About Servo Motors
What is a servo motor?
Servo
motors simply means that a motor has feedback to indicate velocity and /or
position. There are four types of feedback on servo motors . They
are encoders, resolvers, hall sensors, and tachometers. Encoders and resolvers
are generally used for precise position or velocity control as well as sending
signals for the commutation of the brushless servo motor. Hall sensor outputs
are generally used for commutation while tachometers are used for velocity
control. All but the tachometer must be precisely aligned to the rotor for
proper operation of brushless servo motors.
AC
brushless servo motors have permanent magnets attached to the rotor of the motor
and a wound stator similar to a regular three phase induction motor.
Useless information
about servo motors
Most
manufacturers of servo motors outsource the manufacture of the feedback used on
their servo motor. You never know whose servo motor may show up for repair
with an Tamagawa or Onisaki or maybe a Heidenhain, Stegman, Renco, or Encoder
products encoder. Yaskawa, Sanyo Denki, Fanuc, Mitsubishi, and Indramat
are manufacturers who do manufacture their own encoders for their servo motors
and all manufacture serial, absolute encoders. If brakes are used, most manufacturers
outsource their brakes to Stromag, Sterns, or any of many Japanese
manufacturers. Most connectors
for power and feedback are outsourced to Amphenal or another connector
manufacturers. Some manufacturers outsource the rotors
and the stators to manufacturers like Custom Servo, Leeson, Yaskawa, and
Kollmorgen.
Fanuc
servo motors have laminated stators. Over time these laminations will allow
liquid contaminates enter inside the servo motor. At Southeast Servo we take
extra steps to insure that our repaired servo motors don’t allow contaminates to
enter by using extra gasket sealing when attaching the end frames. We fill the
feedback and power connectors with silicone and we paint the servo motor with
epoxy based paint. We have documented that our protection methods work at
several different customers who encounter severe contamination of their servo
motors in their process.
When
power is applied to two of the three windings, for instance from “U” to “V”, the
shaft will snap to a position and will resist efforts to turn the shaft as long
as power is applied. By applying power to the motor leads “V” to “W”, the shaft
will advance (in its forward direction) to another position. By continuing
changing the leads to power “W” to “U”, the shaft will again advance. By
repeating this sequence the shaft will continue advancing. After the sequence
is repeated several times (depending on the number of poles of the motor), one
rotation of the shaft is made.
End
frames are made of aluminum and are subject to wear over time. About ¾ of the
motors we see for repair have worn end frame “fits” allowing the bearing to
wobble or even spin. Some of our competitors punch the end frame to swedge the
bearing into place. Others use stamped steel tolerance rings. At Southeast
Servo, we machine the end frames and press fit a steel sleeve to install new SKF
bearings on each repaired servo motor.
Most
servo motors can be repaired economically.