Basics of a Servo Motor

Servo motors come in many shapes, sizes and types, and are used for applications that require precision speed and positioning control. They provide a fast response to speed change commands while maintaining speed and torque control stability.

For this discussion, we will consider the AC servo motor, the basic parts of which are few. It starts with a rotor that turns surrounded by a stator that induces the magnetic field to turn the rotor. These are held in place by supports and rotary bearings. The rotor is embedded with permanent magnets that oppose the magnetic field created in the stator. By switching on and off different positions in the stator, the opposing magnetics create rotation.

There is also some sort of commutation device such as a Hall Effect sensor, a resolver or encoder to provide precise speed and positioning feedback to the drive. All of these parts are enclosed in the motor housing which is often square, as well as longer and thinner than a typical induction motor of the same rating.

All brands of servo motors have their differences, but there are some basic steps and tests that are performed on all motors. Here at Precision Electronic Services, our reputation for quality servo motor repair is the result of our years of experience and highly trained servo motor technicians. The following is a look into our repair process and how it relates to the basics of a servo motor.

The first test is an inspection to see if there is any physical damage to the mechanical components, looking for anything that is broken, missing, bent or otherwise out of tolerance. This includes items such as the motor shaft, housing, covers, connectors, etc. A check of the motor end bells is also performed. They are cleaned, and repaired if necessary.

Bearings are the hardest working parts of the servo motor and the constant motion and vibration causes great wear and tear to be inflicted on them. Bearings that are even slightly worn can cause all kinds of symptoms in a servo motor from feedback errors to high temperatures while running to speed fluctuations. Bearings that are more worn can cause the motor to lock up completely and not spin at all. For this reason, every servo motor repair includes bearing replacement at PES.

 

 

For the electrical components of the servo motor, the first sign of problems can be sensed by a strong odor coming from the motor, which may indicate burned windings or a brake issue. The motor windings are thoroughly tested to ensure no shorts or insulation breakdown exists between the motor phases or to ground, and these tests are performed by HIPOT insulation testing and surge testing the motor windings.

For feedback device test and alignment, PES uses the Mitchell Diagnostics System. This system allows us to test the operation of servo motors, check and align feedback devices. With this system we are able to view data such as lockup angles, max current, stall torque and more.

The back EMF or voltage test is a test performed to determine the strength of the motors magnets, which affects motor torque. Some servo motors also have brakes, so if applicable, the brake will be cleaned, tested and repaired / replaced as necessary.

When the motor has been repaired with new bearings and all other components repaired or replaced, the motor is fully tested and burned in, then painted, retested and packed for shipment back to the customer.