running a servomotor with VFD

[cokeguy]

We still use a lot of synchronous motors in our plant, driven by VFDs in V/Hz mode, with sizes ranging from 3 to 10 HP approximately. Sometimes a single VFD runs 2 or 3 motors, so the VFDs are usually oversized to allow starting individual motors at normal speed without overcurrent trips.

Since small synchronous motors are increasingly harder to get, we will eventually phase them out and install servomotors and servodrives for those applications, but in the meantime is it possible to connect a brushless AC servo to a VFD (open loop obviously)? I know that essentially a brushless AC servo is just like a PM synchronous motor, but I don't know if an equivalent size brushless servomotor will behave just like a synch motor in open loop. Will it even start up if not ramped up to speed, like synh motors do?

Thanks for your comments

 

[luktek]

You will need to go closed loop and have a resolver input back to the VFD to control the firing pulses of the drive.

We have done this quite a lot lately and it works very well.

 

[cswilson]

Motors marketed as "synchronous" motors intended for open-loop operation will have an asynchronous torque-generation mechanism that permits them to get up to speed and "lock in" at this speed, at which the synchronous torque-generation mechanism takes over.

Brushless servo motors, which are intended for closed-loop operation, do not have one of these asynchronous torque-generation mechanisms, so trying to run them open loop is problematic at best. You cannot simply apply your target electrical frequency and expect one to ramp up to speed.

Curt Wilson
Delta Tau Data Systems