MCQs on Electrical Machines

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01․ In dc machines, the field system has to be provided on stator unlike synchronous machines Where in it could be on any member, because
it reduces field structure iron losses
it gives more uniform air-gap flux distribution
commutator action is not possible otherwise
dc machines are comparatively low rating machines.

To make commutation possible we must place field winding on stator and armature winding on rotor. In synchronous machine we do not require commutation so any winding can be placed anywhere.

02․ In dc machines, the space distribution of air-gap flux density wave at no load is
co- sinusoidal

Air-gap flux density in dc-machine is same in entire polar region whereas it is either increasing or decreasing in interpolar regions. The shape achieved is called flat-topped type.

03․ A commutator in DC machines can 1. provide half-wave rectification 2. Provide full-wave rectification 3. convert ac to dc 4. Convert dc to ac 5. provide controlled full-wave rectification From these, the correct answer is
2, 3, 4
1, 2, 3
2, 3 ,5
2, 3, 4, 5

In a dc machine commutator can convert either dc into ac or ac into DC. We have not any control upon commutator action unlike we have in case of controllable switches like chopper, scar etc. So, commutator provides uncontrolled full-wave rectification.

04․ In dc machines, the space waveform of the air-gap flux distribution affects
torque but not the voltage
voltage but not the torque
both the torque and voltage
neither the voltage nor the torque

Neither voltage nor torque depends upon the space waveform of the air-gap flux distribution.

05․ In dc machines, the field-flux axis and armature-mmf axis are respectively along
direct axis and indirect axis
direct axis and interpolar axis
quadrature axis and direct axis
quadrature axis and interpolar axis

Field flux is along north pole to south pole i.e. direct axis whereas by right hand thumb rule armature flux is along interpolar axis.

06․ The brush-axis of a bipolar dc motor is rotated by 90°. The effect of this rotation on the back emf Eband the torque developed Tdwould be such that
both Eb and Td are unchanged
Eb is zero, but Td is unchanged
Eb is unchanged, but Td is zero
both Eb and Td are zero

Demagnetizing effect takes place due to 2θ region where θ is the brush shift angle i.e. demagnetizing ampere turn Here brush shift is given 90 degree so all ampere turn will act for demagnetizing effect. Hence back emfand torque will be zero.

07․ Consider the following statements regarding the magnetization characteristics of a dc generator : 1. It represents the variation of field flux with field current. 2. Variation of open-circuit terminal voltage with field current is independent of speed 3. It is non-linear from zero to rated field current 4. It helps to determine the critical field resistance at a given speed 5. It always starts from the origin Which of these statements are correct ?
1 and 4
1, 2 and 5
2, 3 and 4
3 and 5

Magnetizing characteristic of a dc generator represents the variation of field flux with field current and it also helps to determine the critical field resistance at a given speed.

08․ A DC shunt generator builds up to a voltage of 220 V at no load while running at its rated speed. If the speed of the generator is raised by 25% kepping the circuit conditions unaltered, then the Voltage to which the machine would build up will
not change and remain at 220 V
increase to 1.25 times 220 V
increase to value lying between 220 V and 1.25 times 220 V
increase to a value greater than 1.25 times 220 V

We know generated emf EgαN So, if we increase speed by 25% then emf will also increase by 25% and that will be 1.25 times of initial voltage.

09․ The armature core of a DC machine is laminated to minimise
hysteresis loss
eddy-current loss
mechanical loss
temperature rise

Eddy currents flow in circular paths in armature core of the machine i.e. it depends upon the resistance of the core. Lamination of armature core increases the resistance in the core which in turn reduces the flow of eddy current through it. Therefore we can say the armature core of a DC machine is laminated to minimize eddy-current loss.

10․ The commutator segments of DC machines are made up of
hard-drawn copper
copper alloy

Wear and tear is one of the main problem associated with commutator action in the DC machine. So,commutator segment s of dc machines are made up of hard-drawn copper to reduce its wear and tear.