Insulation measurement at the three-phase AC motor, motor cable, drive controller

If the drive controller shows the error indication
• OC1 (overcurrent or 'Short circuit' message) or
• OC2 (earth fault)

several times, and you cannot detect an impending cause, the insulation resistance of the motor should be detected. The used insulation resistance tester should correspond to the demands of the EN 61557 part 1, 2 and 4 or DIN VDE 0413 part 1 and 4. The insulation resistance measurement is supposed to be effected with a DC voltage in order to exclude the influence of capacities between conductors, as well as between the conductors and earth.

Procedure for measurement of the insulation resistance at the three-phase AC motor:

Please disconnect all connections at the motor. The interconnection bridges (for star / delta connection) of the windings are supposed to be eliminated.

The insulation resistance tester is supposed to be set to the suitable test voltage:
• Motor rated voltage ≤ 500 V => test voltage 500 V DC
• Motor rated voltage > 500 V => test voltage 1000 V DC

Following points are measured: 1. Resistance from winding to winding
2. Resistance from winding to PE
3. Resistance from winding to KTY/PTC Attention: Before you measure please connect both poles of the temperature sensors!

Evaluation of the measurement result:
In case of a test voltage of
• 500 V the minimum permissible limit value of the insulation resistance is ≥ 0.5 MOhm (plus the measurement tolerance of 30 % ≥ 0.65 MOhm)
• 1000 V the minimum permissible limit value of the insulation resistance is ≥ 1 MOhm (plus the measurement tolerance of 30 % ≥ 1.3 MOhm).

In practice appr. 3 MOhm should be reached.

Notes:
Do not mistake the measurement of the insulation resistance for the high-voltage insulation test of the electrical equipment according to VDE 0160 part 105 (EN 61800-5-1)! The high-voltage insulation test voltage of electrical equipment must be greater than the peak voltage at the three-phase AC motor during operation (√2 · Umains).

Insulation test on the motor cable:

Additionally, the insulation resistance of the motor cable should be measured. For this purpose the motor cable is to be disconnected from the motor and from the drive controller / other components (e. g. motor protection, motor filter).

Insulation test of the electrical equipment/system according to VDE 0160, part 105 (EN 61800-5-1):

The insulation test of the complete electrical device according to VDE 0160, part 105 requires the disconnection of the drive controllers and existing mains filters for protection against destruction of the power semi-conductors and other power components, which should be protected against voltage. This procedure is permissible, as Lenze tests the high-voltage of the drive controllers and main chokes in the framework of the final inspection according to VDE 0160 part 105 (EN 61800-5-1).

Insulation measurements made with connected drive controller or mains choke bears the following risk:
• Due to the reverse current of the semi-conductor components or due to currents caused by eventual RFI capacitors an insulation fault can be simulated.
• Due to overvoltage electronic components can be destructed.

Moreover, all overvoltage protection devices should be disconnected for insulation measurements, as otherwise the test voltage would be limited by the protection components, which would lead to incorrect measurement. If during measurement overvoltage protection devices should still be mounted, they are normally not destructed by the low currents.

Search term: Isolation, DGUV A3

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