LadderLogic said... 3-Phase Motor vs. Brushless Servo Motor

LadderLogic added this to the end of a thread where I thought it might get lost in the clutter.

I think it's a good question and deserves a place of its' own.

LadderLogic said...

"I am having a nagging feeling here... so I will ask an extremely stupid question (and I am profusely apologizing for doing so).

What is the difference between a synchronous permanent-magnet 3 phase AC motor and a brushless servo motor? You know, one of those with 3 phase AC windings and a permanent magnet rotor...

If someone fits the former with a position feedback device (such as an encoder) - would it make it the latter?.."

BTW, LL,
This is not a stupid question at all. It's a pretty good "informed enough to ask the right question" kind of question. (As opposed to those "I don't know enough to ask a reasonable question" kind of questions... as in, "What are PLC's and what are they good for?")

Your everyday, 3-Phase Motors suffer from cronic-slippage. That is NOT necessarily a bad thing. It produces TORQUE... that is good.

However, that motor, or rather the driven device attached to that motor, is not necessarily moving at the speed or in the position that you might expect.

The control of the speed is generally open-looped. That is, nominal is nominal. Some programmers make speed estimates using the nominal speed value... ignoringing that the speed starts at zero-rpm.

Position indications are generally made using actual input sensors telling the location of the driven device. Although, some programmers try to use the speed-time to calculate position.

Servo-Motors, with absolute Rev-Position Indication Feedback, are able to provide the controller with a real speed value and a real position value. Torque is developed by manipulating the timing of the constructed 3-Phase signal to the motor. This works in a way very similar to the method used to automatically advance the timing in an automobile engine.

While the Servo-Motor is turning, the controller, through the encoder built into the motor, is aware of the speed and position of the driven device. Of course, this requires that there be a known relationship between rotary and linear motion. As in, so many revolutions equals so many inches in linear travel.

So... the difference between the two...?

Their ability to control Position is unquestioned (8000 counts per Rev!). A 3-Phase Motor, on a drive, can have an encoder but it can't move to Count=XXXX.

Their ability, through their controller, to manipulate TORQUE on demand is better. 3-Phase can't manipulate TORQUE without additional slippage, but then, in 3-Phase, position control suffers.

They can follow a profile much more accurately and stop on a count (or two).

And now, the other shoe finally drops...
Servo-Motors, with their Rare-Earth Magnetics, are much more expensive. Although the advantage is GREAT,,, in some applications.

Terry, I think you missed something in LL's question (as you can see, it's my question too). The comparison of servo motors was not to a conventional induction motor but to the newer permanent magnet synchronous 3 phase AC motors now appearing for inverter applications.

There is no slip in these devices, being fully synchronous.

I believe the question is---what is the difference between a three phase AC permanent magnet synchronous motor and an AC servo motor?

They both appear to have three phase stators and permanent magnet rotors and both are synchronous in operation.

Terry, I very much appreciate you starting this thread.

Unless I am missing something important, the only difference between the two is the feedback device: "must-have" for a servo, optional for a sync motor.

"Of course, this is just my opinion. I could be wrong"(C) Dennis Miller

I have not used the three phase AC permanent magnet synchronous motor, but from the description, there is no real fundamental difference.

A servo has a feedback device, but so to some normal motors. I think that the main difference is the construction with regard to rotor inertia. Servos are designed with small diameter and longer body to keep inertia down. This allows for higher accelerations and better control. The disadvantage is that most motor-controller combinations like to match load inertia to the rotor inertia, leading to all kinds of tricks to lower the inertia of the load seen by the motor. (reducers, belts, etc.) At least in my field, where we move heavy things.

If the motors in question look like a standard induction motor, their rotor inertia will be high, but that doesn't matter for their application.

Sync motors

Reluctance Synchronous (Synduction) Motors are manufactured to perform better than permanent magnet synchronous motors when being driven by VFDs. The Seimens Motor Division is the owner of the Synduction trademark. Back when VFDs were first getting introduced in industry, most applications were regular 3phase induction motors but a few apps called for exact speed control with out a lot of expensive and troublesome feedback from tachs and the like. So, most users were off to the races with the good old standby, PM Sync motors. But, as luck would have it, depending on the app and drive, many of the old standbys just burned up in a matter of weeks or months. Come to find out, there were way to many harmonics, eddy currents and the like created by the VFD in the presence of the PM rotor. Many drive manufacturers just plain would not guarantee their drive in those apps. However, the main problem was the drive itself, as most in that time were SCR type and also instead of the PWM, they were 6 step approximation drives. Wow, can you imagine what all those square edges were doing to those motors? Luckily, Siemens and others developed the synduction type motor and along with the advent of PWM made a lot of the earlier problems just go away. I have installed over 100 synduction motors running in banks, or single and operated almost exclusively from AB 1336 drives and to my knowledge, they are still there doing their thing. So, in the for what it is worth department, there it is.