Copper loss is negligible at _____.

Electrical or Copper losses (I²R losses):

The copper losses are the winding losses taking place during the current flowing through the winding. These losses occur due to the resistance in the winding.

Copper Loss = I2R

At no load value of current is negligible, so that copper losses are also negligible. 

The copper loss at any load also given by

Pcu = x2 Pcf

Where, x = Fraction of load, Pcf = Full load copper loss,

Hence at no load copper loss is negligible.

These losses are determined with the help of a short-circuit test. 

The short circuit test is performed on the high voltage winding of the transformer or electrical machine while the low voltage side is short-circuited i.e. performed at rated current.

The equivalent resistance, impedance, and leakage reactance are also calculated by the short circuit test.

Important Points

• The no-load test gives information regarding no-load losses also known as constant losses such as core loss, friction loss, and windage loss.
• The no-load test is performed when the rotor rotates with synchronous speed and there is no load torque.
• Rotor copper loss at no load is very less than its value is negligible.
• In this test, a small current is required to produce adequate torque.
• This test is used to evaluate the resistance and impedance of the magnetizing path of induction motor or shunt branch parameters.
• During the no-load test of a 3 phase electrical machine, the machine draws power for core loss and windage-friction loss.