SOLUTION

Heat Run test on induction motor

This test is carried out to determine the temperature rise of different parts of the motor while it is running at rated speed. For motors with the continuous rating and for motors for periodic duty and for continuous duty with intermittent load, the test of heat run should be continued till thermal equilibrium is reached. In the case of intermittent duty cycle motors, the duration of one cycle should be 10 minutes.

The temperature should be measured both while the motor is running and after it is shut down. Whenever thermal equilibrium is reached, the motor should be stopped as quickly as possible. When temperatures are measured after shutdown, the cooling curve should be plotted.

The temperature rise of a part of the motor is the difference between the temperature of the part and the temperature of the cooling medium.

Methods of measuring the temperature of windings

(a) Embedded temperature detector method: These detectors are either resistance thermometers or thermocouples built in the machine during manufacture at inaccessible points such as the slot portion of stator windings.

(b) Resistance method: Where embedded temperature detectors are not used, this method is used. The increase in resistance of the stator windings is measured and then the temperature rise of the windings is determined.

The temperature rise of the winding is found out by the expression

where

t_a = temperature °C of cooling air or gas at the end of the test.

t₂ = temperature °C of the winding at the end of the test.

t₁ = temperature °C of the winding (cold) initially.

R₁ = initial resistance of the winding.

R₂ = resistance of the winding at the end of the test.

Thermometer method: Wherever the last two methods are not applicable, this method is used for measurement of the temperature. This method uses thermometers applied to the accessible surface of the motor. Where bulb thermometers are used in any moving or varying magnetic field, alcohol thermometers should be used in preference to mercury thermometers.

Measurement of cooling air or gas temperature:

Measure the temperature of the cooling medium at the entrance of the motor when using forced ventilation.

In other cases, the cooling air temperature should be measured by several thermometers at various points around the motor at a distance of one or two metros.

Generally, the heat run test on induction motors is carried out by a pump test on two identical coupled machines so that the net energy used during the test is corresponding to the losses in the two machines only and not as per full load on the machines as in that case output is wasted in load.