Whenever load is not connected on the secondary winding of the transformer and the primary winding is energised, the condition is said to be no load condition of the transformer. The primary winding having high inductance and low resistance draws a small current lagging behind the voltage by nearly 90 electrical. The current is said no load current. 10, and the power factor resulted is said the no load power factor, cos . The no load current can be resolved into two components:
(a) Magnetising current (l
(b) Wattful current (1m).
(a) Magnetising current (l p). The magnetising component is lagging behind the voltage V p by 90. It is responsible for the magnetic flux and both I and the flux are in-phase. It is also called the reactive component of the load current. It is equal to 10 sin and is the wattless component of no load current.
Wattful component (). It is known as the working component or active component of the no load current. It is in phase with the voltage applied and is I cos Φ. The component is used to feed the necessary real power to supply the hysteresis and eddy current losses in the iron cores. Here as shown in Fig. 18.12 cos is the power factor at no load and sometimes is called the hysteresis angle of advances.
The magnetising component produces a changing flux, in the iron cores of the transformer, which is actually responsible for producing the voltage in primary winding, the self induced primary voltage and in secondary winding, the mutually and self induced voltages.
What are the different information obtained, from the “no load” condition of the transformer?
These following informations can be obtained from the ‘no load’ conditions of the transformer:
(i) the voltage transformation ratio,
(ii) the no-load current,
(iil) the hysteresis angle of advance.
(iv) Xo and Ro can be calculated for the equivalent circuit of the transformer.