage.
half of the supply voltage.
double the supply voltage.
all of the above.

The maximum power will be developed when

07․ In dc motors, under leading pole tips flux density will--------?
either increase or decrease
none of the above

Main flux and armature flux will aid each other under leading pole tips and for motors it will oppose each other in the trailing pole tips.

08․ In DC motors, under trailing pole tips, flux density will_______ ?
either increase or decrease
none of the above

For DC motor main field flux and armature flux will oppose each other under trailing pole tips. When the armature flux leaves the main pole it opposes the main field flux.

09․ In DC machine torque is proportional to which of the following?
armature current(Ia)
both A and B
none of the above

. So, in DC machines torque developed is proportional to both flux and armature current.

10․ Which motor cannot be started on no load?
shunt motors
series motors
cumulative compound motors
both B and C

DC series motors cannot be started on no load because it will attain dangerous high speed and damage itself. Armature winding will not burn but it will open from the axis due to centrifugal forces. These are best suitable for electric locomotives like hoists, cranes, traction etc.