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How to Troubleshoot a Servo Drive Failure in Under 30 Minutes

by Bryan Hellman



Servo drives rarely “die” without causing chaos. They trip alarms, shut down unexpectedly, or fault only when the machine accelerates or decelerates — which makes the symptoms look like software problems, mechanical problems, and electrical issues all at once.

Because a servo drive sits between the machine, the motor, the feedback system, and the power network, its errors often point to something outside the drive, even when the display makes it look guilty.

The fastest path to recovery isn’t to replace parts blindly — it’s to eliminate the most common external causes first. With a structured approach, most failures can be diagnosed in under 30 minutes without pulling the drive or calling a vendor.

The Goal: Find the Root Cause Before Replacing Anything

Many plants assume a faulty servo drive is “burned out” internally, but statistically, that’s not true. More than half of servo drive failures are caused by:

  • Bad grounding or noisy power lines
  • Incorrect servo parameters after maintenance
  • Moisture or contamination in connectors and cable shields
  • Faulty feedback encoder wiring
  • Repeated stress from mechanical issues (binding, overload, inertia mismatch)

When these issues aren’t caught, replacing the drive just masks the problem temporarily. A new unit experiences the same stress, fails again, and costs the plant twice — once for the part, and once again for downtime.

Finding the root cause ensures the solution is permanent, not just expensive.

Step-by-Step Troubleshooting (30 Minutes or Less)

1. Read the Fault and Exact Conditions (3–5 minutes)

The first rule: don’t clear the alarm immediately. Many technicians lose information that could instantly shorten the diagnosis. Write down:

  • Fault code and subcode if available
  • When it happened (startup, decel, full load, homing)
  • Temperature inside the panel
  • Recent changes (new motor, rewiring, washdown, cleaning, retuning)

The same code can mean different things depending on the circumstances. For example, an “overcurrent fault” during acceleration may indicate a motor insulation issue, while an identical fault during homing can point to incorrect parameters or encoder feedback problems.

Context narrows the fault. The code alone does not.

2. Check Power Quality and Grounding (5 minutes)

Servo electronics are extremely sensitive to unstable power and electrical noise. Even a minor imbalance or loose bonding strap can send false signals into the drive, triggering cascading alarms. In many facilities, welding equipment, VFDs, or poorly grounded cabinets introduce noise that looks like internal drive failure.

Inspect for:

  • Loose or corroded grounding straps
  • Shared grounding with high-noise equipment
  • Unbalanced voltage between lines
  • Weakened power supplies feeding logic sections
  • Rusted terminals from humidity or washdown areas

Noise issues often produce random, non-repeatable faults — the kind that mysteriously disappear and return, misleading technicians into thinking the drive is “intermittently failing.”

3. Inspect Motor, Encoder, and Feedback Cables (5–7 minutes)

Servo systems depend on perfect communication with encoder feedback. If the encoder signal drops for even a millisecond, the drive has no accurate position or speed reference and is forced to shut down to protect itself. This produces errors that mimic drive failure, motor failure, or tuning errors — when the cause is simply a weakened cable shield or bent connector pin.

Look for:

  • Shield continuity breaks (especially near cable chains or flexing points)
  • Connectors with oil residue or oxidation
  • Bent pins on encoder plugs
  • Jacket cuts exposing copper braid
  • Moisture inside connectors from washdowns or humidity
  • Worn insulation where the cable rubs against metal edges

A low-cost connector or cable sleeve can be responsible for a shutdown that costs thousands.

4. Test the Motor Without Load (5–7 minutes)

Running a servo “unloaded” is one of the most powerful diagnostic steps. If a fault disappears with no mechanical resistance, the drive is responding correctly — it’s the mechanical system that’s imposing abnormal torque or inertia.

This test helps identify:

  • Binding linear actuators
  • Over-tightened gearboxes
  • Dry bearings or worn couplings
  • Incorrect inertia matching after retrofits
  • Brake issues applying too much drag

If the motor still faults without load, attention shifts immediately to motor windings, feedback issues, or drive output stage problems. One test instantly splits the problem into mechanical versus electrical.

5. Check Parameters and Restore Backups (3–5 minutes)

Servo drives rely heavily on configuration accuracy. Even small errors — a mismatched motor model, wrong encoder resolution, or incorrect brake settings — change how the drive interprets the machine, causing heat, vibration, noise, and false fault codes.

Confirm:

  • Motor model parameter matches the installed motor
  • Encoder type, resolution, and feedback settings
  • Servo tuning values haven’t been altered by auto-tune unexpectedly
  • Braking resistor settings match the actual hardware
  • Torque and acceleration limits are safe for the load

Parameter mistakes frequently occur after motor swaps, system expansions, or rushed repairs when backups aren’t restored. The result is a perfectly healthy drive that fails anyway.

Common Problems and What They Usually Mean

Below are common symptoms and the issues they most often point to. Each problem should first be checked in the surrounding system before assuming the drive itself has failed.

Failure Symptom Most Likely Cause
Random encoder faults Cable shield failure, pin damage, moisture contamination
Overvoltage on decel Bad or missing braking resistor, high inertia loads
Overcurrent at startup Motor short, drive output failure, incorrect parameters
Over-temp during normal use Fan failure, poor cabinet cooling, contamination
Fault only under heavy load Binding mechanics, motor mismatch, incorrect tuning
Failure right after washdown Moisture wicking into cable or connector

Each issue points externally before it points to the drive itself.

When to Repair vs. Replace (Decision Guide)

When to Repair

Repair often makes sense when the drive still has life left in it and the failure is localized or triggered by external conditions. Consider repair if:

  • The drive powers on but faults during operation
  • The issue originates from cables, encoder, tuning settings, or power quality
  • The output stage damage hasn’t cascaded into logic boards
  • The model is commonly supported (for example Yaskawa, Mitsubishi, FANUC servo drives)

Repairs can include component-level board work, new IGBTs, power section rebuilds, fan replacements, capacitor replacements, or contamination cleaning. A reliable repair with burn-in testing often outlasts OEM units and costs 50–70% less than a new unit.

When to Replace

Replacement is a better option when repair would be slow, risky, or more expensive in terms of downtime. Consider replacement if:

  • The drive is fully discontinued with limited parts support
  • Power section failure has destroyed microcontroller or logic sections
  • The plant cannot wait for a repair turnaround
  • The system is being upgraded and needs future migration compatibility

Replacement is a strategic decision — not just a reaction. When the downtime cost per hour exceeds the price of a unit, replacement becomes the most economical choice.

Example Servo Drives Available from Industrial Automation Co.

If you’ve identified a failing servo drive and need a fast, tested replacement, here are a few popular units we stock regularly from Mitsubishi, Yaskawa, and FANUC:

All of these drives are backed by Industrial Automation Co.’s 2-year warranty and tested to perform in demanding production environments.

Need Support, a Replacement, or a Fast Repair?

Industrial Automation Co. stocks thousands of tested servo drives and motors from brands like Mitsubishi, Yaskawa, FANUC, and more — all backed by a 2-year warranty. If you’re unsure whether your unit needs a repair or a replacement, we’ll help you make the smartest decision for your plant, not the most expensive one.

Get expert help from Industrial Automation Co.

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