APM
All alarm codes
ALARM 21

Power Drive Fault / Insufficient Voltage (Spindle, FR-SF)

FR-SF spindle drive is not getting a clean encoder signal from the spindle PLG (pulse generator). The Z-phase amplifier signal voltage is out of spec.

Controls
T-32, T-Plus
Machine series
SQT, Quick Turn

Causes

  1. FR-SF spindle drive is not getting a clean encoder signal from the spindle PLG (pulse generator). The Z-phase amplifier signal voltage is out of spec.

    • noname777forum
      FR SF spindle, alarm 21, no signal from encoder. Motor has a little box attached, inside you can find little amplifier with A B and sometimes Z phases. If signals are bad, it will stop with alarm 21. Check the voltage, i…

  2. Failing / aged caps in the FR-SF power supply. Drive runs hot, alarms trip more often at higher RPM ramp-ups. Capacitors can be replaced as a set; an auxiliary cabinet fan moving air across the SF-PW helps.

    • cnctoolcatforum
      Most Japanese CNC machine's operate on 200 V internally, so make sure that's what your drives are seeing. If they are seeing more than 220 volts, you can have problems! And yes, the FR-SF power supplies run really hot, a…

  3. Incoming line voltage too high. Japanese Mazaks expect ~200V internally — if the cabinet transformer taps are wrong, drives see >220V and trip overvoltage / power-drive-fault.

    • cnctoolcatforum
      Make sure your incoming transformer taps are set for 240-250 volts incoming. Most Japanese CNC machine's operate on 200 V internally, so make sure that's what your drives are seeing. If they are seeing more than 220 volt…

How to clear it

  1. 1

    Measure the encoder amplifier output (A/B/Z phases) at the small box bolted to the spindle motor. Voltage must be ±1.5 V AC. If it drifts at speed, suspect rear-bearing run-out distorting the PLG head/sensor gear.

    • noname777forum
      Check the voltage, it must be +/- 1.5 AC. You can put FR-SF into open loop and adjust them, say run 300 rpm. The other thing is to check them at full speed, in CLOSE LOOP. See if voltage shrinks or deviates a lot. That m…
  2. 2

    Check the incoming transformer tap setting and verify drives see ~200 V, not >220 V.

    • cnctoolcatforum
      Make sure your incoming transformer taps are set for 240-250 volts incoming.
  3. 3

    If alarms reliably trip during heat-soak / high duty cycle: replace ALL caps in the FR-SF power supply (~$20 in parts) and add a muffin fan across the SF-PW board.

    • cnctoolcatforum
      FR-SF power supplies run really hot, and are know to fail (all capacitors replaced usually fixes). ... I add an auxiliary muffin fan inside the cabinet to move air across the FR-SF power supply (SF-PW).

Additional notes

Alarm 21 'Power drive fault' often shows alongside Alarm 204 'Spindle controller malfunction' on T-32/T-Plus — the underlying problem is the FR-SF spindle drive + its PLG.

Confidence: high · Two independent PM threads (overvoltage QT15N and SQT10MS/spindle-speed-error) both implicate the FR-SF drive caps and the PLG encoder. cnctoolcat (Diamond member) gives a specific, repeatable cap-replacement + cabinet-fan fix.

First sourced: 5/21/2026