PlacementPrepA transformer works on the principle of:
Solution
A changing flux from the primary induces a voltage in the secondary.
Answer: (C) Mutual induction
Technical · Electrical and Electronics (EEE)
Electrical Machines MCQs for placement tests
20 technical questions of moderate difficulty, as asked in campus placement and company tests. Each has a short explanation.
A transformer has 200 primary turns and 50 secondary turns. With 240 V on the primary, the secondary voltage is:
Solution
V₂ = V₁ × N₂/N₁ = 240 × 50/200 = 60 V.
Answer: (D) 60 V
Transformer core losses are reduced by:
Solution
Laminations reduce eddy currents; silicon steel reduces hysteresis loss.
Answer: (A) Using laminated silicon steel cores
The open-circuit test on a transformer measures mainly:
Solution
With no load, the current is small, so copper loss is negligible.
Answer: (C) Core (iron) losses
The short-circuit test on a transformer measures mainly:
Solution
Rated current flows at low voltage, so core loss is negligible.
Answer: (D) Copper losses
A transformer has maximum efficiency when:
Solution
Efficiency peaks when variable loss equals constant loss.
Answer: (A) Copper loss equals iron loss
The synchronous speed of a 4-pole motor on a 50 Hz supply is:
Solution
Ns = 120f/P = 120 × 50/4 = 1500 rpm.
Answer: (D) 1500 rpm
A 4-pole, 50 Hz induction motor runs at 1440 rpm. Its slip is:
Solution
Slip = (1500 − 1440)/1500 = 0.04.
Answer: (C) 4%
An induction motor can never run at synchronous speed because:
Solution
Torque needs rotor current, which needs slip.
Answer: (B) No relative motion means no induced rotor current or torque
Which DC motor is used where very high starting torque is needed, such as in traction?
Solution
Series motor torque is roughly proportional to the square of armature current at start.
Answer: (A) DC series motor
A DC series motor should never be started without load because:
Solution
At no load the field is weak and the speed can become very high.
Answer: (C) Its speed can rise to dangerous levels
The back EMF of a DC motor:
Solution
It is generated by rotation and limits armature current.
Answer: (D) Opposes the applied voltage
In a DC generator, the commutator converts:
Solution
It acts as a mechanical rectifier.
Answer: (D) AC induced in the armature into DC at the terminals
Synchronous motors are:
Solution
They need a starting method, such as damper windings.
Answer: (D) Not self-starting
An over-excited synchronous motor operates at:
Solution
It supplies reactive power and can correct power factor.
Answer: (A) A leading power factor
Which single-phase motor is commonly used in ceiling fans?
Solution
A capacitor stays permanently in the auxiliary winding circuit, giving quiet, efficient running.
Answer: (B) Permanent split capacitor (PSC) motor
The speed of a DC shunt motor can be increased above normal by:
Solution
Speed is inversely proportional to flux.
Answer: (D) Reducing the field current
The efficiency of a large power transformer is typically:
Solution
Transformers have no moving parts and are very efficient, often 98–99%.
Answer: (B) Above 95%
Star-delta starters are used for induction motors to:
Solution
In star, each winding gets 1/√3 of the line voltage, cutting starting current to one third.
Answer: (B) Reduce the starting current
A universal motor can run on:
Solution
It is a series motor whose field and armature reverse together.
Answer: (D) Both AC and DC
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