GEH-5304A
28
Table 13
Effect of Temperat ure on Insu lation Resistance
Winding Temperatur e Multiplying Factor to Obtain Insulation
(Degr ee C) Resistance at 40°C (104°F)
80 10.00
70 5.50
60 3.10
50 1.70
40 1.00
30 0.55
20 0.31
10 0.17
Note that for a 104°F (40°C) decrease in temperature, the insulation resistance is increased by a mul-
tiplier of ten.
The insulation resistance of a machine is affected by its design. The insulation resistance of the ar-
mature cir cuit, corrected to 104°F (40°C), should measure at least 1.5 megohms. If measured value is
below 1.5 m egohms, clean and r e-test.
If the measurements are less than this limit, the machine should be dried or cleaned to attempt to in-
crease the insulation resistance. Regular, periodic measurements of insulation resistance can give a
useful indication of the rate of insulation system deterioration. External connections should be re-
moved to isolate the windings to be tested and megger value logged. A sudden drop or consistent
trend toward low values of insulation r esistance, although possibly caused by moisture or contamina-
tion, generally gives eviden ce the insulation system is deteriorating and that failure may be immin ent.
High-potential tests are not recommended on machin es which h ave been in use. If such a test is made
immediately after installation, the test voltage should not exceed 85% of the original factory test of
two times the rated volts plus 1000 volts.
NOTE: Surge testing and AC impedance tests of windings to detect shorts should be performed by
train ed person nel only.
M. Cleaning of Windings
If windings become con tamin ated, suitable cleaning methods can be used to alleviate the problem.
The machin e should be de-energized and slowly rotated by h an d to permit maximum dust removal. Dry dirt,
dust or carbon should first be vacuumed – without disturbing adjacen t areas or redistributing the con tamina-
tion. Use a small nozzle or tube connected to the vacuum cleaner to enter into narrow openings (i.e., be-
tween commutator risers). A soft brush on the vacuum nozzle will loosen and allow removal of dirt more
firmly attached.
This vacuum cleaning may be supplemented by blowing with compressed air (air pressure should be in ac-
cordance with OSHA standards), which has passed through a dryer to remove moisture before entering the
motor.
Dirt can collect on the inside surface of the drive end coil support an d on the un derside of the ar mature coils.
This dirt can be easily removed with compressed air or a vacuum.
It is importan t to realize that when blowing out a machin e, dir t may settle in a pr eviously cleaned ar ea and it
may be n ecessary to repeat the cleaning process to ensure that a thorough job is done.