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Consider these two statements about transformer cores: Statement 1: The hysteresis loop (B-H curve) for a transformer core material must be narrow. Statement 2: The material used for transformer cores must have high retentivity. Which of the following is correct?

1. Statement 1 is true and Statement 2 is false
2. Statement 1 is false and Statement 2 is true
3. Both Statement 1 and Statement 2 are false
4. Both Statement 1 and Statement 2 are true

Correct answer: Statement 1 is true and Statement 2 is false

Solution

A transformer core undergoes repeated magnetisation and demagnetisation with the alternating current. The energy dissipated per cycle is proportional to the area of the hysteresis loop, so it must be narrow to minimise heat loss. Soft magnetic materials (like soft iron or silicon steel) have narrow loops, low retentivity, and low coercivity. High retentivity is a property of hard magnetic materials (used for permanent magnets), which is undesirable for transformer cores. Therefore Statement 1 is true and Statement 2 is false.

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