Motor Phase UW Short Circuit

What does F42 mean?

This fault detects a short circuit condition between motor phases U and W at the drive output. This implies an insulation failure within the motor windings or a direct short in the cabling connecting the drive to the motor.

• Motor winding insulation breakdown between phase U and W due to overheating, moisture ingress, or mechanical stress.
• Physical damage or abrasion to motor cable insulation between phase U and W conductors, causing a direct short.
• Contamination (e.g., metallic dust, moisture) inside the motor terminal box or drive output terminals bridging phases U and W.
• Defective motor contactor or output reactor causing an internal short between phases U and W before the motor.

Click steps to track your progress.

1. 1

   Isolate power to the drive and motor. Disconnect motor leads U and W from the drive output terminals (T1, T3).

2. 2

   Measure resistance between motor phases U and W at the motor terminal block using a multimeter (ohmmeter). Expect an open circuit or very high resistance (>20 MΩ).

3. 3

   Perform a megger test (insulation resistance test) between motor phases U and W at the motor terminal block at 500VDC or 1000VDC. Expect >100 MΩ.

4. 4

   Visually inspect motor cables for signs of insulation damage, crimping, or water ingress. Inspect drive output terminals for debris or arcing evidence.

5. 5

   If the motor and cables test good, disconnect the motor cable at the drive output and measure resistance between drive output terminals U and W. If shorted, replace the drive.