Servo Motor Rewinding
One common reason for servo motor failure is a bad winding. Windings are the copper coils inside of each motor, be it an AC servo motor or DC Brush type motors. Each motor has a unique set of windings with different wire, a different number of turns, and a different coil shape. If a motor becomes contaminated with coolant or water, the soft copper windings can be among the first things to loose their integrity. This complicates matters, because when a current-carrying conductor breaks down it will either trip the over-current protection in the servo system, or it will explode!
Servo motor windings do not last forever. Even normal operation incurs a modest amount of vibration due to the frequency of the voltage applied to a servo motor. Over time, uncontaminated windings will eventually fail because this small amount of vibration will slowly degrade the insulation and cause a short.
At Servotech we have the capacity to rewind servo motors and brake coils for a servo motor that has failed windings – even for motors that are not in production anymore. Rewinding a servo motor is a time intensive process. Our servo motor rewinding services utilize the best and most professional techniques and processes to ensure your new windings will be protected and will provide long life spans.
More on each step of our servo motor rewinding process:
1. Servo Motor Burn Out & Data Taking
The first step in the rewinding of servo motor is to put the motor in a burnout oven for several hours to melt the old varnish on the windings and make them more pliable. This can be especial difficult because some manufacturers, like Baldor and Yaskawa, as these servos fill their stators with potting material. While the servo is still hot the winder will “take data” – or figure out the winding scheme so that the proper coils and connections may be recreated with new copper.
A typical Yaskawa AC Servo stator after the burnout process. The end turns have been chiseled off showing a cross-sectional view of the coils going into each slot.
2. Proper Cleaning & Prep for Servo Motor Rewinding
Next, the stator is stripped of all its copper and the iron laminations are cleaned and sprayed with insulating paint. Phase paper is cut and installed to line the stator iron and provide protection against ground faults. Top sticks are then cut to secure and protect the motor’s coils once they are installed.
This vacant Fanuc iron will follow the meticulous data taken during the burnout to eventually become a working stator.
3. Servo Motor Rewinding Coil Making
The appropriate coil head is selected to create the right curvature, so that the coils will fit inside the slots of the stator and the end turns will fit inside the servo motor end-bell. Wire size, turns and the number of wires in hand effects the properties of the magnetism created when the windings are under load.
The winding machine with two coils made and clipped. An assortment of different “Winding Heads” mount to the machine for the various sizes and styles of coil.
Once all of the coils have been formed they are placed on racks to keep their shape and integrity while the rest of the coils are being installed.
4. Servo Motor Coil Installation
The servo motor’s new coils are carefully placed inside the slots according to the data obtained during the burn out phase. All of the protective materials, including phase insulation and fiberglass sheathing, are installed concurrently with the coils themselves.
This small Fanuc stator is mid way through having the coils installed back into the iron. Well made coils are installed with ease, and protected by the slot liners, phase paper, phase insulation, top sticks and fiberglass sheathing.
5. Coil Connections for Servo Motor Operating Direction
Once the coils are installed they must be internally connected and have lead wires installed. It is vital that these connections be made correctly and the correct lead assignment remains intact. In a 3 phase motor a mistake means the motor will run in the wrong direction. In a servo winding, it may make it impossible to align the encoder!
After the coils have been installed the internal connections are made with a technique known as braising. Once all of these connections are protected and tied down, the motor itself will have just three leads, one for each phase.
6. Servo Motor Coil Tying
After the coils are installed their ends are tied with a special kind of string and then manipulated to make sure they will not interfere with any of the mechanical portions of the motor. Proper tying ensures that the windings are grouped together neatly. Stray wires or wires that do not follow the curvature of the rest of the winding is the sign of poor workmanship.
The same Yaskawa stator from the top photo, fully rewound, about to be submerged in our custom V.P.I. tank. Once the new windings are saturated with varnish, they are baked and cured.
7. Servo Motor Rewinding Electrical Tests
After the stator has been rewound it goes through a battery of tests, including, a resistance check, surge test, hi-pot, and we even use a small compass to evaluate the poles. A servo winding is never fully tested until it has been reassembled with the rest of the motor so that a Back EMF (Electromotive Force) can be obtained. Back EMF is a voltage that is generated by servo motors at certain speeds and must meet factory specification in order for motors to operate at expected torque.
When the entire motor rewinding process is complete, and all tests confirm proper rewind, the motor can go back to one of the servo departments to complete the refurbishment.
More on our complete servo motor repair process.