Guide · 5 min read · Updated October 2026

How to test a servo motor without its drive

You can find most servo motor faults without the original drive. Insulation, winding balance, brake and back-EMF can be checked with standard instruments; feedback and a run test need a universal servo test platform.

By Suresh Sankaran, founder of Star Automations, servo motor repair since 1998.

01Basic tests

Tests you can do on site

Test	How	Healthy result
Insulation	500 V or 1,000 V insulation tester, each phase to frame	High megohms, steady
Winding resistance	Low-ohm meter, U-V, V-W, W-U	All three equal within a few percent
Brake	Apply rated voltage, usually 24 V DC	Shaft frees with a click; holds when removed
Back-EMF	Turn the shaft, measure phase-to-phase AC	Equal on all three pairs, rises with speed
Bearings	Turn by hand, listen and feel	Smooth, no rough spots or play
Encoder supply	Measure at the encoder connector	Rated voltage, no short

02Needs a platform

Tests that need a test platform

Encoder counts, stuck bits, status and memory
Commutation offset at every pole pair
Resolver signal quality
Run test with the repaired feedback at speed and under load

03Help

When to send it to us

If the basic tests pass but the axis still alarms, the feedback or alignment is the likely cause. Send the nameplate or WhatsApp +91 97866 22233. Related: servo motor load test, motor electrical testing.

Free downloads for your maintenance file

Printable one-page sheets from our bench. More on the resources page.

Servo encoder alarm quick sheet

Common encoder and feedback alarms on Fanuc, Yaskawa, Mitsubishi and Siemens drives, what each usually points to, and what to check on the cable and connector first.

PDF · 1 page · 4 KB · free, no sign-up

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Servo motor encoder quick reference

Encoder families, interfaces, counts per revolution and the motors they sit on: Fanuc pulsecoders (alpha A64, A1000), Mitsubishi OBA/OSA, Yaskawa UTOPH, Tamagawa, Heidenhain ECN/EQN/ERN and SICK SKS/SRS/SRM.

PDF · 2 pages · 8 KB · free, no sign-up

Download PDF

04Enquiry

Send the nameplate or encoder label

A clear photo of the motor nameplate and, if the cover is off, the encoder label is enough to start. We reply with whether it can be repaired and a quote.