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Code DRV-21: Output PhaseLoss

Issue Description & Safety Warning

The Allen-Bradley PowerFlex 750 Drive has triggered a critical Output PhaseLoss alarm. Immediate attention from a qualified operator or maintenance technician is required to prevent secondary damage to the equipment or spoiled parts during the manufacturing cycle. Always ensure standard safety and LOTO procedures are followed before accessing electrical cabinets.

Official Troubleshooting Steps

Type: Configurable 444 [OutPhaseLossActn] Bit 0 “Load Faults” The current in one or more phases has been lost or remains below the threshold set in P445 [Out PhaseLossLvl] for 1 second. Decreasing the threshold makes the drive less sensitive to tripping. A decreased threshold is necessary when the motor is smaller than the drive rating. If TorqProve™ is active, the current in one or more phases has been lost or remains below a threshold for five msecs. The phases are checked at start to be sure that torque is delivered to the load. If the drive is faulting on start, increase P44 [Flux Up Time]. If TorqProve is active, and the brake is slipping, this fault occurs. When TorqProve is used, before the signal to the brake is applied to release it, the flux up time is used to check the three phases. The angle is adjusted to be sure that current is flowing through all three phases. If the motor moves during this test, the brake is not holding and a phase loss can occur. If TorqProve is active, and no brake is present, this fault occurs. Check for an open output contactor.

Source: Allen-Bradley PowerFlex 750-Series AC Drives Programming Manual (750-PM001)

Last reviewed: 2026-07-01 · Verified against the manufacturer’s official documentation.

About Allen-Bradley PowerFlex 750 Drive Alarms

Allen-Bradley controllers include extensive self-diagnostics that flag or halt the machine when a fault is detected. Error code DRV-21 on the PowerFlex 750 Drive identifies one specific fault condition — its documented cause and the official corrective steps are listed above, sourced from Allen-Bradley's service documentation. Always confirm against the manual for your exact machine configuration and follow LOTO procedures before servicing.

Complete Alarm List: 663 Allen-Bradley PowerFlex 750 Drive codes.

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