Prevention Of Megger Reduction and Drying

This post is the 4th part of

COIL AND INSULATION IN HIGH VOLTAGE MOTORS

The second part

Measurement Of Insulation Resistance

The 3rd part  

Insulation Resistance Value Required For Operation

It is desirable that the coil and insulation of the motor is kept clean and dry. The dust accumulated on the surface of the coil prevents heat dissipation, and causes the deterioration of the insulation, though the deterioration depends on the type of insulation. Water penetrates through the joints of the outside lead wire and the cracklings of the insulation, thus reducing the insulation resistance.

Attention to daily cleaning and prevention of moisture absorption leads to the prevention of megger reduction:

 Use of Space Heaters

When the motor is running, its interior is not so humid, and in dry condition, but absorbs humidity at rest.

To prevent absorption of humidity, the space heater is to be energized immediately after the motor comes to stop and make the temperature inside of the motor 3 to 5ºC higher than the ambient temperature. 

Drying

In case that drying is required due to the low insulation resistance caused by absorption of humidity, the drying is to be carried out after the motor has been cleaned. The space heater, the blowing of hot air, or low current heating are to be used for drying. It is safer not to apply current heating when insulation resistance is 0.05 MΩ or less.

In any case, the drying is to be carried out at a temperature of 75~85ºC, raising the temperature at a rate of up to 10ºC per hour.

The drying is to be continued, measuring the insulation resistance every several hours until insulation resistance remain constant for more than 24 hours.

The change of insulation resistance due to the drying is shown in Fig. 1-2 (a-b).

 

Fig.1.2-a Temperature correction curve

Fig.1-2-b : Change in Insulation .Resistance