Maintenance

Insulation Resistance

Check insulation resistance periodically. Use a hand cranked or solid state insulation resistance tester and test with at least 500 volts, but not greater than motor rated voltage.

For motors with newer insulation systems such as MICLAD™ VPI, the insulation resistance after one minute should be greater than 1000 megohms. (Values in excess of 5000 megohms are common.) For older motors, the minimum value recommended in IEEE Standard 43 can be used. The value in

megohms, when corrected to 40°C, is equal to the motor rated voltage in kilovolts plus 1. For example, for a motor with a rated voltage of 2300 volts, the limit value would be:

2.3 + 1 = 3.3 (megohms).

Drying Insulation

If the insulation resistance is less than satisfactory, and the cause is believed to be excessive moisture in the windings, dry the windings by applying heat from:

1.A warm air oven.

2.Electric strip heaters.

3.Circulating currents through the coils.

The heat should be applied slowly so the desired temperature will not be obtained in less than six hours.

Insulation Drying Temperature*
Class “B”Class “F”Class “H”

 

 

 

200°F

245°F*

275°F*

94°C

118°C

135°C

 

 

 

*Class “F” and “H” insulated units should be baked at 70% specified temperature (to avoid steam inside winding) for about six hours, before temperature is raised to drying temperature.

Insulation resistance should be measured before the heat is applied, and every six to eight hours thereafter.

NOTE

Insulation resistance will decrease as the motor warms up; but will begin to increase as the drying process continues.

A uniform temperature must be maintained in the motor to obtain constant resistance readings. When the megger readings remain constant, the drying process is complete and may be discontinued. Check for other causes if readings are still low.

Warm Air Oven Drying

1.Remove bearing housings.

2.Remove rotor.

Bake in oven at temperatures specified in Insulation Drying Temperature table, and follow procedures described for drying insulation.

Electric Strip Heater Drying

1.Remove bearing housings.

2.Remove rotor.

3.Direct a fan on stator to carry away the moisture.

4.Attach temperature indicators to winding and apply heat as specified in the Insulation Drying Temperature table and follow procedures described for drying insulation.

5.Radiant type heaters are not recommended because some parts may become scorched before remote parts reach desired temperature.

Circulating Current Drying

1.Remove bearing housings.

2.Center the rotor in the stator core.

3.Wedge fiber strips into the lower part of the air gap so rotor does not touch stator core, or remove rotor.

4.Direct fan on unit to blow away excessive moisture.

5.Attach temperature indicators to windings. Do not exceed the drying temperatures in the Insulation Drying Temperature table.

6.An external source of current can be used to circulate direct current through the winding of any type of alternating current motor. A portable low voltage motor-generator set, such as is used for welding, is usually suitable.

CAUTION

High temperatures.

May cause damage to insulation.

Avoid hot spots and radiant type heat

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