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## AL.10 Definition
Mitsubishi MR-J4 alarm AL.10 is undervoltage — the main circuit DC bus voltage has dropped below the minimum threshold.
## Voltage Thresholds
For 200V class MR-J4:
- Normal DC bus: 280-340V
- AL.10 triggers below: 200V
For 400V class MR-J4:
- Normal DC bus: 560-680V
- AL.10 triggers below: 400V
## Diagnosis Steps
### Step 1: Check Input AC Voltage
Measure AC voltage at drive input terminals (L1, L2, L3). Should be within +/-10% of rated.
If AC voltage is low: check upstream transformer tap settings, check for voltage sag during press forming stroke.
### Step 2: Check for Voltage Sag During Forming
On servo presses, the forming stroke draws peak current. If the power supply is undersized, voltage sags during peak load and triggers AL.10.
Fix: Add line reactor (3-5% impedance) on drive input. This reduces voltage sag and also protects the drive from line transients.
### Step 3: Check Main Contactor
A worn main contactor can cause intermittent voltage drops. Check contact resistance (should be below 1 milliohm per contact).
### Step 4: Check DC Bus Capacitors
Aged capacitors have reduced capacitance and cannot hold DC bus voltage during transients. Capacitor life is typically 5-7 years.
Measure capacitance with LCR meter (drive powered off, DC bus discharged). Replace if below 80% of rated value.
## AL.10 During Deceleration
If AL.10 occurs during press deceleration (return stroke), the regenerative energy is exceeding the drive's braking capacity. Add a braking resistor or regenerative unit.
mike_chen_eng
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.
mike_chen_eng
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.