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RM = KV + 1
Where
RM = Recommended minimum insulation resistance in megohms at 104° F (40° C) of
the entire winding.
KV = Rated motor terminal to terminal voltage in kilovolts (1000 volts = 1 KV).
On a new winding, where the contaminant causing low insulation resistance is generally
moisture, drying the winding through the proper application of heat will normally increase the
insulation resistance to an acceptable level. The following are several accepted methods for
applying heat to a winding:
1. If the motor is equipped with space heaters, they can be energized to heat the winding.
2. Direct current (as from a welder) can be passed through the winding. The total current
should not exceed approximately 50% of rated full load current. If the motor has only 3
leads, 2 must be connected together to form one circuit through the winding. In this case,
one phase will carry the full applied current and each of the others, one-half each. If the
motor has 6 leads (3 mains and 3 neutrals), the 3 phases should be connected into one
series circuit.
3. Heated air can be either blown directly into the motor or into a temporary enclosure
surrounding the motor. The source of heated air should preferably be electrical as opposed
to fueled (such as kerosene) where a malfunction of the fuel burner could result in carbon