Common Failures in Servo Drives and Motors: Causes, Diagnosis, and Preventive Strategies

About the author

Written by John Yang, Sales manager of Apter Power

John Yang, Sales Manager of Evolo Automation, is dedicated to the industrial control industry and committed to providing end-to-end support for clients throughout the entire cooperation process. He deeply understands the critical role of stable industrial control systems in production operations, so he focuses on listening to clients’ actual needs—whether it’s upgrading existing control equipment, solving system compatibility issues, or customizing automation solutions for special industrial scenarios.

In modern automated factories — from CNC machining centers and semiconductor equipment to packaging lines and robotics — the servo drive and servo motor form the core motion-control unit. Unlike ordinary induction motors, servo systems operate in a closed-loop architecture that constantly compares command signals with feedback signals. This design delivers high positioning accuracy, fast response, and repeatability, but it also means that small faults can quickly escalate into machine shutdowns.

Recent maintenance statistics from several Asian manufacturing plants show that over 45% of unexpected production downtime in precision machinery is related to motion-control systems, and within that category, servo drive and motor faults account for the largest share. Understanding the typical failure patterns is therefore essential for maintenance engineers and automation managers.

Pre-Start Inspection: The Most Ignored but Most Effective Prevention

Before energizing a servo motor, several checks are necessary. Many field failures actually originate before the motor even runs.

Engineers should verify:

Insulation resistance (low-voltage motors should exceed 0.5 MΩ)

Correct power supply voltage and wiring phase sequence

Proper grounding

Condition of fuses and protection devices

Alignment and integrity of the mechanical transmission

Clean and safe installation environment (no flammable materials nearby)

Data collected from maintenance records shows a striking pattern:

approximately 30% of servo drive alarms occur within the first 72 hours after installation, and most are traced to wiring errors, poor grounding, or incorrect voltage levels. In other words, these are not component failures — they are commissioning mistakes.

Bearing Overheating: The Most Common Mechanical Failure

Bearing overheating is one of the earliest warning signs in servo motors.

Internal mechanical causes

Excessively tight fit between bearing rings

Shaft and end-cover misalignment

Incorrect bearing type selection

Contaminated or insufficient lubrication

Shaft current passing through the bearing

External usage causes

Poor installation alignment between motor and load shaft

Over-tensioned belts or pulleys

Expired or hardened grease

Temperature monitoring data shows that once bearing temperature exceeds normal operating range by 15–20°C, bearing life may drop by nearly 50%. In high-speed CNC spindles, this often leads to vibration alarms and eventually over-current trips in the servo drive.

Three-Phase Current Imbalance

A healthy servo motor should draw balanced current across all three phases. If imbalance appears, the drive typically reports overload or over-current alarms.

Typical causes include:

Unbalanced supply voltage

Poor soldering or loose internal connections

Winding short circuits (turn-to-turn or phase-to-ground)

Incorrect wiring

Even a 2–3% voltage imbalance can create over 15% current imbalance, dramatically increasing heating in one winding phase and accelerating insulation failure.

How Servo Motors Actually Control Speed

A servo system is a classic closed-loop feedback control system.

The motor drives a gear set or mechanical load. A position sensor — typically an encoder — converts shaft rotation into an electrical feedback signal. The drive continuously compares:

Command pulse signal (target position/speed)

vs.

Feedback signal (actual position/speed)

If deviation exists, the controller generates correction pulses, causing forward or reverse rotation until the error approaches zero. This is why servo motors achieve precise positioning and constant speed regulation — and also why sensor problems immediately cause system alarms.

Encoder and Feedback Faults

One of the most frequent electronic problems is unstable feedback.

Typical symptoms:

Axis jitter

Sudden speed fluctuation

Position loss alarms

Unstable tachometer signal

Root causes include:

Cracked encoder disk

Loose terminals

Electrical interference

Poor shielding

Field analysis suggests encoder issues account for nearly 25% of servo system troubleshooting cases in CNC machines.

Encoder phase alignment is also critical. When the electrical angle of the encoder does not match the rotor magnetic pole position, the motor may vibrate, overheat, or fail to start. Alignment is usually performed using low-current DC injection and signal observation with an oscilloscope, or automatically stored inside encoder EEPROM in absolute encoders.

Spark Between Brush and Commutator (for brushed servo motors)

Although many modern servos are brushless, brushed servo motors still exist in legacy equipment. Observing spark conditions helps diagnose problems:

Tiny occasional sparks: normal

No sparks: normal

Multiple small sparks: commutator cleaning needed

Large sparks: brush replacement or commutator machining required

A severely uneven commutator surface must be precision-machined; minor damage can be repaired by progressive polishing with fine abrasive paper.

Motion Instability: Hunting, Creeping, and Vibration

Hunting (oscillation during movement)

Occurs when:

Feedback signal unstable

Backlash in mechanical transmission

Excessive servo gain

Creeping (jerky motion at low speed)

Usually caused by:

Poor lubrication

Heavy load

Loose coupling between motor and ball screw

High-speed vibration

Often leads to over-current alarms.

In most cases, the real problem is not mechanical — it is improper speed-loop parameter tuning.

Studies in machine-tool commissioning show that incorrect servo parameter tuning is responsible for roughly 20% of performance complaints despite perfectly healthy hardware.

Torque Drop and Overheating

When a servo motor accelerates to high speed, available torque decreases. However, a sudden torque drop indicates a problem:

Cooling failure

Excessive mechanical load

Overheated windings

Thermal imaging inspections in production lines reveal that motors operating continuously above rated load can experience insulation degradation within months instead of years. Proper load calculation before installation is therefore essential.

Position Error Alarms

Servo drives continuously monitor positional deviation. If the axis exceeds the allowable error band, a position deviation alarm occurs.

Common reasons:

Gain parameters set incorrectly

Contaminated position sensor

Accumulated mechanical transmission error

Tolerance set too tight

In high-precision machining centers, this alarm frequently appears during rapid acceleration or deceleration.

Servo Motor Does Not Rotate

When a servo motor refuses to start, the issue is often control-side rather than mechanical.

Technicians should check:

Pulse and direction signal from CNC controller;

Enable signal (typically +24 V);

Brake release condition;

Drive faults;

Coupling failure between motor and ball screw;

Statistics from field service teams indicate that control signal problems occur more often than actual motor damage in “no-rotation” cases.

Servo systems are precision devices integrating electronics, mechanics, and control algorithms. Their failures rarely occur randomly. Instead, they follow identifiable patterns: installation errors, feedback problems, mechanical misalignment, and improper parameter tuning.

The key lesson for modern factories is clear:

preventive inspection and correct commissioning reduce failures far more effectively than replacing hardware.

As manufacturing moves toward high-speed, high-accuracy production, maintenance is no longer reactive repair — it is data-driven reliability engineering. Understanding the common failure mechanisms of servo drives is therefore not just a technical requirement, but a fundamental element of smart manufacturing.

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