-6- B1091-GB-3212 www.nord.com
3. Information
3.6 Checking insulation resistance
Before putting the motor into operation for the first time, after a lengthy period of storage or
standstill (approx. 6 months), the insulation resistance of the winding has to be determined.
During and directly after measurement the terminals carry hazardous voltages in some cases
and must not be touched.
Insulation resistance
The insulation resistance of new, cleaned, repaired windings against the housing and against
one another is >200 M Ohm.
Measurement
In the case of windings up to an operating voltage of 400 V the insulation resistance against
the housing must be measured with a DC voltage of 500 V. At operating voltages up to 725 V
measure with a DC voltage of 1000 V. When doing this the temperature of the windings
should be 25°C ± 15°C.
Checking
If, in the case of a new, cleaned winding or a repaired motor which has been stored or shut
down for any lengthy period, the insulation resistance of the winding against the housing is
less than 50 MOhm, the cause may be moisture. The windings will then have to be dried.
After any lengthy period of operation the insulation resistance may drop. As long as the
measured value does not fall below the critical insulation resistance of < 50 MOhm, the motor
may continue to be operated. If the value falls below the critical level, the cause must be
established and if necessary, the windings or winding sections must be repaired, cleaned or
dried.
3.7 Commissioning
Note: Electromagnetic compatibility
Emitted interference: If torques are not equal (e.g. drive of a piston compressor) a non-
sinusoidal motor current is bound to develop, the harmonics of which may cause
impermissible mains influence and hence impermissible emitted interference.
If power is fed via an inverter, different levels of emitted interference will occur, depending on
the design of inverter (type, suppression, manufacturer). It is absolutely essential that the
EMC information provided by the inverter manufacturer be observed. If the manufacturer
recommends a screened motor supply lead, screening will be most effective if it is electrically
connected to a large area of the metal terminal box of the motor (with EMC cable gland made
of metal). In the case of motors with built-in sensors (e.g. PTC thermistors) interference
voltages can occur on the sensor line, caused by the inverter.
Interference immunity: In the case of motors with built-in sensors (e.g. PTC thermistors) the
user himself must ensure adequate interference immunity by making a suitable selection of
sensor signal line (possibly with screening, interfacing as with motor supply lead) and the
analysing instrument. Prior to commissioning always observe the information and instructions
in the operating manuals for inverters as well as all other sets of instructions. After attaching
the motors, check them to make sure they are operating properly! In the case of brake motors
also check that the brake operates properly.
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