Encoder alignment explained: why a new or repaired motor still faults

A permanent-magnet servo motor needs to know exactly where its rotor magnets are, so the drive can switch current into the right windings at the right moment. This is called commutation.

The encoder on the back of the motor reports the shaft position. Alignment (also called the encoder offset) is the relationship between the encoder's zero and the motor's magnetic position. If that relationship is wrong, the drive pushes current at the wrong angle.

• Overcurrent or overload faults soon after enabling the axis
• Low torque, or the motor runs much hotter than normal
• The motor jerks, vibrates or runs better in one direction than the other
• High current while the motor is standing still
• The axis runs away or will not hold position

Alignment is usually set when the motor is built. It can be lost whenever the encoder and shaft are separated:

• The encoder is replaced or removed
• The motor is rebuilt: bearings, shaft, seals or rewinding
• The encoder coupling slips after an impact or a crash on the machine

That is why a motor that has had "just new bearings" somewhere else can come back worse than it left.

Absolute encoders know the shaft position at power-up. Incremental encoders count pulses and rely on extra commutation signals or a reference to find the magnetic position. Both need correct alignment. Some drives can identify the offset themselves; many motors have to be aligned on a test setup before they go back in the machine. See the glossary for both types.

A misaligned motor can move unexpectedly when enabled. Never test an axis with people near the machine, and follow your lockout and the manufacturer's instructions. If in doubt, send the motor to a repair centre that can align and test it under drive control.

Every servo motor we repair has its encoder alignment checked, and the motor is tested under drive control on our own jigs before it ships with a test report and 12-month warranty. Encoder repair & alignment.