The GATE Grind

GATE 2014 EE (EE1) – Question 22

Electrical Machines · Single phase transformer: equivalent circuit, phasor diagram, open circuit and short circuit tests, regulation and efficiency · 1 mark · Multiple choice

At specified transformer input voltage and frequency, the equivalent core radius is halved. By what factor should primary turns change to maintain the same no-load current?

  1. 1/4
  2. 1/2
  3. 2
  4. 4

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (C) 2

Explanation

Halving equivalent radius quarters core cross-sectional area, increasing magnetic reluctance by4when path length and permeability are treated as fixed. Magnetizing inductance is N²/reluctance. Maintaining the same no-load current at fixed voltage and frequency requires the same inductance, so Nmust increase by√4=**2,C**. This is the ideal magnetizing-current model.