1937

Item 1937

DESIGN: Electrotor-SloMo - Rotorhead - Synchronization - Electrical

Initial consideration:

These are electrically Synchronous Motors. With a 36-pole motor and 3-phase control there are 12 physical sectors of 30º each, This means that for the rotors to be out of synchronization one rotor will have to be physically 30º (or some multiple of the 30º) ahead or behind the other rotor. I think that this will mean that a rotor must get 15º ahead or 15º behind the other rotor to be electro-magnetically pulled out of synchronization by the preceding or following electrical 360º. If less than this, the motors will want to be pulled into back synchronization. Concern: it might be less than 15º if the leading rotor has acquired greater rotational inertia than the lagging rotor. Also, it has been mentioned that 'catch-up' force is weaker than the 'maintain' force due the angle of the flux path through the air gap.

This might be functional by the use of only one set of Hall sensors on only one rotor. This will mean that the rotor without the Hall sensors must work to keep in sync with the rotor that has the Hall sensor. This may be a dumb idea.

A better idea may be to have 2 distinct power circuits in each motor. This will involve 24 power transistors. It may also involve 4 Hall Effect sensors. The Port and the starboard motors can then be varied with their separate redundant pair of circuits by the pulse width modulation. Also use regeneration if necessary. See DESIGN: Electrotor-SloMo - Rotorhead - Synchronization - Motor-Generator

Another alternative consider having Hall sensors in both rotors. A small electronic device will average the timings from the two sensors and present it to the single controller that drives both motors. This approach is probably superior to the above one in that it has both rotors working at maintaining synchronization plus it may allow for a greater deviation.

Ref. Hall Effect Sensor:

Regenerative braking: Would it be advantageous to include the regenerative braking for use on the leading rotor when the 'out of sync' exceeds a certain angle?

Polyphase: (> 3 phases) More phases will increase the arc of the sectors, while decreasing the number of sectors. This might help with synchronization by providing a larger angle in which to have a magnetic influence.

36 coils / 3-phase = 12 sectors of 360º / 12 = 30º.

36 coils / 4-phase = 9 sectors of 360º / 12 = 40º. This may not work because 9 is not divisible by 2 into an integer. Consider 48 or 72 coils.

Question. Does this mean that fewer permanent magnets with larger arcs should be used?

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Sensorless BLDC:

Idea: I wonder if the back EMF from both motors can be combined and then the timing measured from this combination????

Notes:

Should one rotor advance and then physically 'push' the other rotor this push might temporarily decrease the electrical torque of the pushed rotor, which would decrease the pitch of the pushed rotor, which will allow it to accelerate; perhaps. Albeit with a slight loss of thrust; perhaps.

Remember the rotors MUST retain synchronization if there is a loss of power in one or both rotors.

This is a critical argument for total electrical redundancy; in each motor, or,

Redundancy due to tow motors and a 100% mechanical interconnection.

Rotary-Wing Forum:

Originally Posted by Rotor Rooter http://www.rotaryforum.com/forum/showthread.php?p=376398 - post376398

Like you, "generate a voltage with small brushless alternator" but eliminate the rectifier and the DC controller. The control would simply consist of a 3-pole On-Off switch. The 3 poles would be 1 for each phase.

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Reply.

Dave, in French, an electric motor with permanent magnet, without electronic controller is named "synchronous motor"
These engines have a serious drawback: beyond the maximum torque, they stop abruptly and restart only when they are exactly resynchronizes (relative to the three-phase supply)
At startup, you can not "hang" synchronization (because the starter is stopped when the propeller is already running at idle): With rotor inertia, would require a huge capacity of torque.
This problem disappears with electronic rectifier and controller (only 0.5 lbs more)
Jean Claude

Question to Eng-Tips: August 23, 2010 Synchronization of two brushless PMDC motors using one controller

Synchronization of two brushless PMDC motors using one controller.

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.

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swilson (Electrical)

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

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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

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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

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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

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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

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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?

sread

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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, 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

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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.

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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

Thoughts:

Consider testing idea with the small controller and two of the basic F&P motors.

This will involve building a comparator/timer on a board.

Re the averaging of the hall sensors, or the combined null of the sensorless method. The inputting of the control signal one cycle later than a conventional input may not be detrimental. This is because the relatively high inertia of the aero-rotors should mean that any change in the rotational speed would be relatively small in respect to cyclic frequency. Note that the action of the torque-pitch mechanism is not significantly subjected to the high inertia of the aero-rotor; which is bad. However the rotors angular change for maximum torque changes is under 5º and the cycle change is 360º/12 = 30º; which is good.

Could be fun to play with.

Excessively Fast Change Possibility:

The maximum pitch change on the blades will be < 15º. The reasons for a sudden large angular change might be; a loss of power in one or both rotors and the fast application of full throttle. Both would change the torque before the RRPM. This 15º change in pitch will be the result of a 4.83º physical change in the motor-rotor to hub and this equates to a 57.96º electrical change. Is this too much?

Initial Consideration for Hall Signal Synchronization:

Outside Information:

DSP-Based Two-Synchronized Motors Control System Using External FPGA Design Of no value.

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Related to DESIGN: Electrotor-SloMo - Rotorhead - Synchronization - Motor-Generator

Initially displayed: August 12, 2010 ~ Posted on Rotary Wing: Aug 8,2010 ~ Posted to Eng-Tips: Aug 23, 2010 ~ Last Revised: October 3, 2011

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