# Question about BLDC comutation



## phoenixosu (Jul 12, 2009)

I am looking into building a few prototype controllers of my own and have run into a question that I haven't been able to find answers for. Is there a not insignificant difference between using the standard "120degree, square wave, block commutated, etc" mode and more advanced such as sine wave commutation or vector control?

I speak in terms of a motor in the range of 1-10KW, Voltages around 36-144V, the question is a bit general but there is a method to it.

I do understand that sin wave commutation and vector control provide a smoother ride, and vector control especially is more efficient. Though an ICE is always rougher than a square wave commutated BLDC motor. That leaves efficiency, and I have not found any concise answers thus far and so I am asking here if anyone knows?


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## samborambo (Aug 27, 2008)

Square wave commutation will NOT work in a traction application. Its fine for a fixed speed system but since back EMF from the motor is proportional to speed, at low speed you'll have an overcurrent situation.

You need some way of efficiently modulating current. The only practical means of doing this is in high frequency PWM. Theoretically, you could put a square wave commutation stage after a DC motor controller to acheive this. It won't give you any of the benefits of vector control though.

Read lots of books on AC motors to get a good understanding of how they work and what is required to control them. 

Sam.


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## phoenixosu (Jul 12, 2009)

Ah, I see that I have not explained my question enough. I was referring to the way in which the duty cycle of the PWM changes as a function of the rotors electrical position (through one electrical RPM). In square wave commutation each phase PWMs (synchronous or asynchronous) at a constant duty cycle. The duty cycle can also be modulated sinusoidally through out each hall "state" or 1/6 of an electrical revolution. In vector controll the duty ratio of the PWM is modulated such that the combination of the three magnetic fields created by the three phases is always at a 90 degree angle to the rotors electrical position. (electrical and physical RPM and position are different, scaled by the number of poles in the motor). In square wave commutation the angle of the net magnetic field varies +/- 30 degrees off of the 90 degree ideal angle, in sine wave PWM it stays a bit closer to 90, and in vector control it is ideally right on 90 degrees.

Therefore my question was how much of a difference does the modulation scheme make in lower power applications (1-10KW)?


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## samborambo (Aug 27, 2008)

phoenixosu said:


> Ah, I see that I have not explained my question enough. I was referring to the way in which the duty cycle of the PWM changes as a function of the rotors electrical position (through one electrical RPM). In square wave commutation each phase PWMs (synchronous or asynchronous) at a constant duty cycle. The duty cycle can also be modulated sinusoidally through out each hall "state" or 1/6 of an electrical revolution. In vector controll the duty ratio of the PWM is modulated such that the combination of the three magnetic fields created by the three phases is always at a 90 degree angle to the rotors electrical position. (electrical and physical RPM and position are different, scaled by the number of poles in the motor). In square wave commutation the angle of the net magnetic field varies +/- 30 degrees off of the 90 degree ideal angle, in sine wave PWM it stays a bit closer to 90, and in vector control it is ideally right on 90 degrees.
> 
> Therefore my question was how much of a difference does the modulation scheme make in lower power applications (1-10KW)?


Ah, I see what you're getting at now. You've found the difference between what is referred to BLDC and PMSM (permanent magnet synchronous motor). Essentially the two types are the same but vary in construction that is optimised for either a square wave or sine wave, respectively. eg: Feeding a PMSM motor with a square wave will produce a torque ripple at the 4th harmonic.

With a closed loop current/torque control, this shouldn't matter anyway as the controller is minimising the torque ripple by compensating the current continuously through the waveform.

Have a look at Microchip's application note on PMSM FOC drives: http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en530042

Sam.


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## phoenixosu (Jul 12, 2009)

Thanks, that's a good app note. Given the motor I'm trying to build a controller around, 10" 8 (I'm guessing based on o-scope experiments) pole, that ripple will be a "smooth" vibration at all but extremely slow speeds, especially given the weight of a vehicle (though I'm not a mechanical engineer), I think its mass would filter a good deal of it at anything beyond just a few miles per hour.

I did a bit of math in excel, using vector control as a reference, assuming vector control is 100% efficient (leaving out other factors), and that inefficiency is due to the net magnetic field being off 90 degrees, I assumed a sinusoidal distribution, so at 0 degrees or 180 efficiency is 0, and at 270 it would be -1, then adding up and averaging for efficiency over a 360 degree rotation, vector control gets a 1, and surprisingly square wave (or as Microchip calls it trapezoidal), 0.95. I can live with 5% losses in a first pass version.

You said something interesting about BLDC and PMSM motors, that BLDC are wound to accept the "trapazoidal" PWM modulation scheme, is this an industry standard terminology? I've had a great deal of difficulty with terms that seem to vary from place to place, TI might call it one thing and then Toshiba calls it something else etc. If all BLDC motors are truly wound for trapezoidal input, then I might be in the clear to go ahead as planned. Current control of course as you said makes this a bit of a pointless exercise.

I read an article that mentioned BLDC motors can be wound sinusoidally or trapezoidally, the later being more difficult. Would the back EMF made by spinning the motor and measuring the outputs of the phases (no controller connected) tell you how it is wound? The manufacturer claims it is trapezoidal, but on the scope it is sinusoidal.

Apologies for my disorganization, its getting late.


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## samborambo (Aug 27, 2008)

Yeah, if the scope shows a BEMF that is sinusoid, its probably going to give less torque ripple with a sine drive. That said, it's probably a moot issue as the drivetrain will have a high moment of inertia and will act as a flywheel.

It's still going to be quieter than an ICE 

Sam.


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