BEE Electrical Machines

Question 1

A transformer has 200 primary turns and 400 secondary turns. If the primary voltage is 120V, what is the secondary voltage?

Accepted Answer

240V

Answer

60V

Answer

120V

Answer

480V

Question 2

The efficiency of a transformer reaches its maximum when:

Accepted Answer

Iron losses equal copper losses

Answer

Copper losses are zero

Answer

Iron losses are zero

Answer

The load is at rated full capacity

Question 3

In a DC motor, the back EMF (Eb) is related to supply voltage (V), armature current (Ia), and armature resistance (Ra) by:

Accepted Answer

Eb = V - IaRa

Answer

Eb = V + IaRa

Answer

Eb = IaRa

Answer

Eb = V / IaRa

Question 4

The synchronous speed of a 4-pole induction motor operating on a 60 Hz supply is:

Accepted Answer

1800 RPM

Answer

900 RPM

Answer

1200 RPM

Answer

1500 RPM

Question 5

At synchronous speed, the slip of a three-phase induction motor is:

Accepted Answer

0

Answer

1

Answer

0.5

Answer

-1

Question 6

Which type of DC motor provides the highest starting torque and is most suitable for traction (railway) applications?

Accepted Answer

DC series motor

Answer

DC shunt motor

Answer

DC compound motor

Answer

DC permanent magnet motor

Question 7

The speed regulation of a DC shunt motor compared to a DC series motor is:

Accepted Answer

Much better (lower regulation)

Answer

Much poorer (higher regulation)

Answer

About the same

Answer

Negative for shunt, positive for series