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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.