26.07.2026 by Viktor Siebert

Unexpected production downtime often results in high costs, delayed deliveries and significant interruptions in manufacturing. This was exactly the situation one of our industrial customers faced when their CNC machine repeatedly stopped during operation due to Alarm 32. The affected unit was a Mitsubishi MDS-CH-V2-3535 AC Servo Drive Unit responsible for controlling multiple servo axes within the machine.

Initially, the fault occurred only occasionally. After a short operating period, Alarm 32 appeared and the machine had to be restarted. Over time, the intervals between failures became shorter until reliable production was no longer possible.

During the initial technical discussion with the customer, we suspected that, in addition to an electrical fault, thermal influences might also be contributing to the problem. The drive unit was therefore shipped to our repair facility for a complete inspection.

Initial Diagnosis

After the drive arrived at our workshop, the first step was a comprehensive visual inspection.

Even before carrying out electrical measurements, one important defect became immediately apparent.

The internal cooling fan was completely seized and could no longer rotate. Significant dust and contamination had accumulated over time, severely restricting the airflow through the cooling system. As a result, the power electronics were no longer able to dissipate the generated heat efficiently.

Following the visual inspection, the unit underwent a complete electrical evaluation.

Among other things, the following assemblies were examined:

• Power supply sections
• DC bus
• Power stage
• IGBT power module
• Gate driver circuits
• Current measurement circuits
• Insulation values
• Connectors
• Heat sinks
• Cooling fan supply

During these tests, the customer’s reported Alarm 32 could be reproduced.

According to the Mitsubishi documentation, Alarm 32 indicates “Power Module Error (Overcurrent).” The alarm is generated whenever the power module detects an overcurrent condition. The manufacturer’s service manual lists possible causes including short circuits, failures within the power module itself, defective motors or damaged motor power cables.

In this particular repair case, however, our inspection also revealed a clear connection between the alarm and the failed cooling system.

Root Cause Analysis

After completing all electrical and mechanical inspections, the following damage pattern became evident:

• Completely seized cooling fan
• Severely reduced cooling performance
• Long-term thermal stress on electronic components
• Elevated operating temperatures inside the power stage
• Power module overcurrent shutdown resulting in Alarm 32
• Early ageing of heat-sensitive electronic components

Our technical assessment was therefore as follows:

The seized cooling fan caused insufficient heat dissipation over an extended period. As a consequence, the temperature of the power electronics increased significantly. Under higher machine loads, the power module operated outside its optimal temperature range until its internal protection system finally detected an abnormal operating condition and triggered Alarm 32.

This diagnosis perfectly matched the physical condition of the drive. While Mitsubishi generally classifies Alarm 32 as an overcurrent fault of the power module, the failed cooling system was identified in our workshop as the most probable underlying cause that initiated the failure sequence.

Repair Process

After customer approval, the Mitsubishi Servo Drive underwent a complete industrial repair.

The following work was carried out:

• Complete disassembly
• Professional industrial cleaning
• Replacement of the seized cooling fan
• Inspection of all power semiconductors
• Verification of all gate driver circuits
• DC bus inspection
• Inspection of the power supply sections
• Examination of all connectors
• Re-soldering of thermally stressed solder joints
• Preventive inspection of heat-sensitive components
• Cleaning of all cooling surfaces
• Renewal of thermal interfaces where required
• Final electrical safety inspection

Special attention was paid to every assembly that may have suffered from long-term thermal stress to ensure maximum long-term reliability after the repair.

Final Testing and Return to the Customer

After completing the repair, the Mitsubishi MDS-CH-V2-3535 Servo Drive Unit underwent extensive testing on our dedicated industrial test bench.

Among other things, the following functions were verified:

• Power-up behaviour
• DC bus voltage
• Current consumption
• Servo axis control
• Power output performance
• Temperature development
• Long-term load test
• Protection functions
• Communication
• Cooling fan operation

The newly installed cooling fan operated flawlessly throughout the entire endurance test. Even under increased load, the temperature of the power electronics remained completely stable.

Alarm 32 did not occur again at any point during the complete testing procedure.

Only after successfully passing all functional and load tests was the drive released for shipment back to the customer.

To mentioned Mitsubishi Drive: Mitsubishi MDS-CH-V2-3535 Servo Drive

More details about our Mitsubishi repair services can be found here:
Mitsubishi drive Repair by Industrypart

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Technical Specifications

Property	Value
Manufacturer	Mitsubishi Electric
Model	MDS-CH-V2-3535
Device Type	AC Servo Drive Unit
Series	MDS-CH-V2
Year of Manufacture	May 2010 (Nameplate)
Power Supply	Part of the Mitsubishi MDS multi-axis drive system
Cooling	Forced-air cooling via integrated cooling fan
Power Stage	35 / 35 Servo Axes
Application	CNC Machine Tools
Diagnostics	LED display with alarm numbers
Protective Functions	Overcurrent, overvoltage, overload, cooling fan monitoring, temperature monitoring, regeneration protection, communication monitoring, detector monitoring

Operating Conditions

Parameter	Value
Installation Location	Electrical cabinet
Environment	Low-dust industrial environment
Adequate Ventilation	Required
Regular Cleaning	Recommended
Heat Sink	Must be kept free from dust
Cooling Fan	Should be inspected regularly
Humidity	Condensation must be avoided
Mechanical Stress	Low vibration

Cooperation with Other Devices

The Mitsubishi MDS-CH-V2-3535 Servo Drive Unit is part of the modular Mitsubishi MDS drive system and operates together with various components within a CNC control system.

Compatible components include:

• Mitsubishi MDS-C1 Power Supply Unit
• Mitsubishi MDS-D Power Supply Unit
• Mitsubishi AC Servo Motors
• Mitsubishi Spindle Motors
• Mitsubishi CNC Controllers
• MELDAS CNC Systems
• Serial Encoders
• Absolute Encoders
• Incremental Encoders

Communication between the CNC controller, power modules and position encoders is continuously monitored. Communication errors, detector errors or parameter errors are immediately detected and displayed as alarms.

Functional Description

The Mitsubishi MDS-CH-V2-3535 Servo Drive Unit is responsible for the highly dynamic control of connected servo axes in modern CNC machine tools.

Based on the command values received from the CNC controller, the drive continuously calculates current, torque and speed commands for the connected servo motors. At the same time, the encoder feedback signals are permanently evaluated and compared with the commanded values.

During operation, the unit continuously monitors numerous protective functions, including:

• DC bus voltage
• Motor current
• Power modules
• Power electronics temperature
• Cooling fan operation
• Communication between controller and drive
• Encoder communication
• Regenerative energy
• Overload conditions
• Power supply voltage

If any operating limits are exceeded, the system automatically protects itself by disabling the power output stages or by performing a controlled deceleration of the axes. This protects not only the drive itself but also the motors and the machine from consequential damage.

Alarm and Fault Codes

Alarm	Fault Name	Description	Reset	Corrective Action
10	Insufficient Voltage	DC bus voltage too low	PR	Check the power supply
17	A/D Converter Error	Current feedback fault	PR	Inspect the drive
23	Excessive Speed Error	Excessive speed deviation detected	NR	Check motor and parameters
24	Grounding	Motor power cable grounded	PR	Inspect motor and power cables
30	Over Regeneration	Excessive regenerative energy	PR	Check braking resistor
31	Overspeed	Motor speed exceeded the allowable limit	PR	Check encoder and control loop
32	Power Module Error (Overcurrent)	Overcurrent detected in the power module	PR	Inspect the power stage, motor, cables and power module
33	Overvoltage	DC bus voltage too high	PR	Check power supply
38	Motor Overload	Electronic motor overload protection activated	NR	Reduce the mechanical load
45	Fan Stop	Cooling fan failure detected	PR	Inspect or replace the cooling fan
46	Motor Overheat	Motor or thermal protection overheated	NR	Check cooling system and motor
50	Overload 1	Overload detected	NR	Reduce the machine load
51	Overload 2	Continuous high motor current detected	NR	Inspect mechanics and load
61	Power Module Overcurrent	Overcurrent detected in the power supply module	PR	Inspect the power module
71	Instantaneous Power Interruption	Temporary power failure detected	NR	Check the power supply
73	Over Regeneration	Regenerative energy exceeded the allowable level	NR	Inspect the braking resistor
75	Overvoltage	DC bus overvoltage	NR	Check power supply and regenerative circuit