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U.S. ELECTRICAL MOTORS
INSTALLATION AND MAINTENANCE
(C) Insulation resistance readings must not drop below the value indicated by the following formula:
Rm = Kv + 1
Rm = minimum insulation, in megohms, at 40°C
Kv = rated motor voltage in kilovolts
(D) Dielectric Absorption Ratio:
In addition to the individual test reading, a dielectric absorption ratio may be required. The dielectric absorption
ratio is obtained by taking megohmmeter readings at a one minute and ten minute interval or when hand
powered megohmmeters are used, at a thirty second and sixty second interval. The voltage should be the
same as outlined in Part A of this procedure.
The ratio is obtained by dividing the second reading by the first reading and is based on a good insulation
system increasing its resistance when subjected to a test voltage for a period of time.
The ratios are as follows:
10 minute : 1 minute 60 second : 30 second
Dangerous = less than 1.0 Poor = less than 1.1
Poor = 1.0 to 1.4 Questionable = 1.1 to 1.24
Questionable = 1.5 to 1.9 Fair = 1.25 to 1.3
Fair = 2.0 to 2.9 Good = 1.4 to 1.6
Good = 3.0 to 4.0 Excellent = Over to 1.6
Excellent = Over 4.0
If a low insulation resistance reading is obtained in either the individual test or dielectric absorption ratio test,
thoroughly clean and dry the windings. Recheck insulation resistance and dielectric absorption ratio.
NOTE: Slightly lower dielectric absorption ratios may be acceptable when high initial insulation resistance
readings are obtained (1,000 + megohms). Refer any questions to U.S.E.M. Product Service
Department.
For additional information on insulation testing, refer to IEEE Transaction No. 43.
4. Start-up Preparations after Storage.
(A) Motor should be thoroughly inspected and cleaned to restore to an "As Shipped" condition.
(B) If motor has been in storage for less than 6 months, remove grease drain plugs at each end of the motor.
Remove a small quantity of grease with a scavenger and replace drain plugs. If any moisture or
contamination is evident in the grease, it must be completely changed by disassembling the unit and
repacking per Section XI "Lubrication".
(C) If motor has been in storage for 6 months or more, grease must be completely changed by disassembling
the unit and repacking per Section XI "Lubrication".
(D) The winding must be tested to obtain insulation resistance and dielectric absorption ratio as described in
Part 3 of this section.
(E) If storage has exceeded one year, the U.S.E.M. Quality Assurance Department must be contacted prior
to equipment start-up.
Storage
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