Electrical Machines: 26 MCQs with answers

A · Stator

The stator is the stationary part of a DC machine, providing the magnetic field. The armature and commutator are rotating parts.

B · Mutual induction

A transformer works on the principle of mutual induction, where a changing current in one coil induces a voltage in another coil.

C · Single-phase induction motor

Single-phase induction motors are widely used in household appliances because they operate on single-phase AC supply, which is readily available.

A · Alternator

An alternator is a synchronous generator, operating at synchronous speed determined by the supply frequency and number of poles.

B · False

A transformer changes voltage levels but does not change the frequency of the AC supply.

C · To change voltage levels

The main function of a transformer is to change voltage levels, either stepping up or stepping down the voltage in an AC circuit.

B · The rotating part of the machine

The armature is the rotating part of electrical machines where EMF is induced.

A · To conduct current to the armature

The brushes conduct current to the rotating armature in a DC motor, allowing it to operate.

C · Induction motor

An induction motor operates on the principle of electromagnetic induction, where current is induced in the rotor.

C · To change the direction of current

The primary purpose of a commutator in a DC machine is to change the direction of current in the armature winding.

C · Alternating Current (AC)

A transformer operates with Alternating Current (AC) to transfer electrical energy between circuits.

A · Primary winding

The magnetic field in a transformer is created in the primary winding when an AC voltage is applied.

C · Aluminum or Copper bars

The rotor in an induction motor is typically made of aluminum or copper bars, which help in inducing current.

C · To provide a magnetic field

The stator provides a magnetic field in electrical machines, which is essential for operation.

A · Generator

A generator is primarily used for converting mechanical energy to electrical energy.

A · Inversely proportional to field flux

For a DC shunt motor, speed is inversely proportional to field flux (N ∝ 1/Φ), assuming constant armature voltage and negligible armature resistance drop.

A · 800 V

Using the turns ratio: V_s/V_p = N_s/N_p = 400/100 = 4, so V_s = 4 × 200 V = 800 V.

C · Convert AC to DC in the external circuit

The commutator acts as a mechanical rectifier, converting the alternating current generated in the armature coils into direct current in the external circuit.

A · Less than synchronous speed

An induction motor operates at a speed slightly less than synchronous speed; the difference is called slip, which is necessary for inducing rotor current.

A · Windage loss

Windage loss is a mechanical loss due to air friction, which occurs in rotating machines like motors and generators, but not in static devices like transformers.

C · Maximum flux in the core

In the EMF equation, $\Phi_m$ is the maximum value of the alternating flux in the core, and the factor 4.44 arises from the sinusoidal waveform.

B · Rugged and maintenance-free operation

Three-phase induction motors are rugged and require less maintenance because they have no brushes or commutator, unlike DC motors.

A · True

The commutator acts as a mechanical rectifier, reversing the connections to the external circuit at the right instants to produce unidirectional current.

D · It will run at dangerously high speed

In a DC series motor, field flux is proportional to armature current. At no load, current is very small, flux is very small, and speed becomes dangerously high (N ∝ 1/Φ).

A · Very low lagging

On no load, a transformer draws a small magnetizing current that lags the voltage by nearly 90°, so the power factor is very low lagging.

A · Slip times supply frequency

The rotor frequency $f_r = s f$, where $s$ is the slip and $f$ is the supply frequency. At standstill, $s=1$ and $f_r=f$.