Siemens 680 Benutzerhandbuch

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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). 

 

 

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. 

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 
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. 

 

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. 

 

High temperatures. 
May cause damage to insulation. 
Avoid hot spots and radiant type heat