The power factor of a transformer on no load is poor due to

A. Magnetising reactance of the transformer 
B. Open circuited Secondary 
C. Low Primary Winding Resistance 
D. Low No-load Current 

Answer: A. Magnetizing reactance of the transformer 

When the transformer is operating at no load, the secondary winding is open-circuited, which means there is no load on the secondary side of the transformer and, therefore, the current in the secondary will be zero.

While primary winding carries a small current I0 called no-load current which is 2 to 10% of the rated current.

This current is responsible for supplying the iron losses (hysteresis and eddy current losses) in the core and a very small amount of copper losses in the primary winding. 

The angle of lag depends upon the losses in the transformer. The power factor is very low and varies from 0.1 to 0.2.
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