Difference Between VFD, Servo Drive Repair

Servo drive repair differs from VFD repairs in several ways, including being a bit more difficult to troubleshoot. Also, servo drive repair usually requires that the servo be connected back to its intended system to ensure it’s repaired correctly (something that’s extremely difficult to replicate in repair shop conditions).

A VFD (variable frequency drive) is generally used to control a squirrel cage type motor, where both stator and rotor are of a wound type to create the magnetic flux.

Servo drives are used to control permanent magnet motors. They are called permanent magnet motors because they use rare earth magnets in the rotor to create a much higher magnetic flux for their given size. This enables the motor to create more torque in a much smaller motor size. This means the motor has a lower inertia to accelerate and decelerate much more dynamically than that of the asynchronous squirrel cage type motor.

On the controller side, the servo controller can calculate a complex path and maintain the position along that path with varying loads and speeds. Many servo controllers are multi-axis or can be daisy chained to make multi-axis moves to follow complex paths.

Because of these differences, a Servo drive repair is much different than troubleshooting a standard VFD.  When isolating the issue with a servo drive, you must consider the entire system, drive, cabling and motor.  It’s difficult to troubleshoot a servo without all three of the main components present.  And since servo drives are designed to run a specific magnetic flux motor, it is close to impossible to test them outside their designed system.

Check These Prior to Servo Drive Repair

We’ve compiled a list of the top simple issues that should be checked before sending in your unit for servo motor repair. Please note this is not meant to be an insult: I very often spend time trouble shooting over the phone to eliminate simple errors or problems. Double check everything before electing to send in a motor for repair, otherwise time has been lost in both the shipping and evaluation.

1. Occam’s Razor: The simplest explanation is most often correct. Make sure everything is plugged in. Try turning your drive and controller off, and reboot.
2. Check All Servo Motor Cables. Look for items such as Broken Wiring, Loose Connections, Dirty and Corroded Connectors – bad connections can interfere with the power and signals that are vital to proper servo operation.
3. Check Overall Cable Conditions
4. Check Grounds and Shields. Just because a cable is in good shape doesn’t mean it is properly grounded. Keep in mind that grounds and shields are important for protecting signal wires from harmful noise that can disrupt feedback communication. Motor power grounds are important because they facilitate the tripping of over current protection devices. If a motor has an inconsistent ground that is not continuous with a drive ground the power cable can throw an unmanageable amount of noise onto a properly shielded feedback cable.

If all this checks out, then you probably have a servo drive issue that requires shop servo motor repair.  Use a repair shop that can help you isolate the problem, but as mentioned above understand it could require some additional testing at your sight AFTER it’s worked on at the shop. If your repair facility also has industrial field service technicians, so much the better. They can can help you with onsite troubleshooting of servo systems.

Marc Phelps is manager of the Innovative-IDM repair facility in Houston. You can contact him at [email protected]