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A servo press uses servo motors for precise force and position control during forming operations.

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Use our free tonnage calculator at servopress.club/tonnage-calculator.html

Mitsubishi MR-J4 AL.10 undervoltage - what to check?

derek_tooling

Mitsubishi MR-J4 showing AL.10 undervoltage alarm. Our incoming power looks fine on the meter — 208V at the drive terminals, steady. But the alarm keeps popping up, usually during the forming stroke on heavy jobs.

Press is a 200T, about 6 years old. The alarm started maybe 2 weeks ago and it's getting more frequent. Yesterday it happened 5 times in one shift.

I checked the obvious stuff — input voltage is within spec, main contactor looks OK, no loose connections. The DC bus voltage reads normal at idle (about 290V) but I haven't been able to catch it during a fault because it clears too fast.

Anyone know what else to check? The drive is the original unit, never been serviced. Starting to wonder if the capacitors are going.

mchen_servo

AL.10 undervoltage — one thing I've seen cause this specifically during the forming stroke: the press is on the same electrical panel as other high-draw equipment (welders, large motors). When those start up simultaneously with the press forming stroke, the voltage sag triggers AL.10. Solution: dedicated transformer for the press, or at minimum a line reactor on the drive input.

toolndye_dave

One thing that catches people out with AL.10 on the MR-J4: the undervoltage threshold is checked on the DC bus, not the AC input. So you can have 200V AC at the terminals and still trigger AL.10 if the rectifier or capacitor bank is degraded. I've seen this on presses that are 8+ years old — the bus caps lose capacitance and can't hold voltage during the inrush of a heavy die.

Quick check: measure DC bus voltage at the drive terminals during a press cycle with a scope or fast-logging meter. If it dips below ~240VDC on a 200V system during the forming stroke, you've found your problem. Capacitor replacement on the MR-J4 is a depot repair, but some integrators do it in the field.

pressroom_tom

Adding to what toolndye_dave said about the bus caps — that's the #1 hidden cause on older MR-J4 units in my experience. Here's how I diagnose it systematically:

1. Monitor parameter Pr.52 (DC bus voltage display) on the MR-J4 during a full-speed forming stroke. If it dips more than 30% from idle, your caps are suspect. Healthy caps on a 200V system should hold above 260VDC even under peak load.

2. Check the cap ESR if you have an ESR meter. Mitsubishi doesn't publish a spec, but as a rule of thumb, if ESR is more than 2x the original value (usually printed on the cap label), replace the whole bank. Don't cherry-pick individual caps — if one is gone, the others are close behind.

3. The sneaky one: ambient temperature. MR-J4 caps are rated for 85°C typically. If your cabinet cooling fan died 6 months ago and nobody noticed, you've been cooking those caps at 50-60°C instead of 35°C. That halves their life per the 10°C rule.

4. Timing matters too. If AL.10 only happens on the first stroke after a long idle (like Monday morning), the caps might be fine but need a soft-start charge cycle. Some older MR-J4 firmware versions have a known issue where the pre-charge relay timing is too short after extended power-off. Mitsubishi released a firmware update for this around 2019 — check if you're on the latest.

One last thing — if you're getting AL.10 intermittently and can't reproduce it, check your incoming power during peak plant load times. We had one that only triggered between 7:00-7:15 AM when the whole shop was starting up simultaneously. A 3% voltage sag at the utility level was enough to push the bus below threshold during a heavy forming stroke.

carlos_mx

Check your caps. 6 years on an MR-J4 in a warm cabinet, they're probably at end of life. We replace ours proactively at 5 years now after getting burned twice.