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Maintenance
Rotor Cleaning
Remove rotor. Inspect and clean.
Stator Cleaning
Micalastic™ form wound VPI (vacuum pressure
impregnated) insulated coils may be cleaned with
solvent using lint free cloths or steam cleaned with
low-pressure steam, then the entire stator oven
baked at 170°F for 6 hours and then 245°F for 12
hours.
The stator winding insulation resistance should be
measured before and after any cleaning operation.
Micalastic™ is a Siemens trademark.
Hazardous voltage.
Will cause death, serious injury,
electrocution or property damage.
Disconnect all power before working
CAUTION
High Voltage.
May damage semi-conductors, small transformers,
voltage regulators, and other devices.
Disconnect from circuit before testing insulation
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
Micalastic™ 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 + 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.
Insulation resistance will decrease as the motor warms
up; but will begin to increase as the drying process
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.
(27 pages)
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