19.05.2026 by Viktor Siebert

An unexpected machine shutdown in a Traub CNC lathe led to the shipment of a Mitsubishi MDS-A-SP-220 spindle drive unit for technical analysis. The operator reported recurring failures during longer machining cycles. The machine reproducibly stopped with error code 3B “Power module error (Overheat)”. Despite a functioning cooling fan and clean heatsinks, stable operation was no longer possible.

The affected spindle unit was operating in a highly dynamic production environment with frequent acceleration and braking cycles. Due to the thermal shutdowns, production interruptions and scrap risks occurred on already started workpieces.

The technical basis of the analysis was derived from Mitsubishi MDS-D/DH alarm tables as well as the provided documentation and alarm overviews.

Initial Diagnosis of the Mitsubishi MDS-A-SP-220

During incoming inspection it was immediately noticeable that the unit had been exposed to high thermal stress. Slight discoloration was visible in the power section and thermally stressed PCB areas.

The first technical analysis included:

• inspection of the power modules
• verification of cooling fan functionality
• examination of thermal coupling between heatsink and power section
• inspection of the IGBT power stage
• ESR measurement of DC bus capacitors
• analysis of gate driver supply voltages
• inspection of transition resistances at power terminals
• thermal measurements under load simulation

The cooling fan operated mechanically correctly. Nevertheless, under load the power module area heated up unusually fast. According to the Mitsubishi alarm table, alarm 3B clearly indicates overheating of the power module or integrated power stage.

Additionally, elevated thermal transition values between the power semiconductor and heatsink were detected. Heat dissipation had significantly deteriorated.

Actual Root Cause

The detailed fault analysis identified several interconnected causes:

• aged thermal interface materials between power semiconductors and heatsink
• thermally fatigued IGBT power stage
• increased power dissipation under dynamic spindle load
• degraded heat transfer despite functioning fan
• partial capacitance loss in several DC bus capacitors
• increased ripple current in the DC bus
• additional thermal stress on the power module
• long term thermal cycling caused by aggressive acceleration ramps
• elevated internal cabinet temperatures

The alarm description in the Mitsubishi MDS-D/DH documentation confirms that alarm 3B is triggered when the thermal protection function of the power module is activated.

The documentation also explicitly references:

• cooling fan failure
• contaminated heatsinks
• ambient temperatures above 55°C
• thermal overload of the power section

as typical causes.

Repair Measures on the Mitsubishi Spindle Module

After completion of diagnostics, the MDS-A-SP-220 underwent a complete overhaul.

The repair included:

• replacement of thermally stressed power semiconductors
• renewal of thermal interface materials
• replacement of critical electrolytic capacitors
• rework of thermally stressed solder joints
• cleaning of all airflow channels and heatsink surfaces
• verification of cooling fan current consumption
• load simulation under rated operating conditions
• thermal endurance testing
• stress testing with dynamic acceleration profiles
• verification of protection functions
• inspection of DC bus voltage stability
• insulation testing of the power stage

Particular attention was paid to thermal stability under continuous load. Especially in older Mitsubishi MDS spindle amplifiers, aging effects frequently occur within the power modules.

Final Testing and Return Shipment

After completion of repairs, the unit was tested on a dedicated test bench under realistic CNC operating conditions.

Testing included:

• simulated spindle acceleration cycles
• continuous load operation
• thermal behavior under load
• DC bus voltage monitoring
• current consumption monitoring
• verification of all protection functions
• communication testing
• thermal long term testing

Alarm 3B no longer occurred during the complete testing phase. Temperature values remained stable within the permissible range.

After successful final testing, the Mitsubishi MDS-A-SP-220 was returned to the customer on short notice, allowing the Traub machine to resume production quickly.

Customer Feedback

“The machine is running stable again in three shift production. Most important for us was the fast analysis and the transparent technical documentation of the root cause.”

To mentioned Mitsubishi Drive: Mitsubishi MDS-A-SP-220 Spindle Drive Unit

More details about our Mitsubishi repair services can be found here:
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Technical Specifications Mitsubishi MDS-A-SP-220

Specification	Value
DC Bus Input Voltage	270V to 311V DC
Continuous Rated Output	22.0 kW
Output Voltage	155V AC, PWM controlled
Main Circuit Method	BiMOS Sinus PWM Inverter, IGBT
Control Method	Digital Closed Loop / Vector Control
Speed Control Range	35 to 4500 rpm depending on motor
Cooling Method	Fan cooling with external heatsink type D1
Ambient Temperature	0°C to +55°C
Dimensions HxWxD	380 x 150 x 300 mm
Weight	10.0 kg
Communication Interface	Mitsubishi High Speed Serial Link

Technical alarm description according to Mitsubishi MDS-D/DH documentation:

Operating Conditions

Environment	Recommendation
Ambient temperature	below 55°C
Humidity	non condensing
Cabinet ventilation	properly dimensioned
Contamination	regular cleaning required
Heatsinks	keep free from dust and oil
Vibration	minimize vibration exposure
Power supply	stable and symmetrical

Thermal load and cooling fan recommendations:

Interaction with Other Devices

The Mitsubishi MDS-A-SP-220 typically operates together with:

• Mitsubishi CNC controls
• Traub CNC machines
• Mitsubishi spindle motors
• MDS-D / MDS-DH power units
• external braking resistors
• DC bus modules
• axis amplifiers of the MDS series
• encoder and feedback systems

Functional Description

The Mitsubishi MDS-A-SP-220 is a CNC spindle amplifier designed for precise control of spindle motors in machine tools.

The unit performs:

• speed control
• torque regulation
• power management
• overcurrent protection
• temperature monitoring
• DC bus management
• communication with the CNC control
• protective shutdown functions

The integrated protection logic continuously monitors:

• power modules
• DC bus voltage
• motor temperature
• current consumption
• cooling fan operation
• communication status

In critical operating conditions, an alarm is automatically triggered and the spindle is shut down safely. The alarm number is displayed directly on the 7 segment display of the module.

Alarm and Error Codes Mitsubishi MDS Series

Alarm	Abbreviation	Error Name	Meaning	Reset
12	ME1	Memory error 1	RAM / ROM checksum error	PR
13	SWE	Software processor error	Software processing error	PR
17	ADE	AD error	A/D converter fault	PR
21	NS2	No signal	No encoder signal detected	PR
23	OSE	Excessive speed detection	Speed exceeded the specified value	PR
31	OS	Overspeed	Motor speed exceeded 115%	PR
32	PMOC	Power module overcurrent	Overcurrent in IPM power module	PR
34	DP	CRC error	CNC communication CRC error	PR
35	DE	Data error	Incorrect movement command data	PR
36	TE	Transmission error	Communication transmission interrupted	PR
37	PE	Parameter error	Parameter exceeded allowable range	PR
38	TP1	Protocol error 1	Frame communication error	PR
39	TP2	Protocol error 2	Information communication error	PR
3B	PMOH	Power module overheat	IPM module overtemperature detected	PR
40	KE1	TK unit changeover error	TK unit switching procedure error	PR
41	KE2	TK communication error	Communication fault with TK unit	PR
44	CAXE	C axis changeover alarm	C-axis changeover fault	NR
46	OHM	Motor overheat	Motor thermal overload detected	NR
50	OL	Overload	Motor overload condition	NR
52	OD	Excessive error	Excessive position tracking deviation	NR
5C	ORFE	Orientation feedback error	Orientation feedback pulse error	NR
82	NSP	Power supply no signal	Power supply signal missing	PR
E1	WOL	Overload warning	Overload warning detected	AR
E7	NCE	NC emergency stop	Emergency stop command from NC	PR

Source: Mitsubishi MDS-D/DH alarm tables and troubleshooting guide.

Preventive Measures for Operators

To avoid recurring thermal failures, the following preventive maintenance measures are recommended:

• regular cleaning of heatsinks
• scheduled cooling fan replacement
• thermal inspection of power modules
• ESR testing of aging capacitors
• monitoring of cabinet temperature
• annual maintenance of power units
• inspection of cabinet airflow
• tightening of power terminals
• inspection of braking resistors
• preventive maintenance of older CNC drives

Especially in older Mitsubishi MDS systems, preventive maintenance significantly reduces unplanned production downtime.

Conclusion

The 3B error on the Mitsubishi MDS-A-SP-220 was caused by a combination of thermal aging and deteriorated heat transfer within the power section. Although the cooling fan and heatsinks appeared normal externally, internal thermal stress caused repeated shutdown of the power module.

Through targeted repair measures, thermal rework and extensive load testing, the spindle module was restored to stable operating condition.