Mitsubishi MR-J4 AL.50: Overload 1 during operation, motor turns poorly, what to check?

Question

I'm getting an AL.50 (Overload 1) on my Mitsubishi MR-J4 servo amplifier. It happens during operation, and the motor seems to turn poorly. What should I check first?

Top answer · Written by InduQT from the fault guide and manufacturer manuals cited below · Oct 28, 2026

Alarm AL.50 on a Mitsubishi MR-J4 servo amplifier indicates Overload 1, meaning the load exceeded the amplifier's overload protection characteristic. If this occurs during operation and the motor turns poorly, it suggests a sustained high effective load. It's important to address the cause and allow the equipment to cool for about 30 minutes before resetting, as repeated resets can damage the amplifier and motor.

Likely causes and checks

1Motor power cable disconnected or U/V/W wired wrongly

Points to it: If the motor power cable is disconnected or the U/V/W wiring is incorrect, an abnormal current can flow, leading to an overload alarm and poor motor performance. This is especially likely if the alarm appeared soon after wiring work.
Check: Check the servo motor power cable and the U/V/W wiring for correct connection.

2Continuous current above the amplifier rating

Points to it: A sustained high effective load can cause the continuous current to exceed the amplifier's rating, triggering the overload alarm. This might be indicated by a high effective load ratio or the alarm appearing later in the cycle.
Check: Check the effective load ratio in MR Configurator2. Consider reducing the load or using a larger capacity motor if the ratio is consistently high.

3Electromagnetic brake not released

Points to it: If the electromagnetic brake is not releasing during operation, the motor will be working against it, causing it to run hot and potentially leading to an overload. You might not hear the brake click when it should release.
Check: Check that the electromagnetic brake is released during operation.

4Servo system unstable and resonating, or hunting in servo-lock

Points to it: An unstable servo system that is resonating or hunting can cause excessive current flow and vibration, leading to an overload alarm. This might be accompanied by noise or vibration from the axis.
Check: Check for resonance or hunting and adjust the gains in the servo system.

Next check: Given the symptom of the motor turning poorly and the alarm occurring during operation, the most immediate and impactful check is to verify the servo motor power cable connections and the U/V/W wiring. This often resolves issues that manifest with poor motor movement and overload alarms.

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