VFD running a AC servo?

[DennisCNC]

Will a 3PH VFD run a AC servo?
Will it run all of them or only some?

I'm thinking it will, but making sure.

[HuFlungDung]

Good question, and I don't really know the answer.

One difference that I can think of, is that a servo amplifier depends on the existence of a feedback positional error between its current position and the commanded position, in order for the amp to output a current to move the motor towards 'zero error'.

A VFD on the other hand, more or less runs a motor to an infinite position, since it never stops running (until the drive is commanded to stop), so I do not know if it is using an error loop to stimulate the output of a current.

Al or some of the other guys likely could tell you how an AC servo provides feedback to the amp via some kind of Hall effect transistors or something, and I am not sure a regular VFD can accept that kind of feedback signal.

[DennisCNC]

Forgot to mention: I just want to run the servo as a spindle motor. I think that servos are built better than most 3ph motors so they will last longer and go faster. I got a 400Hz VFD and wana see the 400hz being used

[ESjaavik]

AC Servo?
If it's a Permanent Magnet type it must be run synchronously. You could probably make it run using a VFD, but not using it's full torque. Reason is that the VFD does not know if the motor is in synch (it has no resolver or encoder input to verify this). So if you load the motor until it misses synchronism, it will not be able to recover and it will stop. If you know how a PM stepper works, the PM servo (BLDC) can be regarded as a stepper with very few steps/rev, and feedback to the drive.

[unterhaus]

ESjaavik, that's the way I like to think of brushless servos. I hooked a brushless servo up to an AMC control that will run without feedback and it spun it just fine. I didn't think it would do it.

There are VFD's that will run brushless servos, but there are a lot more that will not.

[DennisCNC]

I thought that a brushless AC servo is like a 3ph motor

[Al_The_Man]

A AC servo has a permanent magnet rotor, which in a servo should be kept in synchronism with the rotating field, hence the feedback sensors.
A 3ph induction motor induces the magnetic field in the rotor by means of its own magnetic field. The position of which changes as the rotor increases in speed, An ac servo can run in synchronism with the supplied AC, an ordinary squirel cage induction motor can never reach synchronism.
Al.