Question

Questions regarding an open circuit transofrmer test:

1-What causes the primary current to be non zero when the secondary winding is open circuied?

2-What is the main type of power loss being measured in an open circuit test?

3-The back-EMF equation tells us that the peak flux density is proportional to the AC supply voltage. Now knowing that the current which flows in the transformer under open-circuit conditions is the current (or mmf) which produces the magnetic flux density, explain why the peak open-circuit current increases nonlinearly whereas the applied voltage increases linearly.

4-Calculate the rated primary current of the following transformer ratings and express it as a percentage of the rated primary current.

A single-phase transformer (240/120V, 1kVA). Note: this transformer actually has two identical secondary windings, each rated at 1kVA resulting in a total transformer rating of 2kVA. However we are only considering one winding in the experiment and so the effective transformer is only 1kVA.

•AC mains supply (240V, 50Hz)

•An autotransformer (240V, 8A, 50Hz)•A 50? (5A) resistor with an unknown inductance (available on dc switchboard)

•A voltmeter (1000V) and an ammeter (5A), both available on the ac switchboard•Voltage and current transducers (available on the dc side of the main switchboard, but can be used to measure the instantaneous values)

Answer 1

1 - A practical (non-ideal) transformer has finite permeability as poopsed to an ideal transformer. Hence, non-zero current is reuired to produce magnetomotive force to generate required flux. When the secondary winding of a practical transformer is open circuited (i.e. no load) and an alternating voltage source is applied in the primary winding, two phenomena cause the primary currnt to be non zero, namely:

i) magnetization of the core: reactive component lagging the applied voltage by 90^o

ii) core loss : resistive component in phase with applied voltage

No load current is the phasor sum of magnetizing component and core loss component.

2) Main type of power loss measured by open circuit test is core loss which has two components, namely hysteresis loss and eddy current loss.

3) Open circuit test is mainly focused on measuring the magnetizing current of the transformer. The transformer is designed to keep the core loss at minimum.

The material used for the transformer’s magnetic circuit is steel which has a high magnetic permeability but reaches magnetic saturation at relatively lower value of magnetic field strength which is proportional to magnetizing current. Because of this, the flux density, which is proportional to the input voltage, displays a non-lenar relationship with magnetising current. As the magnetising current increases, the increase in flux density slows down.