PowerFlex 525 fault F004 (UnderVoltage) — causes and how to fix it

Symptoms

The drive stops, the display shows F004 UnderVoltage, and the motor coasts. The fault tends to come in "waves": several drives in the same panel or on the same supply branch trip at the same time, often coinciding with a large machine starting, a supply changeover, or a brief dip in the mains. If the drive has auto restart enabled, the fault may clear itself and leave a trace only in the fault history.

According to Rockwell (520-UM001O, Troubleshooting chapter, pp. 159–161), F004 means the DC bus voltage has fallen below the minimum value. It is a Type 1 fault — resettable and configurable for auto reset — unlike F003 Power Loss (phase loss under load) or F005 OverVoltage (bus overvoltage, for example from the motor regenerating during deceleration).

What "below the minimum" means

The trip levels are given in 520-TD001I (Protection table, p. 10), on the DC bus, with the equivalent mains input voltage:

• Drives with a 200…240 V AC supply: bus 190 V DC — equivalent to the mains dropping to about 150 V AC.
• Drives with a 380…480 V AC supply: bus 390 V DC — equivalent to the mains dropping to about 275 V AC.
• 525…600 V AC drives: 487 V DC (P038 = 600 V) or 390 V DC (P038 = 480 V).

In other words, on a 380…480 V drive the mains has to fall to ~275 V before it trips — a dip of a few tens of volts does not cause the fault. If F004 appears while the measured mains is normal, check places other than the mains: supply wiring, fuses, the input contactor contacts, or the voltage recorded at the moment of the fault.

Common causes

1. Brief mains voltage dips — a large machine starting on the same branch, generator/mains changeover, lightning. The drive only tolerates short supply interruptions: power ride-through is 100 ms (520-TD001I p. 10); anything longer gives F004 or F003.
2. Loss of one phase or a poor contact at the input: one blown fuse, a loose terminal screw, pitted or burnt contactor contacts. A 3-phase drive can still run on 2 phases at light load, but the bus ripples and drops when the load increases → F004 or F003 depending on load.
3. Supply cable too long or too small for the inrush current of the whole panel: the voltage at the drive drops even though it measures fine at the distribution board.
4. Many drives powering up at once after a power outage: simultaneous capacitor charging current pulls the branch voltage down.
5. Degraded DC bus capacitors in a drive that has run for many years or has been stored for a long time without power — a rarer cause, to be concluded only after the four items above have been ruled out.

Checks at the panel

Step 1 — read the data from the moment of the fault, before resetting it. The PowerFlex 525 stores the last 10 faults together with the bus voltage at the time of each fault:

• b007 [Fault 1 Code] … : fault codes, most recent first.
• F651…F660 [Fault x BusVolts]: DC bus voltage at the time of the fault — the most important figure. A bus of 380 V on a 480 V drive that trips at 390 V → a slight dip, suspect the mains; a bus of ~0 V → complete loss of supply, suspect fuses/contactor.
• F611…F620 [Fault x Time-hr] and F621…F630 [Fault x Time-min]: the time of the fault in the drive's powered-up time (d362/d363) — compare with the operating log to find out which machine was starting at that moment.

Step 2 — measure at the drive's L1/L2/L3 terminals, not at the distribution board: measure all three phase-to-phase voltages at no load and under load, and compare with b005 [DC Bus Voltage] shown on the drive (bus ≈ 1.35 × line voltage). Phase imbalance > a few % or a bus voltage abnormally low compared with the measured voltage → check fuses, tighten terminals, check the input contactor.

Step 3 — check whether the configuration is hiding the fault. A541 [Auto Rstrt Tries] and A542 [Auto Rstrt Delay] set to non-zero mean the drive may have reset F004 itself many times before anyone noticed — and a repeated fault is recorded only once in the fault history (p. 79), so the history does not show how often it occurred. A548 [Power Loss Mode] = 1 (Decel) helps the drive hold the bus during a short supply loss by decelerating; A549 [Half Bus Enable] allows the drive to keep running at 50% input voltage during a short dip — but Rockwell requires a minimum source impedance (equivalent to a 5% transformer rated at 6 times the drive) when it is enabled, so enable it only after the supply conditions have been checked.

Step 4 — look at the whole branch. If several drives report F004 at exactly the same moment → the cause is in the mains/branch, not in the individual drives. Note which machines were starting at that time and whether there was a supply changeover.

When to replace the drive

• No need to replace the drive when F651–F660 show the bus dropping exactly when the mains dipped, phase measurements show imbalance, or the fault comes in waves across several drives. Fixing the supply side (fuses, terminals, contactor, cable, start-up sequence) is enough.
• Suspect the drive when: only one drive in the panel reports F004 while the voltage measured at its own terminals is adequate; b005 shows a bus much lower than 1.35 × the measured voltage; or the drive has run for many years or been stored for a long time. The bus capacitors and charging circuit then need to be checked: contact Rockwell or a repair centre authorised by Rockwell; do not disassemble the drive (520-UM001O pp. 13, 159).
• Further investigation is needed when the dips are brief (only continuous voltage recording will catch them), when the fault is linked to the start-up sequence of a whole line, or when the A541/A542/A548 configuration has to be settled for the application (conveyors, pumps and fans react differently to auto restart). When a machine is down, TDL provides troubleshooting on site or remotely: see breakdown support and maintenance.

For faults F005 (DC bus overvoltage), F007 (motor overload) and F013 (ground fault), see PowerFlex 525 faults F005, F007, F013.