Why is the transformer's power factor so low when there is no load on it?

A transformer’s power factor is normally very low when there is no load applied because the primary winding’s only current flow is the magnetizing current. The transformer core magnetic field is produced by the magnetizing current, which is mostly an inductive current. Inductive currents do not produce real power (watts), as they trail the voltage by 90°.

As a result, a transformer’s power factor when there is no load is often relatively low, between 0.2 and 0.4.

The two main factors for a transformer’s extremely low power factor when there is no load:

  • Induction can be observed in the magnetizing current. Because inductive currents are 90 degrees behind the voltage, this indicates that the inductive currents do not contribute to the actual power (measured in watts).

  • The current that magnetizes the object is not very potent. The full-load current of a transformer is much higher than the magnetizing current, which is just a small percentage of the full-load current. This is due to the fact that the magnetic field within the core of the transformer only needs to be as strong as is necessary to induce the desired voltage within the secondary winding.

By adding more load to the transformer, the power factor can be increased. The magnetizing current stays constant while the load current increases as the load on transformer increases. As a result, the power factor is enhanced when the phase angle between the current & voltage decreases.

Further to the magnetizing current, a small amount of the leakage current also circulates in a transformer’s primary winding when there is no load. The inadequate insulation between the primary & secondary windings is the source of leakage current. Leakage current, which is likewise inductive in nature, lowers a transformer’s power factor when there is no load on it.

Engineers of power systems must take into consideration a transformer’s power factor. Low power factors can increase power system losses and make it much harder to maintain a constant voltage. The power factor of transformers can be increased using a variety of methods, including the use of

  • Capacitors and

  • Power factor correction coils.