Insulation resistance shows whether the insulation between the winding and the frame is sound and dry. Before shipment, HEM motors undergo a routine winding-to-frame insulation check with a 500 V megger. On site, the measurement should be taken:

  • Before first commissioning
  • On motors that have been stored or idle for a long time (the catalogue makes the test mandatory in this case)
  • After flooding, heavy condensation or wash-down
  • During periodic maintenance, to see how the value trends over time

When measuring:

  • Isolate and lock out the motor; disconnect the supply cables from the terminals.
  • Take inverters, soft starters or capacitors out of the circuit; the test voltage can damage them.
  • Do not apply megger voltage to PTC or PT100 sensor leads.
  • Measure between winding and frame; afterwards, briefly connect the winding to the frame to discharge it.

If the value is low, the winding has usually absorbed moisture; the motor is dried and the test repeated. For the acceptance value, refer to the standard used at your plant or consult our service team. For motors that have been idle for a long time, see the article on motor storage and idle periods.