Running BLDC motors at constant speeds.

[x88x]

For a couple projects I'm planning, I'm trying to figure out the simplest way of running a BLDC motor at a constant speed. Obviously using a brushed DC would be the simplest way because I would just dump whatever voltage I want across the terminals, but I would like to take advantage of the decreased noise, increased efficiency, lack of consumable parts (brushes), and absence of sparks that BLDC motors have.

Now, I could, of course, use a controller with a resistor across the throttle input at whatever speed I want, but that seems needlessly complicated and expensive to me. So, my question to all the motor gurus out there; is there a simple, reliable, cheap, way to run a BLDC motor at a constant speed? I've heard of people using inverters to run AC motors at a constant speed; is there something similar I could do with BLDC?

In case anyone is curious, the projects in question are a cordless electric chainsaw in the near future and an A/C compressor in an electric conversion a few years down the road.

 

[amberwolf]

The simplest way without starting from scratch is indeed to just use an existing BLDC controller with a constant voltage on it's speed control input.

If you were to do it from scratch, you can make a very simple controller using the analog MC3303x series chips, which even have a feedback loop control input to set and keep a constant specific speed. Jeremy Harris' Simple BLDC Controller thread shows how you can use this chip effectively in a complete controller.


You still have to use a controller of some sort, because otherwise there's no way to do commutation necessary to spin the motor.

A "simple" option for the controller is just another BLDC motor with it's phases tied to the drive motor's phase inputs, and driving the first BLDC motor as a generator, by using a brushed DC motor (with just a switch for the controller) to physically spin the shaft of the generator-setup BLDC motor. But it is still mechanically complex and probably a lot heavier than you want in a portable tool like that.

 

[x88x]

Hmm, ok, not exactly what I was hoping to hear, but it makes sense.

Thinking of cheaper options, I know the ESCs that RC people use to run their BLDC motors tend to die horribly in our LEVs. Is that because of the resistance or the constant power? Probably wouldn't work for the chainsaw, but for the various motors on the car conversion I have planned, the load might be low enough that it wouldn't blow one.

 

[jonescg]

Couldn't you just come up with a simple oscillator circuit that uses heavy-duty components? They would produce a square wave, or more likely a saw-shaped wave, which might ruin your motor, but that would be one way of doing it. Hang on, does the BLDC motor you have in mind need three phases? If so, I'm out of ideas :lol:

 

[x88x]

I don't actually have a specific BLDC motor in mind. I just would rather use BLDC than brushed, mainly because they run quieter and don't need to have brushed replaced, no matter how infrequently. Not so important for the chainsaw really, but with the conversion I'm planning, having a few different small motors I would like to keep them as maintenance-free as possible, and since my current plans are to use a BLDC drive motor, I would like to have all the other motors be as silent as possible. Kinda like how Nissan had to find quieter motors for the windshield wipers on the Leaf because you could hear them when they turned on; I don't want to be driving down the road in silence and suddenly hear my A/C compressor kick in, the wipers turn on, or whatever.

 

[Hillhater]

Why wouldn't you simply pick a motor with a KV rating to give you the rpm you wanted from a nominal battery pack voltage.
Then run it at 100% throttle on an ESC /controller at max efficiency.
EG a 200kv motor with a 37v battery = 7400 rpm...about right for a ICE replacement ?
Use controller settings to control ramp speed etc.
ESC's are cheap and work well at 100% throttle.
Personally i believe a chain saw needs variable speed.

 

[x88x]

Well, the idea was to avoid any unnecessary electronics, but if ESCs will work, they're cheap and small enough that shouldn't be a problem. I'm not sure that having a variable speed chainsaw would be a great benefit, but if I go with BLDC and an ESC it would be easy to at least try it out. I'm just thinking that most saws make the cleanest cuts when the blade is moving as fast as possible.

 

[TylerDurden]

FWIW, my corded electric chainsaw is on/off, not variable. (McCullough)

Even the big boys are getting in the game...

http://www.popularmechanics.com/home/reviews/outdoor-tools/stihls-battery-powered-tools-get-the-job-done-stihl-bga-85-cordless-blower#fbIndex1

 

[Hillhater]

ESC's seem to not like being run at part throttle with a high load.
High load at max throttle is a much better state for them.
As was said before, you will need some form of pwm generator to run the motor, so it may as well be a $20 ESC.

 

[x88x]

Hmm, nice to see the professional companies getting into cordless. Hopefully they can help turn some of the negative stereotype that battery powered outdoor tools have somehow acquired.

This quote really sums up why I personally want the chainsaw I buy to be electric.

Starting up the chain saw was an odd sensation, since it dispensed with the ritual that I've become so familiar with over the last 30 years or so. There was no fuel to mix or add, no choke fiddling, no pull-and-start sequence. There was no revving the saw to get it warmed up. I simply grabbed the saw, walked into the woods and turned it on.

I was helping my dad cut down/up a tree in my parents' yard a couple years back, and after messing with his ICE chainsaw (that had been sitting unused for ~7 years) for over an hour we just gave up and used a bowsaw and axe. You leave an electric chainsaw alone for 7 years and it'll most likely work the second you turn it on. Guess what he has now? A corded electric chainsaw. Unfortunately, I will need mine to work much further away from my house than he does, so corded really wouldn't work for me, so cordless electric it is.

ESC's seem to not like being run at part throttle with a high load.
High load at max throttle is a much better state for them.

Ok, good to know. I'll go that route then. ..well, unless I just hack one of the brushed DC motors I have lying around onto it instead.