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

By the turns ratio formula V2/V1 = N2/N1, so V2 = 120 × (400/200) = 240V.

Question 2

The efficiency of a transformer reaches its maximum when:

Accepted Answer

Iron losses equal copper losses

Answer

Maximum efficiency occurs at the load where variable copper losses equal the constant iron (core) losses.

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

Applying KVL around the armature circuit gives V = Eb + IaRa, so 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

Synchronous speed Ns = 120f/P = 120 × 60 / 4 = 1800 RPM.

Question 5

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

Accepted Answer

0

Answer

Slip s = (Ns - Nr)/Ns; when rotor speed Nr equals synchronous speed Ns, slip equals zero.

BEE - Bachelor of Electrical Engineering Practice Test