6 Permanent Magnet AC motors driven by one VFD 1

[ebarba]

Hi all,

because space constraints we are considering to drive six 4 kW, 400 V identical 10-pole, PM AC, sensorless motors with only one VFD. The applied torque on each of the six motors is NOT the same and varies with time.

I understand the logic that the VFD will fail to determine the rotor(s) position with the usual high frequency current injection, because the initial rotor position of the six motors won't be the same at start-up... but what if we use the following starting strategy:
[ol 1]
[li]remove all load from the six motors (this we can do very easily)[/li]
[li]start feeding a very low frequency (say 1 Hz) for a sufficiently long period (say 10 s). This should allow for the rotors to start moving and to the six of them to achieve the same phasing.[/li]
[li]then gently ramp the motors up to speed, to avoid inertia to overcome the ramping speed.[/li]
[li]then apply load.[/li]
[/ol]

Is all this just horribly wrong and plain madness?

Thanks!

 

[EdStainless]

It won't work.
People make multi drives system.
One drive is a full unit, the others are just motor power electronics and they are slaves.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[ebarba]

but in that case we still need to bring separate cables for each motor, which is what we need to avoid.

From school days, I remember that the issue with synchronous motors was the starting, once you got them up to speed, they behaved "normally". The other issue was not to overload them as slipping would occur with awful vibrations, often destructive.

Now, wouldn't the procedure I'm suggesting allow for an orderly ramp-up of the six motors with the same VFD?

 

[jraef]

No, because they are PM AC motors. The drive must closely control the output, even more so than on a squirrel cage motor and does do via a closed loop current monitoring system, akin to Encoderless Vector Control, but even more critical. The drive will be unable to do that on multiple motors. You cannot equivocate this type of motor to any other motor model you have seen or used.

So what drove you to use PMAC, then get yourself in a situation where you can’t run separate cables to each motor?


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[ebarba]

Well, PMAC was chosen for its higher energy density, we needed more power in less space. There is no particular need for speed and or position control.

The issue about the separate cables was something we mechanic guys totally overlook: we have a similar machine with 20 squirrel-cage motors driven by only one VFD and works wonderfully. Sometime during the design phase we lightheadedly asked "will these motors work with a normal VFD" and they replied that most new VFD's can control PMAC motors and we settled ourselves on the idea that it was just the same.

jraef, I understand what you say, the sensorless position control system won't be able to work on six separate motors. What I don't understand is what makes a VFD-driven synchronous motor so different from the same motor powered by mains, that requires such complicated control loops.

On the mains-powered machine, there is no position/current control and it just works until you overload it and it starts to slip and do horrible things... but while power is within operational limits, there is no difference with a squirrel cage, other that the speed... or am I wrong?

 

[jraef]

Yes, you are wrong. A PMAC motor cannot operate Across-the-Line (DOL), it NEEDS the ability of the VFD to ramp it up to speed, otherwise the motor just sits there and vibrates. That's why PMAC motors didn't exist until the VFD technology facilitated it.

Let me see if I can adequately describe this: please forgive me though, it's new(ish) to me as well.

Torque = the vector of stator MMF x rotor flux x sin(theta), where sin(theta) is the angular displacement between the two, so you achieve maximum torque when the displacement is 90 degrees, and zero torque at zero degrees. When connected DOL, the "grid" has no way to change the displacement angle relative to the rotor position, so over the course of a cycle the average displacement will be zero, so the torque will be zero. What the VFD then does is constantly monitor the rotor position and that displacement angle to achieve whatever the desired effect is that you are looking for (speed or torque or both). It has no encoder, so it monitors that rotor position my "watching" the effect the rotor flux has on distorting the stator current waveform. If you put multiple motors on that VFD, the mixed distortion signals become essentially noise and the VFD cannot do what it needs to do to make the motors function.

By the way, it's also untrue that "most new VFDs will control PMAC motors". It takes a high level of control sophistication (meaning mProcessor power) and current measurement accuracy to accomplish this. That's why even though VFDs have been around for 25+ years, PMAC motors are very very recent. VFD technology had to evolve enough to get there, and only the high-end / newest VFDs are actually capable of it.

Despite the power density and efficiency issues, if you need one cable to operate all of the motors, you will have to go back to squirrel cage motors. But that makes me question another aspect of what you are describing. How are those motors being protected if they all come from one cable? Each motor must have its own running overload protection because again, the VFD cannot discriminate one from another if it only sees the entire group as a unit. So if you were using small squirrel cage motors with integral protection, you will also find that feature not available in PMAC motors, at least not that I have seen, because the motor mfrs KNOW that you must have one VFD for one motor, so they don't bother offering integral OL protection.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[John2025]

Jraef, Great description. These kinds of posts are the reason I end up "wasting" so much time on this site.
John

 

[cswilson]

I'll put forward a more nuanced take on it. In theory, it could work...IF everything goes well. However, the failure modes look pretty catastrophic to me.
Fundamentally, you are driving 6 giant stepper motors in open-loop mode with identical command signals. (Most stepper motors are PM AC motors commanded open-loop, but are seldom over 200W.)

I think it can be made to work (with everything going well), PROVIDED you characterize your system very carefully and make sure you have adequate margins.
jraef is correct that you cannot do a DOL start with these synchronous motors, but you realize this, and are planning a slow acceleration of unloaded motors through a gradual ramping up of voltage and frequency from the VFD. This part should be manageable, but you must be much more careful of the ramp-up rate than with induction motors.

As jraef shows, a PM AC motor produces torque proportional to the sine of the electrical "lag" angle. In steady-state conditions, each motor will lag the no-load angle by the arcsine of the ratio of the load torque to the maximum possible generated torque (while matching the frequency, as these are synchronous motors). Of course, this only works for lag angles of 0 to 90 electrical degrees -- if the load torque ever exceeds the maximum, you will get a nasty, unrecoverable error. The stepper motor people call this "stalling", as the motor will likely quickly grind to a complete stop with lots of vibration.

By contrast, induction motors slow down (increase slip frequency) with increasing load. If the load torque ever exceeds the maximum ("breakdown") torque, you can get a pretty rapid deceleration, but it will be smoother than for a PM AC motor, and with the potential for recovery when the load torque reduces, so it can often ride out a momentary high load.

Obviously, you want to make sure the maximum torque each motor can produce is substantially larger than the maximum load torque you ever expect to see. In my experience, most stepper motor users employ a 2:1 margin -- that is, the maximum torque (at 90e degree lag) is twice that you ever expect to see in normal operation.
You would have to look very carefully at how you engage the loads, as it is possible that the transients could cause a loss of synchronicity.

I think what makes most posters here very nervous about your proposal is the more violent and unrecoverable nature of exceeding maximum torque capabilities with PM AC motors. Can you really be sure that this won't ever happen, even years down the line when something (e.g. more friction) has increased the loading? This kind of "stalling" on a 24 kW system is much more severe than with 100W steppers.
And with all motors chained off one VFD, think of what happens electrically when one motor stalls. The VFD is providing full voltage, but that motor has no back EMF to speak of, so its armature is almost a short circuit. For the motor itself, this may well exceed the instantaneous current rating of the motor, quickly causing demagnetization of the PMs and permanently ruining the motor. You would have to analyze very carefully what happens to the drives and the other motors in the case of a stall.

Also, you must consider the effects on your mechanical system of a stall. Again, I emphasize that this is more severe than having exceeding the breakdown torque of an induction motor. I don't know if countermeasures, such as quickly detecting this (if possible) and disengaging the load, would be sufficient.

 

[ebarba]

Thanks to all you guys!

So, the conclusion is that the strategy is possible, albeit complicated. An effective and FAST way of monitoring the speed of each of the motors has to be implemented and the control system has to disengage power and load quickly if any of the six motors loses sync.

We will now have to balance the above against having to redesign a huge part of the machine to allow for a more normal 6-VFD architecture.

Thanks again!

 

[waross]

It would be so much simpler to use half size induction motors at double the frequency and double the voltage.
A 2 kW, 200 Volt, 50 Hz motor will put out 4 kW at 400 Volts and 100 Hz.
That may give you the size saving that you were looking for originally.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ebarba]

we already considered that option, but we can't go at double frequency (the speed would be too high) and in any case, the increase in power would not be enough, unfortunately.

 

[jraef]

Cswilson,
It sounds as if you may have talked to one of my clients! They tried that with a bank of 4 high speed 5HP blowers. For years we did it with induction motors all running from one VFD at 400Hz, then they were sold (by the blower mfr) on the PMAC motors because they were physically smaller and more efficient for the same pressure and flow. But they didn't ask us up front about running all of them one one VFD, they just looked at the fact that the VFD we had switched to using was advertised as being PMAC capable, then did it. Thankfully it was a beta test unit to verify proof of concept.

During startup one of the blowers stalled about 3 seconds into the ramp and the VFD immediately tripped to protect itself. Nothing was actually damaged (except reputations), but they only called me in AFTER it failed to find out why. Your post has definitely given me more insight so thanks. But it also proves that theory is one thing, implementation is another.

They are still debating the PMACs though, because they can go with a slightly larger control panel to house the 4 small VFDs vs one larger one plus separate protection, but real estate in the tool working area is more valuable and they need to show better energy efficiency overall.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[sreid]

Only as a suggestion you could use 10 ea. Mitsubishi Serve Drives, one for each motor.

https://us.mitsubishielectric.com/f...elservo-j3/servo-amplifier/j3-servo-amplifier

One can be the master to control velocity and the other 9 would operate in Torque mode slaved to the master amplifier.

The drives at 5 HP are not that large.