Applicable Systems
PowerFlex 753
Symptoms
The input circuit breaker (molded-case) feeding the PowerFlex 755 trips on short-circuit. The breaker handle is in the tripped position (mid-travel) and the trip indicator is visible. The drive loses all power.
Possible Causes
- Internal rectifier or input choke short-circuit in the drive
- Lightning strike or voltage surge causing input stage failure
- Incorrect breaker short-circuit rating for the available fault current
- Failure of the drive pre-charge circuit causing inrush current exceeding breaker instantaneous
Troubleshooting Steps
- Step 1: Do not reclose immediately. Visually inspect the drive for smoke, burnt smell, or damaged components
No visible damage, swelling, or leakage from capacitors or chokes - Step 2: Megger the drive input terminals (L1, L2, L3) to ground with a 500V megohmmeter
Insulation resistance >5 MOhm; if lower, the drive input stage is likely failed - Step 3: Verify the breaker short-circuit rating (Icu) exceeds the available fault current at the drive location
Breaker Icu > available Isc per NEC or IEC coordination study - Step 4: If the drive is suspect, isolate it and energize a temporary load to test the breaker alone
Breaker holds with a resistive test load; confirms the drive is the fault source
How to Reset
Reset the breaker by moving the handle to OFF then ON after confirming the fault is cleared. Do not reclose if the drive is damaged; replace or repair the drive first.
Common Replacement Parts
- 140M-C2E-B16: Allen-Bradley 140M motor protection circuit breaker (Verify frame size and magnetic trip range match the drive input current)
Frequently Asked Questions
- Q: Why did the breaker trip but not the drive's own fuses?
The breaker may have a lower instantaneous threshold or faster response than the semiconductor fuses, or the fault was on the input side before the fuses.
- Q: Can I use a standard breaker instead of a motor-protection breaker?
A standard breaker may not protect the drive input semiconductors properly. Use a Type 2 coordination breaker or semiconductor fuses as specified by Rockwell.
Source: Rockwell Automation PowerFlex 750-Series Installation Manual, Publication 750-IN001
Category: Electrical