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An interesting BLDC controller concept

Hi:

An idea has been floating in my head for a few weeks, ever since building an analog block commutation drive for a brushless DC (BLDC) motor.

It is a way to implement a sinewave drive on a BLDC motor which lacks an encoder. The sine drives that I have seen so far require a high resolution encoder to provide angular information from which to synthesize the sinewave.

My idea is to use a PLL to multiply the hall sensor (or back EMF derived position information if a non-sensor motor) frequency by some multiple to create a sine generation clock.

Of course, there would be no dependable angular frequency to lock on before the motor is running. So startup would be performed with simple block (6-step, trapezoidal, so many terms...) commutation.

Once the motor was running and the PLL could track, the drive could morph to sinewave mode.

Acceleration and deceleration of the motor would result in small glitches in the sinewaves, but would still probably produce much less harmonic current than plain block commutation. Very strong acceleration could perhaps use block commutation temporarily.

Anyone ever tried this? Obviously not applicable to position servo applications.

I just wanted to get it out in the public domain in case it hadn't been patented yet ;-)

Good day!

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_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
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CC
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In message , dated Sun, 6 Aug 2006, CC writes

What has it got to do with Lebanon? (;-)

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OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
Reply to
John Woodgate

Sorry Chris. Save the champagne for the next idea.

You've just re-invented the microstepped stepper motor. If you are careful about your acceleration and deceleration profiles, you don't really need the encoder.

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Bill Sloman, Nijmegen
Reply to
bill.sloman

Interesting.

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_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5
Reply to
CC

How much power are you talking about? Why the fascination with sine waves? There are a variety of techniques, with working ranges from milliwatt to megawatt. Not all techniques are suitable to all power ranges. See also variable frequency drives (VFD) and adjustable frequency motor controls (AFMC). Now, what is it you really want?

--
 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
  --Schiller
Reply to
joseph2k

Doesn't matter, the technique is applicable to any power level.

Why the fascination with sine waves?

Torque ripple and harmonic currents which increase losses (heating) are high using block commutation in BLDCs. Sinewaves are the form of the back emf waveforms, so sinewave drive results in minimal harmonic currents, as well as smooth torque.

BLDC motors are a special class of synchronous motors and by definition operate using a form of VFD. VFD as a commercially used term are mainly applicable to induction motors.

The point is that the market offers block commutation BLDC motor drives, as well as sinewave drives. The sinewave drives exist for the reasons mentioned and so are considered for applications where torque ripple must be minimized and/or harmonic current related losses must be minimized. However, they require extra cost and complexity of the motor, which must be coupled to a high resolution encoder.

What I want is to explore these concepts and drives both theoretically and experimentally for learning purposes, as well as for some present applications with which I am involved. I have already constructed a block commutation BLDC drive, as well as employing some commercial drives.

My next interest is in understanding harmonic currents in more detail, and how to reduce them. This may possibly entail constructing a sinewave drive.

Good day!

--
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5
Reply to
CC

True, but usually because is much cheaper, those kind of motor are using pseudo-sinewaves (trapezoidal or rectangular with sinus shape only). That's because all motors have inductive coils and the shape is integrated.

Using a PLL to generate a power sinusoidal signal (with a serious load) will be a difficult task due the PLL working principle. Any spike or load variation will track the PLL ouside the capture band and everything will finish into a mess. I hope I'm wrong (for your future patented methode). :)

greetings, Vasile

Reply to
vasile

There are (I believe) two main forms of windings for this sort of three phase brushless DC motors - BLDC windings, optimized to be run by trapezoidal waveforms, and PMSM windings, optimized for sinusoidal waveforms. Of course, you can use either waveform for either type of motor winding, at a slight efficiency loss, but there is no point in striving to get a nice sinewave unless you are sure your motor matches it, and unless you can be sure of very smooth running.

As for using a PLL based on the hall effect sensors - you do realise that the 6 switching points from a typical motor hall effect sensor setup are not accurately at 60 degree steps? You are going to get some serious jitter effects if you try and multiply the step rate, unless you just use a single step (i.e., one signal per electrical revolution).

Reply to
David Brown

Are you referring to slotted vs. slotless stators? If so, which of your designations "BLDC" and "PMSM" is slotted/slotless? Also, what does the abbreviation PMSM mean?

I had a discussion with a sales engineer at

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about slotless/slotted motors. It wasn't clear from this discussion though which should be driven with trapezoid vs. sine.

There also doesn't seem to be clear guidance from motor manufacturers about this.

One thing is certain, my Maxon motors produce perfect sinewave back EMF waveforms. I wonder if a slotted motor would produce something more "trapezoidal?"

Yes, they are not perfectly accurate. I think I was inclining just toward looking at a single pulse/rev. Though now it brings up an interesting refinement:

A controller could calibrate itself to the hall misalignment by running the motor in a PLL and measuring the hall timings. Now I have to think about how this information could be used later to correct the hall timings...but there must be a way. Probably would have to be a digital PLL implementation of some sort.

--
Good day!

________________________________________
Christopher R. Carlen
Principal Laser&Electronics Technologist
Sandia National Laboratories CA USA
crcarleRemoveThis@BOGUSsandia.gov
NOTE, delete texts: "RemoveThis" and
"BOGUS" from email address to reply.
Reply to
Chris Carlen

ST has some interesting app notes using sine-wave drive on BLDC motors. Here's one:

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look at some of their others:

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#Application%20Note

Are you trying to build flexible chopper wheels for your LASER systems? I've built a phase-lockable motor controller that lets you go up/down in speed and allows for nearly arbitrary phase offsets for such systems (DC motor though, not BLDC).

Regards. R.

Reply to
John

I haven't heard the terms "slotted" and "slottless", so I'm afraid I can't help you.

"PMSM" is "permanent magnet synchronous motor".

That's certainly true, at least for all the motors I've seen.

Actually Maxon motors can be driven well with either form, and it is not clear from their documentation (that I've read) which is optimal. I think they work well enough with either kind of driving.

Reply to
David Brown

Slotted motors have laminated stators with actual slots to hold the windings. Slotless motors have no such slots. How the winding is done in a slotless motor looks something like Maxon's ironless rotor design for permanent magnet DC motors. Pictures of this and the slotless BLDC construction appear in their catalog.

Yes, but what is the physical difference between BLDC and PMSM?

-- Good day!

________________________________________ Christopher R. Carlen Principal Laser&Electronics Technologist Sandia National Laboratories CA USA snipped-for-privacy@BOGUSsandia.gov NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.

Reply to
Chris Carlen

Freescale have some quite good overviews:

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The physical difference between the BLDC and the PMSM is a minor change in the winding topology to make the former most efficient with trapezoidal waveforms, and the later most efficient with sinusoidal waveforms.

Reply to
David Brown

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