Troubleshooting Output Phase Loss (LF)
Topic Description

An output phase loss fault (LF) will occur when the current of an output phase has dropped below 5 - 10 % of the drive rated current.

Possible Causes

  • A loose connection or splice is present.
  • An external device between the motor and drive (such as a contactor) has failed.
  • An open phase is present in the motor.
  • A single phase motor is being used.
  • A significantly smaller motor than the drive rating is being used.
  • The drive has a failed/damaged output transistor or DCCT.
Resolution

Possible Solutions

CAUTION: After removing power to the drive, it is recommended to verify 10VDC or less is present on the drive's DC bus before performing any service work.

  • If a significantly smaller motor is being used, resize the drive appropriately based on the full load current of the motor.*
  • Check for loose connections or splices at both the drive output terminals (U/T1, V/T2, W/T3) and the motor terminals (peckerhead).
  • Check any external device(s) (such as a contactor) located between the drive and motor. Wire the motor directly to the drive output terminals for testing.
  • Test the motor windings with a multimeter (phase-to-phase resistance) and use a megohmmeter to check insulation resistance. Refer to this document: Performing an Insulation (Megohmmeter) Test on a Motor with a Short Video
  • Verify a single phase motor is NOT being used.
  • Disconnect the motor leads from the drive, run at 60 Hz, and verify correct output voltage. If an output phase loss (LF) fault still occurs with the motor leads disconnected, the drive is then the likely issue and should be repaired or replaced.

* In the event of a significantly smaller motor being used, disabling Output Phase Loss (L8-07 = 0) may be the only option. This is NOT recommended and therefore an external device should be used to monitor for output phase loss protection.

Note:
Output phase loss protection is automatically disabled when the drive is in Speed Search mode or the output frequency is less than 1Hz.
Parameter L8-07 is not available in the J1000 drives.


Related Documents
Input Diode and Output Transistor Check Procedure on a Variable Frequency Drive (VFD) with Short Video

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Last Modified Date: 03-03-2025ID: 10471