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U.S. ELECTRICAL MOTORS
INSTALLATION AND MAINTENANCE
(H) All motors must have the shaft rotated a few turns once a month to maintain a lubricant film on bearing
races.
3. Periodic Maintenance/ Insulation History
The only accurate way to evaluate the condition of the winding insulation is to maintain a history of the
insulation readings. Over a period of months or years these readings will tend to indicate a trend. If a
downward trend develops or if the resistance drops too low, thoroughly clean and dry the windings, retreating
if necessary.
The recommended insulation resistance test is as follows:
(A) Using a megohmmeter, with winding at ambient temperature, apply DC voltage (noted below) for 60
seconds and take reading.
Rated Motor Voltage Recommended DC Test Voltage
600 and less 500 VDC
601 to 1000 incl. 500 to 1000 VDC
1001 and up 500 to 2500 VDC
(2500 VDC optimum)
(B) For comparison the reading should be corrected to a 40°C base temperature.
This may be done by utilizing the following:
R40C = Kt X Rt
Where R40C = insulation resistance (in megohms) corrected to 40°C
Rt = measured insulation resistance (in megohms)
Kt = temperature coefficient (from Graph 1)
GRAPH 1
(ADAPTED FROM IEEE 43)
Installation Resistance Temperature Coefficient: Kt
Storage
3
Winding Temperature °C