Question

Can a 50 Hz transformer be used as 25 Hz, if the input voltage is maintained constant at the rated corresponding to 50 Hz?
1. Yes, since the voltage is constant, current levels will not change
2. No, flux will be doubled which will drive the core to excessive saturation
3. No, owing to decreased reactance of transformer, input current will be doubled at load
4. Yes, at constant voltage, insulation will not be overstressed

Answer 1

Correct Answer - Option 2 : No, flux will be doubled which will drive the core to excessive saturation

Concept:

• If frequency of the transformer is decreased (assuming its applied voltage is same), magnetizing current in the primary of the transformer increases as it is inversely proportional to frequency and directly proportional to the applied voltage.
• This magnetizing current is used to set up flux in core of the transformer. If magnetizing current goes beyond certain limit, the transformer core may saturate.
• Therefore, it is required to reduce applied voltage along with frequency in same proportion to keep the magnetizing current same.

 

V/f ratio must be constant.

Application:

\({B_{max}} \propto \frac{V}{f}\)

Here v is constant, f is decreased to half. Then B_max increased to double, which will drive the core in to deep saturation and also I_μ is very high to create double the rated flux.