How to read a servo motor nameplate: torque, speed, feedback, brake and fit

A servo motor is one half of a matched system. The motor – usually a permanent-magnet synchronous motor with a feedback device on the back – is paired with a servo drive (or amplifier) that closes the loop on torque, speed and position. That is why a servo replacement is less forgiving than an induction-motor swap: the drive has to know the motor's electrical data, read its feedback and supply the right current. Whether the motor is from Allen-Bradley (paired with Kinetix drives), Fanuc, Yaskawa, Siemens or Kollmorgen, the nameplate carries the same kinds of information, even if the symbols differ.

What to copy from the nameplate

Photograph the whole nameplate before anything else; on a motor buried in a machine it may be the only chance. Then record the fields below. Symbols vary by maker – European labels often use M for torque (M0 for stall torque) and n for speed – so read them together with the manufacturer's data sheet.

Torque, speed and current

Stall torque, rated torque and peak torque

Continuous stall torque is what the motor can produce indefinitely at or near zero speed. Rated torque is the continuous figure at rated speed and is usually a little lower, because losses rise with speed. Peak torque is available only for short bursts. When comparing a replacement, match or exceed the continuous figures first; a motor with the same peak torque but lower continuous torque will overheat on a busy axis.

Rated and maximum speed

Rated speed is where the rated torque and power are specified. Some motors are offered in several windings with the same frame and torque but different speeds; the winding changes the current, Kt and Ke, so two motors that look identical can behave differently on the same drive. Compare the full model number, not just the frame.

Voltage class and insulation

Servo motors are wound for a drive voltage class, commonly 200–240 V or 380–480 V. The class tells you which drives the motor belongs on: a low-voltage winding on a high-voltage drive is overstressed, and a high-voltage winding on a low-voltage drive will not reach its speed. The insulation class (for example F or H) and the thermal sensor (a PTC, KTY or PT1000 type, for instance) tell you how the winding is protected; the drive must be able to read the sensor that is fitted.

Feedback: encoder or resolver

The feedback device is often the deciding factor in a replacement. The main types are:

• Incremental encoders, which count pulses from a reference point and need homing after power-up;
• Absolute encoders, single-turn or multi-turn, which report position directly – some multi-turn systems keep their count with a battery;
• Resolvers, rugged analog devices common on older and heavy-duty motors.

Modern absolute encoders talk to the drive over a serial protocol. Some protocols are published and used by several makers (EnDat, HIPERFACE and BiSS are examples); others are specific to one manufacturer, such as Siemens DRIVE-CLiQ or the serial pulse coders on Fanuc motors. The drive must support both the protocol and the resolution, and the feedback cable must match. If the motor's number has an option code for feedback, a single character can separate an incremental motor from an absolute one.

Brake, flange, shaft and protection

Holding brake

A servo holding brake is usually spring-applied and released by power, often 24 V DC. It holds a stopped axis – typically a vertical one – when the drive is disabled or power is lost. It is not designed to stop a moving load in normal operation. Check the brake voltage, how it is switched (by the drive or by the machine's control), and whether the motor you are buying has a brake at all: brake and no-brake versions often differ by one character.

Frame, flange and shaft

The frame or flange size (often given as the square flange width in millimetres) sets the mounting. Check the pilot diameter, bolt circle, shaft diameter and length, and whether the shaft is keyed or plain. A gearbox or coupling built for one shaft will not accept another.

IP rating and connectors

The IP rating describes the housing: the first digit covers solids, the second water. Some motors reach their stated rating only with an optional shaft seal, so check the option code. Note the connector type and whether the connectors are fixed, angled or rotatable – a different orientation can make a cable impossible to route.

Matching a motor to a drive

1. Start with the same model number as the failed motor, option codes included. That is the only true drop-in.
2. If it differs, confirm that the drive maker lists the motor as supported with that drive and firmware.
3. Check current: the drive's continuous and peak current must cover the motor's.
4. Check feedback and cables: protocol, resolution and the correct power and feedback cables.
5. Load the motor data – many drives read it from the encoder; others need a motor file or selection in software – then tune the axis.
6. Re-establish the reference position on absolute systems, and test the brake before running a vertical axis.

Buying a servo motor as surplus

For discontinued motors, surplus is often the fastest source. Every item we stock is photographed, so you can read the nameplate before you buy: compare the full model number and option codes, look at the shaft and connectors, and ask whether the unit was removed from working equipment. For Fanuc part numbers such as A06B, see our guide to finding obsolete Fanuc parts; for induction motors on VFDs, see reading a motor nameplate and sizing a VFD. Browse servo motors, servo drives and encoders, or send us the model number for an official written quote in under 24 hours on business days.