minimum PWM frequency for BLDC controller

Thread Starter

strantor

Joined Oct 3, 2010
5,158
Let's say you have an 8 pole BLDC motor with a maximum RPM of 5000. for 8 poles, that's 4 pole pairs, so it would take 4 electrical cycles to get 1 mechanical revolution. In order to spin this motor it 5000 RPM, it would take 20KHz of pure DC pulse, but since it need to be PWM, it should be at least 10X higher frequency than that*, so 200KHz is what PWM Frequency I need to use? That seems really high; Am I doing it right?

*Microchip AN885:
As a rule of thumb, the PWM frequency should
be at least 10 times that of the maximum frequency of
the motor.
 

shortbus

Joined Sep 30, 2009
7,078

Thread Starter

strantor

Joined Oct 3, 2010
5,158
I know I've bothered you a lot about this stuff, but you might take a look at this, there is a link to a couple of papers from MIT on the physics of BLDC motors and controllers. And a link to a paper by a guy named James Mevey with formulas. Just please read the first page of this link and it will send you to the others.

http://www.instructables.com/id/Make-Your-Own-Miniature-Electric-Hub-Motor/step2/The-Brushless-DC-Motor/
Thanks for that shortbus! What I've been dreaming up, is after I get my controller built, I may try to make my own brushless motor. I have about a dozen 3phase fractional hp motors, on up to 2hp and one or more of them are just begging to be frankensteined. I will find this info invaluable when the time comes for that. I'm going to print out that mammoth BLDC book at work on monday because I think it will help me in my controller design.
 

shortbus

Joined Sep 30, 2009
7,078
That was my thoughts too, to convert a tree phase to run on DC. I started another thread and even went to a little mom and pop rewinding shop. But the guy that owned the shop died and the wife sold it. Getting a motor done now means sending it out and I've got other projects that are closer to getting done. The guy at that shop would give me magnet wire roll ends that weren't long enough for him to use, for scrap price.

Are you sure that a VFD doesn't give the same out-put wave form that a BLDC drive does? By just changing the mosfet/IGBT on the out put as needed, it would make things easier. Or even get one of the drivers they use with RC plane motors and modify the out-put devices to suit the motor your going to use. The circuit up to the mosfets should be the same, just needs to have higher rated mosfet.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,158
That was my thoughts too, to convert a tree phase to run on DC. I started another thread and even went to a little mom and pop rewinding shop. But the guy that owned the shop died and the wife sold it. Getting a motor done now means sending it out and I've got other projects that are closer to getting done. The guy at that shop would give me magnet wire roll ends that weren't long enough for him to use, for scrap price.

Are you sure that a VFD doesn't give the same out-put wave form that a BLDC drive does? By just changing the mosfet/IGBT on the out put as needed, it would make things easier. Or even get one of the drivers they use with RC plane motors and modify the out-put devices to suit the motor your going to use. The circuit up to the mosfets should be the same, just needs to have higher rated mosfet.
I have a drive like this: http://www.ebay.com/itm/YASKAWA-EMS-CIMR-G5U43P7-VARIABLE-FREQ-AC-DRIVE-NIB-/250294030927?pt=LH_DefaultDomain_0&hash=item3a46afd24f#ht_282wt_128

I went out to the garage and took it apart and found this:

see datasheet attached

as I thought, it's high voltage (1200V) low amps (15A) IGBT.
If you're going to rewind your motor for a lower voltage (higher amps) you would need to change out the IBGT brick.
Also check out the datasheet on pg2, it shows a full wave X3 rectifier bridge. that would give you +DC bus and -DC bus but no 0v bus .... but then you CAN hook up a HV battery to the DC bus terminals and run the drive, so I dont really know.

here's worth looking at; pg13, it's what I was describing about the variable duty cycle to construct a sine wave. ironically its for a "sinousidal BLDC controller", that's another curve ball for me
VFD waveforms:



BLDC waveform:

I've perused through the RC BLDC (airplanes) stuff and I think what they are doing is open loop with no hall sensors. I think they don't need the halls because they run with a constant load, so the driver just blindly send out pulses at a proven frequency and trusts that the motor is keeping up.not 100% sure on that.

When you're talking about modifying the RC controller to use bigger FETs/IGBTs, you are (I assume) going to want to update the FET drivers, and provide some position feedback for closed loop. Doing that is probably going to be harder than starting from scratch, which is where I am.
 

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shortbus

Joined Sep 30, 2009
7,078
This stuff is getting beyond me now:) different companies are using different ways to get to the same result. My training is in machining and took this type of stuff to late in life. Always had an interest in electronics but didn't have the time to learn when I was young.

I have learned more from all the discussions on AAC than from all the other sites combined. People here don't treat you like your completely stupid and will take the time to help with no egos involved!!!
 
Last edited:

tom66

Joined May 9, 2009
2,595
I've perused through the RC BLDC (airplanes) stuff and I think what they are doing is open loop with no hall sensors. I think they don't need the halls because they run with a constant load, so the driver just blindly send out pulses at a proven frequency and trusts that the motor is keeping up.not 100% sure on that.

When you're talking about modifying the RC controller to use bigger FETs/IGBTs, you are (I assume) going to want to update the FET drivers, and provide some position feedback for closed loop. Doing that is probably going to be harder than starting from scratch, which is where I am.
Most model aircraft ESCs use the back-EMF to feedback information on the position of the rotor. It's precise enough to be able to adapt to a different prop size or loading conditions, but it will give stability problems at low speeds (the motor will often stall, and the ESC will restart it, but it will momentarily speed up on start-up as it tries to stabilise the speed.) That's not usually a problem for model aircraft, though, as the motor is often run at 50-100% full speed.

Most model aircraft ESCs in the sub 25A range do not bother with drivers and find it is cheaper to just mount a big heatsink on the MOSFETs.
 
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