Section 1 - Electrical Machines

Question 1

A transformer works on the principle of:

A. Self induction

B. Mutual induction

C. Electromagnetic force

D. Electrostatic induction

Question 2

The core of a transformer is generally laminated to:

A. Reduce copper losses

B. Reduce eddy current losses

C. Reduce hysteresis losses

D. Increase mechanical strength

Question 3

The hysteresis loss in a transformer core is minimised by using a core material with:

A. Wide hysteresis loop

B. Narrow hysteresis loop

C. High resistivity only

D. Low permeability

Question 4

For a transformer, the copper loss varies with load as:

A. Directly proportional to load current

B. Proportional to the square of the load current

C. Independent of load current

D. Inversely proportional to load current

Question 5

A transformer's iron loss (core loss) is:

A. Proportional to load current

B. Proportional to the square of load current

C. Practically constant at rated voltage and frequency

D. Zero at no load

Question 6

The maximum efficiency of a transformer occurs when:

A. Iron loss equals copper loss

B. Copper loss is zero

C. Iron loss is zero

D. Load current is maximum

Question 7

The open-circuit test on a transformer is used to determine:

A. Copper losses and equivalent series impedance

B. Core (iron) losses and shunt branch parameters

C. Regulation only

D. Insulation resistance

Question 8

The short-circuit test on a transformer is performed to determine:

A. Iron losses and magnetising current

B. Copper losses and equivalent leakage impedance

C. No-load current only

D. Turns ratio only

Question 9

An autotransformer differs from a two-winding transformer in that it has:

A. No magnetic coupling

B. A single winding that is common to both primary and secondary sides

C. Two magnetically independent windings

D. Only a DC output

Question 10

The all-day efficiency of a distribution transformer is defined as:

A. Ratio of output kWh to input kWh over 24 hours

B. Ratio of full-load output to full-load input

C. Ratio of iron loss to copper loss

D. Ratio of primary current to secondary current

Question 11

The back EMF of a DC motor is:

A. Independent of speed

B. Directly proportional to speed and flux

C. Inversely proportional to speed

D. Zero at running speed

Question 12

The speed of a DC shunt motor can be increased above rated speed by:

A. Increasing armature voltage above rated

B. Weakening the field flux (field control)

C. Increasing the load

D. Adding series resistance in the armature only

Question 13

A DC series motor should not be started without load because:

A. It draws very small current

B. It may run at dangerously high speeds (racing)

C. It cannot develop torque

D. It reverses direction

Question 14

The torque developed by a DC motor is proportional to:

A. Flux only

B. Armature current only

C. The product of flux and armature current

D. The square of armature current only

Question 15

Armature reaction in a DC machine refers to:

A. The reaction of load torque on the armature

B. The effect of armature MMF on the main field flux

C. Mechanical vibration of the armature

D. Heating of the armature winding

Question 16

Interpoles in a DC machine are used to:

A. Increase the main field flux

B. Improve commutation by neutralising reactance voltage

C. Reduce the number of brushes

D. Increase the mechanical output

Question 17

The synchronous speed of a 3-phase induction motor is given by:

A. Ns = 60 f / P (where P is number of pole pairs)

B. Ns = 120 f / P (where P is the number of poles)

C. Ns = f * P

D. Ns = 60 P / f

Question 18

The slip of an induction motor is defined as:

A. (Nr - Ns) / Ns

B. (Ns - Nr) / Ns

C. Ns / Nr

D. Nr / Ns

Question 19

At starting (standstill), the slip of a 3-phase induction motor is:

A. 0

B. 0.5

C. 1

D. Infinity

Question 20

The rotor frequency of a 3-phase induction motor is:

A. f (supply frequency)

B. s * f (slip times supply frequency)

C. f / s

D. Independent of slip

Question 21

A single-phase induction motor is not self-starting because:

A. It has no rotor conductors

B. The single-phase winding produces a pulsating (not rotating) magnetic field

C. The rotor is too heavy

D. The supply voltage is too low

Question 22

In a synchronous motor, the speed of rotation is:

A. Slightly less than synchronous speed

B. Exactly equal to synchronous speed at steady state

C. Independent of supply frequency

D. Proportional to the load

Question 23

A synchronous motor operated at over-excitation and light load can be used as a:

A. Lagging power factor load

B. Leading power factor load (synchronous condenser) for power factor correction

C. DC generator

D. Frequency changer

Question 24

The V-curve of a synchronous motor shows the relationship between:

A. Armature current and field current at constant load

B. Torque and speed

C. Voltage and frequency

D. Power factor and slip

Question 25

Two alternators can be operated in parallel only if:

A. Their terminal voltages, frequencies, and phase sequences are matched

B. Their internal resistances are equal

C. Their prime movers are identical

D. Their armature currents are equal

Questions

Section 1 - Electrical Machines - Set 1