The magnitude of the applied dc test voltage
only slightly affects the value of the insulation resistance
and the use of a 500 Volt or 1000 Volt megger for stator
windings (and a 500 Volt megger for rotor windings) is
suitable for machines covered by this Instruction Manual.
The environmental conditions of moisture and surface
cleanliness, together with the ambient temperature,
largely determine the value of insulation resistance. The
insulation must be clean and dry and the measured value
must be corrected to 40C. This value is then compared to
a minimum acceptance criteria. Moisture and dirt will
decrease the insulation resistance of a winding and these
conditions must be corrected in order to increase it.
The insulation resistance of a winding measured
by a 500 Volt or a 1000 Volt megger, with the test
applied for 1 minute, should not be less than.
R = KV + 1
where : R = Insulation Resistance in megahoms,
corrected to 40C base
KV = rated voltage of the winding in kilovolts
To convert the actual insulation resistance reading
of the megger, R
, taken at an ambient winding
temperature in degree Celsius, to R, make the following
The temperature correction factor, K
determined for any specific winding or a reasonable
approximation can be used. Both methods will be
To determine the temperature correction factor for
a specific winding, make several measurements (at least
five) at several different temperatures, all of which are
above the dew point. Then plot the results, with
measured insulation resistance on a log scale and
winding temperature on a linear scale. The results should
approximate a straight line, from which the value of
insulation resistance at 40C can be determined.
A more general method, with reasonable
accuracy, is to use the curve, Fig. 1, to determine K
function of the winding temperature at the time of
measurement. It is based on doubling the insulation
resistance for each 10C reduction in temperature, for
conditions above the dew point. It has been found to be
reasonable for new windings.
GE Energy Motors GEEP-427-I
R = K
, can be
GEEP-427-I Pegasus, Horizontal Induction Motor
The polarization index is frequently helpful in
evaluating the cleanliness and freedom from moisture of
a winding. The polarization index is a measure of the
change is insulation resistance with the time duration for
which the test is applied. It is conducted by applying the
megger for 10 minutes and determining the insulation
resistance at 1 minute and 10 minutes. The polarization
index in the ratio of the 10-minute insulation resistance
reading to the 1-minute insulation resistance reading,
both readings haven been corrected to a 10C temperature
base. Clean, dry windings should exhibit a polarization
index of 2 or more.
Each winding of each unit will have its own
insulation resistance history which is unique to it. It is
recommended that the insulation resistance be measured
and recorded at least every six months, and more often if
feasible, and that the polarization index be measured and
recorded at least once a year. This accumulated
information will provide a data base which will be
helpful in managing preventative maintenance.
The user is referred to IEEE Standard 43, IEEE
Recommended Practices for Testing Insulation
Resistance of Rotating Machinery, for a more complete
discussion of the complete subject of Insulation
Before the machine is started for the first time, a
pre-start inspection should be made. The following are
some of the items frequently overlooked.
1. Measure the insulation resistance of the
windings. For machines located in or near salt air or
other corrosive environments, a polarization index
should also be taken.
2. Verify that the voltage and frequency
corresponds to the nameplate.
3. Verify that the phase sequence of the applied
voltage is correct for the desired direction of rotation.
Verify that the desired direction of rotation agrees with
4. For totally-enclosed. water-cooled machines,
verify that the cooling-water temperature does not
exceed the value on the nameplate.
5. The lubricant used should agree with the
nameplate and this instruction book.
Copyright 2009, The General Electric Company, USA 17
TEWAC, Sleeve Bearing