Question

A transformer on no-load is switched on to a source of voltage. It will draw a current _______
1. which is the same as the steady-state magnetizing current
2. which is several times the steady-state magnetizing current, depending upon the initial state of the residual flux in the transformer core
3. which is several times the steady-state magnetizing current, independent of the initial state of the residual flux in the transformer core
4. which is twice the steady-state magnetizing current provided the core has no residual flux

Answer 1

Correct Answer - Option 2 : which is several times the steady-state magnetizing current, depending upon the initial state of the residual flux in the transformer core

Transformer on no-load:

• Whenever a transformer is on no-load, the secondary winding has no load connected to it.
• The transformer must actually withdraw zero current from the primary side.
• But practically even on no-load, a small amount of current is drawn from the primary side, to set up the required magnetic flux in the magnetic core.
• At no load, the transformer draws some current (Im) in order to establish the flux in the transformer core which lags the applied voltage by 90° ( since flux is required to create a magnetic pool in order to transfer energy from one port to another) called as magnetizing current.
• It is several times the steady-state magnetizing current, depending upon the initial state of the residual flux in the transformer core.
• This magnetizing current (no-load current) is about 3-5% of the full load current and it accounts for the losses in a transformer.