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Motors, Part 18

Motors, Part 18Can you shunt or bypass the overload protection (not speaking of the overcurrent protection, here) during motor starting? Yes, you can [430.35]. But only under certain conditions.

If the motor’s not automatically started:

  • The shunting (or cutout) device can’t be left in the starting position.
  • Overcurrent protection devices (OCPDs) rated (or set) at not over 400% of the FLC are operative during the starting period.

If the motor’s automatically started, the motor starting period must exceed the time delay of available motor overload protective devices. And a listed means must be provide to:

  • Sense motor rotation and automatically prevent shunting or cutout if the motor fails to start.
  • Limit the time of shunting/cutout to less than the motor’s locked rotor time rating.
  • Provide for shutdown and manual restart if running condition isn’t reached.

The reason you might undertake the complicated task of meeting all these requirements is the motor has more inrush current than the protection system can handle. This solution addresses the symptom, but what about addressing the cause instead?

Installing a soft starter eliminates the inrush problem, and not only gets you out from under these requirements but also reduces stress on the electrical supply to the motor. A variable frequency drive (VFD) provides this advantage and a few others.

If torque is the reason for not using a soft start or VFD, consider a torque-multiplying gear drive or fluid drive on the load side of the motor.

« Part 17 | Part 19 » | Source: Mark Lamendola | Mindconnection