AC servo motor: working, problems and maintenance (India)

An AC servo motor is a motor that is run together with a servo drive and a feedback device (encoder or resolver) in a closed loop. The drive sends current to the motor, the encoder reports the shaft position and speed, and the drive corrects the current many times a second. This lets the motor follow a commanded position, speed or torque very accurately.

An AC servo motor (usually a permanent-magnet synchronous or brushless motor) has no brushes, needs little maintenance and gives higher speed and power for its size. A brushed DC servo motor uses carbon brushes that wear and need replacement. Most industrial machines today use AC servo motors.

Both exist. Industrial servo motors on CNC, packaging, plastics and robotics machines are mostly AC (brushless) types. Small brushed DC servos are found in older or low-power equipment. Check the nameplate and the drive type.

It depends on the model. Industrial AC servo motors are commonly supplied by a drive on a 200 V class or 400 V class supply, and small ones may run at low DC voltage. The nameplate and the drive manual give the rated voltage for your motor. Never connect a servo motor directly to the mains.

Torque equals power divided by speed. As a rough guide, a 1.5 kW motor rated at 3000 rpm gives about 4.8 N·m, and at 2000 rpm about 7.2 N·m. The exact rated and peak torque are on the motor nameplate or datasheet.

With the drive off and the power locked out, check the nameplate, cable and connector, turn the shaft by hand (a brake motor needs the brake released) and look for rough or tight spots, oil or dust, and burnt smell. Then read the alarm on the drive. A full check of the winding, encoder and bearings needs test equipment, so send the nameplate to a repairer if the motor is suspect.

Common signs are an alarm on the drive, the motor not turning or jerking, high temperature, grinding or buzzing noise, loss of position, weak torque and burnt smell. Some of these point to the drive, cable or settings instead of the motor, so the alarm code and a proper test are needed to be sure.

Typical causes are a drive alarm or disabled drive, a wrong or loose cable, an encoder or feedback fault, a brake that does not release, a mismatch between motor and drive settings, or a seized bearing. Read the alarm code first and check the cable and brake before changing parts.

Overload or duty that is too high, poor cooling or dust, a worn bearing, a damaged winding, a wrongly aligned encoder that makes the motor draw high current, or drive settings that do not suit the motor. Persistent heat damages the winding and magnets, so find the cause early.

A worn bearing gives a grinding or rumbling sound. A humming or buzzing sound with vibration often points to the drive tuning, a loose coupling, or the encoder and rotor alignment. Stop the machine if the noise is loud or the motor is hot.

Position errors, encoder or feedback alarms, jerky or runaway motion, a motor that cannot start, and alarms that appear only when the motor heats up or the cable moves. Encoder faults are among the most common servo motor faults.

Worn bearings, encoder faults, winding insulation breakdown from heat or moisture, oil or coolant entering the motor, brake failure, cable and connector damage, and overload. Regular inspection of cables, cooling and noise catches many of these early.

Yes, in most cases. Bearings, winding, brake, encoder or resolver, oil seals and connectors can be repaired or replaced, and the encoder is aligned and the motor run-tested. Some motors with severe magnet or shaft damage may not be economical to repair. Send the nameplate and we confirm repairability before you courier.

There is no single interval, as it depends on duty, dust, heat and the manufacturer's manual. A practical approach is a quick visual and sound check at regular intervals, a more careful inspection of cables, connectors and cooling every few months, and a bearing and encoder check at overhaul. See our servo motor maintenance checklist.