Question

Open Circuit test in electrical machines in done to calculate:
1. Iron Losses
2. Copper loss
3. Both iron and copper losses
4. None of these

Answer 1

Correct Answer - Option 1 : Iron Losses

Open circuit test:

• It is done by keeping one of the windings open (without load, usually high voltage winding is open) and applying rated voltage to other winding (usually low voltage winding because it is easier to apply rated voltage).
• The current drawn from this terminal is the no-load current at low power factor corresponding to the core loss component. Since the no-load current is very small it doesn’t contribute to the copper loss. Core loss is calculated by multiplying the applied voltage and no-load current.
• As the secondary side is open, the entire coil will be purely inductive in nature. So, the power will be lagging due to the inductive property of the circuit. So LPF (Low Power Factor) Wattmeter is used in the open circuit test of the transformer.

 

It is used to find

• The core/iron loss of the transformer
• The no-load current
• Equivalent resistance referred to metering side
• Shunt branch parameters i.e. magnetizing impedance

 

Short circuit test:

• It is done by shorting one of the winding terminals (usually low voltage terminal) and applying a small voltage across the other winding terminals (high voltage terminal because the current in HV terminal will be less and easy to handle) and using a wattmeter to measure the power dissipated in the LV terminal. Wattmeter will indicate the full load copper loss.
• In the short circuit test, the secondary winding of the transformer is short-circuited. As the secondary side is short-circuited the entire coil will be purely resistive in nature. So, the power factor will be High or unity.

 

Short circuit test is done to find

• The full load copper loss or ohmic loss
• Short circuit current