Yaskawa A1000 LF3 Fault — What It Means
The LF3 fault on a Yaskawa A1000 variable frequency drive indicates Power Unit Output Phase Loss 3. The drive has detected a phase-loss condition on the output side of the power unit, meaning one of the three phases traveling from the drive to the motor is missing, open, or has abnormal resistance. This is not an input power problem but a problem in the path from the drive’s output terminals (typically labeled U/T1, V/T2, W/T3) to the motor.
Yaskawa’s troubleshooting documentation points first to wiring errors, disconnected output cables, and abnormal resistance between motor lines as the most common causes. The fault may also indicate motor winding damage or, if all external checks pass, an internal drive output stage or gate drive board failure. Yaskawa recommends cycling power as the first immediate action, then continuing troubleshooting if the fault returns.
Before You Replace Anything
Technicians sometimes replace the entire drive or control board before checking the motor cables and motor windings. A simple visual inspection of output terminals and a resistance check between motor lines will identify open or damaged wiring in minutes and save hundreds of dollars.
Common Causes
- Loose or open motor output cable (~50%) A disconnected, damaged, or loose conductor between the drive output terminals and motor causes one phase to drop under load.
- Miswired motor leads (~20%) Incorrect termination at the drive or motor end results in an incomplete circuit for one phase.
- Motor winding damage (~15%) An open or high-resistance winding inside the motor creates abnormal phase resistance and triggers the output phase-loss detection.
- Drive output power stage or gate drive board failure (~10%) If external wiring and motor checks pass, a fault in the drive’s internal power unit or gate drive circuitry may prevent one output phase from conducting.
- Damaged motor cable insulation or short (~5%) Insulation breakdown or a short in one phase conductor can create enough imbalance to trip the phase-loss fault.
Quick Diagnosis
Answer these to narrow it down fast.
Does the LF3 fault disappear after cycling power and stay off during a test run?
No: The fault is persistent. Continue to inspect output wiring and motor connections.
Are all three motor output cables firmly terminated at both the drive and motor, with no visible damage or missing conductors?
No: Repair or replace the damaged cable and re-terminate all connections, then test again.
Do resistance measurements between motor lines show balanced values with no open circuits?
No: The motor has an open or damaged winding. Replace or rewind the motor and clear the fault.
Step-by-Step Fix
- Cycle power on the A1000 drive by switching off the main disconnect, waiting 30 seconds, and switching back on to see if the LF3 fault clears and stays off.
- Inspect the output wiring from the drive’s U/T1, V/T2, and W/T3 terminals to the motor for loose connections, broken strands, or disconnected phases.
- Verify motor cable terminations at both ends are tight and correctly landed, with no missing phase conductors or reversed connections.
- Measure resistance between motor lines (U-V, V-W, W-U) using a multimeter to identify an open winding or significant imbalance that would indicate motor damage.
- Check motor insulation resistance to ground using a megohmmeter if the motor has been exposed to moisture, heat, or suspected winding failure.
- Reconnect or replace damaged cables if you find an open or damaged conductor, then re-test the drive under load.
- Consult Yaskawa service or replace the drive if all external checks pass and the LF3 fault persists, indicating an internal power unit or gate drive board failure.
Parts Often Needed
| Part | Notes |
|---|---|
| Motor output cable (appropriate AWG for drive frame) | Amazon | Only if cable is damaged, open, or undersized for the application. |
| Replacement motor | Amazon | Only if winding is open or insulation has failed and repair is not economical. |
| Yaskawa A1000 gate drive board or power unit | Amazon | Only if internal drive fault is confirmed after all external checks. Match frame size and part number to your specific A1000 model. |
When to Call a Pro
Call a qualified electrician or drive technician if you are not trained to work on industrial three-phase motor circuits. High DC bus voltages (up to 800 V or more depending on drive size) remain inside the A1000 even after input power is removed, and improper handling can cause injury or death. A technician should perform all internal drive diagnostics, gate drive board replacement, or power unit service. If motor winding tests or insulation checks require specialized equipment like a megohmmeter or if you are unsure how to safely measure resistance on a three-phase motor, professional service is the safest choice.
Rough cost: A pro service call runs about $150-600.