What’s Killing Your Servo Motors? Heat, Dirty Power, and Oversizing Mistakes

by Bryan Hellman November 14, 2025

Servo systems rarely fail at random. Motors do not just “wear out” one day. They fail because something is stressing them over time—usually heat, dirty power, or a mismatch between the drive and motor. The good news: once you understand these root causes, you can prevent most failures long before a motor burns up or a drive trips offline.

This guide walks through the three biggest servo killers and how proper drive–motor pairing can protect your equipment, boost reliability, and extend service life.

Heat: The Silent Servo Killer

A servo motor does not have to feel extremely hot on the outside to be overheating internally. Even modest increases in winding temperature can shorten insulation life, raise resistance, and accelerate bearing wear. Over time, that means more current, less torque, and eventually a motor that can’t hold position under load.

Common sources of servo heat include:

Overloaded or stalled axes where the drive is constantly trying to force torque
Poor airflow in the cabinet, causing the baseline motor temperature to creep up
Repeating high-torque, high-acceleration moves with little cool-down time
An undersized drive that must push current hard just to meet basic motion demands

Modern servo drives such as Mitsubishi MELSERVO MR-J2 and MR-J2S units use internal thermal models and encoder feedback to estimate winding temperature and trigger alarms before a failure occurs. When those warnings are ignored, heat continues to build until the insulation, bearings, or magnets are permanently damaged.

Examples of Mitsubishi drives that provide this level of control and protection include:

When you see repeated overtemperature warnings on drives like these, it is the system telling you the motor is being thermally stressed. Treat those alarms as early warning, not background noise.

Dirty Power: Spikes, Imbalance, and Electrical Noise

Even a correctly sized servo system can be slowly damaged by poor power quality. Voltage spikes, phase imbalance, harmonics, and ground noise all create extra stress on servo windings, insulation, encoders, and bearings.

Typical symptoms of dirty power include:

Intermittent encoder or position faults that “go away” when the machine is idle
Motors running hotter than expected even at light mechanical load
Overvoltage trips during deceleration, especially on vertical or high-inertia axes
Multiple drives faulting at once when large loads switch on or off elsewhere in the plant

Servo drives are designed to be more than just amplifiers. A well-matched drive helps regulate voltage, manage regeneration energy, and filter noise before it reaches the motor. Yaskawa Sigma-series drives are a good example: they provide tight current control, bus management, and protection features that shield SGDH and SGDV servo motors from upstream disturbances.

Representative Yaskawa drives you might standardize around include:

On the FANUC side, servo drives such as the FANUC A06B-6089-H105 AC servo drive are built to monitor and adjust motor performance continuously, helping maintain stable positioning and reduce the impact of line disturbances on A06B-series motors.

Oversizing Mistakes: When “More Torque” Hurts You

Oversizing is one of the most expensive and least obvious servo mistakes. Choosing a much larger motor and drive than the application actually needs might feel safe, but it often creates new problems.

Problems caused by oversizing include:

Low efficiency at very light loads, causing motors to run warmer than expected
Inertia mismatch between the motor rotor and the driven axis, leading to oscillation or “hunting”
Higher shock loads on couplings and gearboxes during starts and stops
More expensive drives and cables than the application truly requires

Servo manufacturers publish inertia matching guidelines for a reason. If the motor inertia is far larger than the load inertia, the servo loop struggles to control position smoothly. The result is chatter, buzzing at standstill, or drives that refuse to settle without heavy filtering and reduced bandwidth.

A better approach is to select the smallest motor and drive that can comfortably handle your peak torque, speed, and duty cycle. For example, within the Mitsubishi MR-J2S family, you can choose from compact amplifiers like the MR-J2S-40 or MR-J2S-100B instead of jumping straight to the highest power rating in the catalog.

Similarly, within Yaskawa’s SGDH and SGDV lines, choosing a correctly sized unit such as the SGDH-04AE or SGDV-2R8A01A for a modest axis will typically result in better tuning, cooler operation, and longer system life than using a much larger servo package “just in case.”

How Proper Drive–Motor Pairing Prevents Failures

The drive and motor should be treated as a matched pair, not two independent parts. When they are correctly paired and sized, the drive can actively protect the motor from all three of the major failure drivers: heat, dirty power, and oversizing.

Good drive–motor pairing provides:

Thermal modeling and current limiting to prevent overheating under heavy load
Matching encoder feedback and control algorithms for stable, precise motion
Regeneration handling and braking to avoid bus overvoltage during deceleration
Continuous monitoring of torque, speed, and error so you can catch problems early

Examples of matched ecosystems include:

Mitsubishi MR-J2 / MR-J2S servo drives matched to compatible Mitsubishi servo motors
Yaskawa SGDH and SGDV drives matched with Sigma servo motors for multi-axis motion
FANUC A06B servo drives and A06B AC servo motors designed to work as a single integrated system

For FANUC users, A06B-series motors such as the A06B-0233-B200 AC servo motor, A06B-0273-B200#0100 AC servo motor, and A06B-2063-B003 AC servo motor are designed to deliver high precision and performance when paired with compatible FANUC drives in demanding machine tool and automation applications.

When to Repair and When to Replace a Servo System

Even with good practices, some servos will eventually reach end of life. The question then becomes whether to repair what you have or replace it with a newer, more available drive–motor set.

Repair or replace decisions often hinge on:

Insulation test results and winding condition
Bearing health, especially if there is evidence of electrical fluting
Encoder condition and cable integrity
Availability and cost of the existing drive and motor series

When a legacy servo drive is still widely supported and your spare stock is strong, repair can be a cost-effective option. When parts are hard to find or the system has chronic power or heat problems, moving to a newer platform can pay for itself in reduced downtime and easier support.

Industrial Automation Co. stocks a variety of legacy and current servo drives and motors, including:

Stop Servo Failures at the Source

Most “mysterious” servo failures trace back to heat, dirty power, or sizing and pairing mistakes. By focusing on proper drive–motor matching, clean power, and realistic application sizing, you can dramatically extend the life of your servo systems and cut down on nuisance faults and unplanned downtime.

If you are dealing with repeated servo faults, hot motors, or drives that seem impossible to tune, it may be time to step back and evaluate whether the drive–motor pairing is really right for the job.

Industrial Automation Co. can help you:

Select compatible Mitsubishi, Yaskawa, or FANUC servo drives and motors for new projects
Identify drop-in replacements for discontinued servo amplifiers
Plan a migration path that reduces risk instead of creating new failure points

Contact Industrial Automation Co. for servo troubleshooting and replacement support and get help choosing drive–motor combinations that keep your lines running instead of burning up.

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