RM = KV + 1
Where
RM = Recommended min im um in sulation resistance in megohms at 104° F (40° C) of
the entire wind ing.
KV = Rated motor terminal to terminal voltage in kilovolts (1 000 volts = 1 KV).
On a new winding, where th e c ont aminant causing low insu lation resistance is gen erally
moisture, drying the wi nding through the proper ap plication of heat will nor m al ly increase the
insulation resistan ce to an acceptable level . The following are several accepted methods for
applying heat to a windin g:
1. If the motor is equipped with space heaters, they c an be energized to heat the wind ing.
2. Direct current (as from a welder) can be p assed through the winding. The total current
should not exceed appro ximately 50% of rated f ull load current. If the moto r has onl y 3
leads, 2 must be connect ed together to form one cir c ui t 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 an d 3 neutrals), the 3 phase s sh oul d be connected into one
series circuit.
3. Heated air can be either blo wn directly into the mot or or in to a temporary enclosu re
surrounding the mo tor. The source of heated air shoul d preferably be el ectrical as opposed
to fueled (such as kero s ene ) where a malfunction of the fuel burner could resul t in carbon