Is it possible to drive AC servo motor with AC inverter? 1

[Hakankocak]

Hi,
As you probably know, there are Permanent magnet AC synchronous motors ( not servo motors, no feedback) for synchronous applications..

Is rotor construction of this PM motors same as AC servo motors?

Is it possible to drive a AC servo motor with a standart inverter like a PM motor? ( Without feedback ) It is necessary for speed synchronization, not for position control..

 

[Skogsgurra]

The question is very general in nature. There are servo motors and there are PM motors. There are also different breeds of inverters. Some work together very well and some do not.

So, the answer would be Yes and No.

The Yes part of the answer comprises high-speed PM motors for machining, compressors and centrifuges. They all work with PM motors and inverters and very often without any encoders or Hall switches for speed feedback or commutation.

There are also drive systems with PM motors. One system, that I worked with recently, has a parameter that can be set to chose between asynchronous motor and synchronous PM motor.

The No part contains combinations of servo motors and some inverters that cannot handle the PM motor. Mostly because you get oscillations and sometimes because the VFD doesn't understand the "no magnetizing current" characteristic of the PM motor.

You will have to ask the supplier. Or experiment.

Gunnar Englund

http://www.gke.org

 

[jraef]

I would add that if an inverter mfr does not specifically boast about being able to do PM synch motors, the chances are they can't. The ones that can are fairly proud of that fact and tend to make sure those that need it know about it.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[cswilson]

The key issue with driving a synchronous motor from an inverter without some kind of feedback is establishing the synchronization. One way is to incorporate some sort of asynchronous torque generation mechanism into the motor rotor (PM servo motors do not have this). For example, the rotors of most synchronous reluctance motors can operate as induction motor rotors until synchronous speed is established, at which point the reluctance torque effect takes over.

The other way is to give the inverter enough smarts (and internal feedback) to let it vary the electrical frequency gradually enough to let the rotor keep up. Ultimately, this is a "sensorless vector" controller, with roughly the same level of smarts as one for an induction motor. As jraef says, if the drive has this capability, the vendor will brag about it.

But without one of these two approaches, it won't work. You cannot just apply "X" Hertz all of a sudden to a synchronous motor and expect it to lock in to this frequency. (The exception is the stepper-like "slow-synchronous" motor that runs mechanically so slowly that it can accelerate to synchronization without a special mechanism.)

Curt Wilson
Delta Tau Data Systems