Synchronization of two brushless PMDC motors using one controller 2

[Intermesher]

Is it possible to drive two mechanically unconnected brushless PMDC motors from a single controller? The idea is for both motors to have their own Hall effect sensors. Their signals would be feed to a sort of comparator-timer where the mean-time of the two signals would be added to the time since the previous mean-time and then feed into the single controller.

This controller would simply output the 3-phase power, which would then go to both motors.

I am assuming/hoping that the force vectors within each motor will be capable of maintaining a synchronization of the two motors as the desired speed is changed.

Is this;

practical / viable / possible / unrealistic / stupid

?


Thanks
Dave J.

 

[Compositepro]

A permanent magnet DC motor fed by 3-phase power. Wouldn't this normally be called a stepper motor? If that's the case, then the answer to your question is yes, but. Otherwise the answer is probably not.

 

[cswilson]

This is done all the time, not just velocity synchronization, but position synchronization as well. It's at the heart of control of robots, machine tools, etc.

But it's very different from off-the-line control. The 3-phase power you supply the motors must come from a smart inverter stage, with quickly controllable magnitude, phase, and frequency, separately controlled for the two motors. Fundamentally, you're talking servo control here.

Curt Wilson
Delta Tau Data Systems

 

[Intermesher]

compositepro.
Thanks for the suggestion however I do not think that the stepper motor will meet the requirements.

Curt,
From what you are saying it appears that a single inverter/controller that is feed the averaged Hall position signal will not work, or at least not work with a sufficient authority to maintain a continuous synchronization.
Thanks.


Dave

 

[cswilson]

I think that even if you could somehow combine the Hall signals from the two motors into an "average signal" (and I don't think there is a simple way of doing that), you really couldn't count on any better control than running them open loop (which is as a glorified stepper motor).

If, say, one of the motors hits a momentary obstruction that causes it to fall behind, you need to tweak the control for that motor to optimize its response. Otherwise, you need to leave the margins typical of open loop control, significantly limiting performance.

Curt Wilson
Delta Tau Data Systems

 

[Intermesher]

Curt.

Thanks for your additional comments.

The two motors are to serve a pair of aerodynamic rotors, with, one motor per rotor. The speed of the rotors is fairly constant. The intent is to provide 'soft' mechanical restraints to assure that the 3-phase motors cannot get more than 360 / (3 * 1/2) = 60 electrical-degrees out of phase.

The intent is that the above mechanical restraint will stop any external perturbation from causing a rotor to jump an electrical cycle, and the electro-magnets will work toward keeping the rotors in absolute synchronization.

Crazy, or not quite crazy?

Dave

 

[cswilson]

Now that you have some mechanical coupling, even if soft, I would put this in the category of "not quite crazy" (but pretty darned close...).

You still have some interesting issues. You think you have some automatic electromagnetic correction, but I'm not so sure. When you run a synchronous motor like this open loop, you run at a torque angle a lot smaller than 90 degrees. As the load increases, the torque angle increases, and the torque, being at least roughly proportional to the sine of the torque angle, automatically increases to compensate. But you only get this effect if you are significantly short of 90-degree torque angle.

When you run the motor closed loop, you use the feedback to maintain a torque angle as close to 90 degrees as possible, maximizing the torque per unit current, and therefore available torque. In this range, you lose any signficant automatic electromagnetic correction, and past 90 degrees it goes negative!

And as I said before, I have no idea how you would combine the Hall signals from the two motors to create a single "average" signal. You might have to try it using just one motor's signal, and see whether your soft coupling keeps the other motor in sync.

Curt Wilson
Delta Tau Data Systems

 

[Intermesher]

Curt thanks for your knowledge, which certainly far exceeds mine.

The help you have given will be digested and looked into further. As you suggest, we may electrically couple two motors in various ways and then subject them to different physical conditions. It may be a fun experiment.

Thanks for all.

Dave

 

[blacksea]

For keeping both motor torques close to max with such 6-step commutation, both Halls must be with the same phase, i.e. motor's rotors must be aligned mechanically.
The problem can with with Power Up - you need rotate one of them for reach low phase tolerance between motor's Halls.
You need rotate them in the same direction also.

 

[Intermesher]

blacksea,

Sorry for the delay in responding.

The rotors are always to rotate in the same direction and at the same speed, as you mentioned.

The motors have 36 wound poles in their stators. This represents 10-degrees of mechanical rotation per phase change. I am hoping that the electromagnetic forces will attempt to pull the rotors back into alignment as long as the rotors are mechanically restrained from getting more +/- 4 to 5-degrees out of alignment.

This is getting close to simply having a full mechanical connection but the hope is that any perturbation on an individual rotor will not be excessively great and the mechanical connection only serve as a 'back up'.


Dave

 

[sreid]

Curt,

Good to see you back. How about "Phase Locking" [PLL] the Hall Signals between the two motors? The performance requirements are non demanding so the loop could be primitive, maybe Fuzzy Control?

 

[cswilson]

sreid -- Never really been gone, just not many questions this summer that were up my alley...

My understanding is the the OP wants a single common power waveform paralleled to both motors. Given that, there is not much that can be done with electronic feedback loops, whether in hardware (e.g. PLL) or software, linear or "fuzzy".

I also pointed out that if he used rotor feedback as most people do to optimize torque per unit current, he would lose the natural electromagnetic feedback that people who run synchronous motors open-loop depend on. It would be possible in theory to lag the waveform from the optimum torque angle to get some of that feedback.

Curt Wilson
Delta Tau Data Systems

 

[Intermesher]

sreid & Curt

Thank you for taking the time to comment on my perhaps novel requirement.

I am under the impression that;

1/ The two motors will hold synchronization as long as; they are very close to sync. AND that any moment that attemps to disrupt this sync. is relatively small.

2/ The two motors are close together, therefore the addition of interacting permanent magnets on both rotors would contribute to their synchronization, without consuming valuable battery power.

A curt Yes or No would be appreciated, (no pun intended)

Dave

 

[cswilson]

Intermesher:

The answer to (1) could very well be yes, but I consider this a "research" rather than a "development" project.

The answer to (2) is no. I've tried to explain this a couple of times, but apparently not succeeded so far. I'll try one more time.

The torque per unit current of this type of motor is proportional to the sine of the (electrical) angle between the rotor field from the permanent magnets and the stator field from the windings. When running open loop with a certain AC current magnitude, you end up at an angle such that the generated torque matches the load torque (as long as you are in the 0 to 90 degree range).

If the load torque then increases, this decelerates the motor, increasing the angle, and therefore the generated torque (again, in the 0 to 90 degree range, where sine increases with angle). This is the electromagnetic correction you are interested.

The gain of this correction is the derivative of the torque curve with respect to this angle -- the cosine of the angle. So the gain decreases as the torque increases, and disappears completely at maximum torque per unit current.

People generally use separate feedback sensors like Hall sensors and create an electronic feedback loop so they can operate at the maximum torque per unit current point. This looks like it would be especially important for you in a battery-powered application. But at this point you have lost the use of the built-in electromagnetic feedback.

While it would be possible for you to operate off the peak point even with feedback, to get any significant corrective "gain" from the magnets, you would need to reduce significantly your torque per unit current, which means consuming a lot more current to get the needed torque.

Curt Wilson
Delta Tau Data Systems

 

[Intermesher]

Curt,

Your explenations were very clear and I thank you for them, including your later elaboration.

My last posting was the one that lacked clarity.
Item 2/ was for the addition of a separate permanent magnet coupling between the two rotors to enhance the synchronization.
[

http://www.unicopter.com/1946.gif

]


Dave