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BLDC motor questions

Awright youse motor folks, I need info. I have, over the last couple years goofed around with BLDC motors made for RC hobbies. I even made a couple just to learn winding techniques. The motors are rated in part by the kv of the motor, which is how fast it will spin when powered at a given voltage. The kv stands for thousand revolutions per volt. A hand wound motor, or any motor for that matter, can be checked for its kv by spinning the motor at a known rpm and measuring the voltage it produces when operated as a generator. Anyway, I now have a practical use for a BLDC motor. But it needs to fit in a certain space and spin slowly. So I am going to wind a motor by hand on laminations that I will mill myself. I have looked online at sites all about these motors and nobody is trying to make a really slow motor. What I need to know is if the kv is really that important. Can't the motor be told to spin at whatever rpm by the motor controller? No matter what the voltage is (whithin reason)? The motors I would have a pretty high kv and the wire I use to wind them is pretty fine. I can use finer wire but only so fine because it will get too delicate for me to hand wind and because the current capability will drop too much. The motor I want needs to spin slow but have pretty good torque for its size. Higer torque means thicker wires to handle higher current but thicker wires also means fewer turns of the windings which means higher kv. Thanks, Eric

Reply to
etpm
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You may have to consider a gear box.

Reply to
Tom Biasi

snipped-for-privacy@whidbey.com prodded the keyboard with:

Try a small stepper motor.

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Best Regards: 
                      Baron.
Reply to
Baron

No matter what wire size you use to wind with, as long as you're filling about the same amount of space with copper the amount of torque that you can generate per watt of I^2-R losses in a permanent-field motor is roughly constant. That's because as the turns go up the wire gets smaller in area and longer in length roughly proportionally with the number of turns, so the resistance goes as turns squared. The torque per current is proportional to the number of turns. So for an N-turn motor with good packing, resistance is roughly proportional to N^2, and the current needed to generate the same torque is proportional to 1/N. (1/ N)^2 * N^2 = 1.

What that means is that the torque that you can develop is a function of the motor's mechanical and magnetic design -- try to get more torque out of it and you'll just burn up the motor. Winding the motor differently is just a convenience to match whatever power rail you're running off of, and your motor controller's ability to deal with a mismatch between the motor's back EMF and the power rail.

You can sort of test this theory by getting onto the Pittmon or Maxon or Faulhauber motor sites and looking at data sheets -- they'll have lines of motors that are identical mechanical assemblies with different windings, and a bit of math will show you that the torque (and power dissipated) at maximum-rated continuous current is roughly constant across the middle of the voltage range, and trailing off at the ends (because at the low end it's hard to wind a few turns to fill a space, and at the high end you start ending up with a lower ratio of copper to insulation).

So, I think you need a gearbox. The only alternative is if your space allows for a very nontraditional diameter (as in -- BIG) and a custom motor. A gearbox is probably better.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!
Reply to
Tim Wescott

You can spin a motor slow, but you also lose output power rating if the motor wasn't made to run slow. Harping on speed constants of a motor won't solve this problem. It's only part of the issue.

You need to identify the speed and torque rating you need. Everthing else comes after that.

Reply to
Cydrome Leader

My speed range is from 30 rpm to 220 rpm. I dunno how much torque yet, but pretty low. I'll be measuring the torque wirh an inch pound torque wrench pretty soon. As others have said a gearbox is probably in order. I don't have a lot of room and so can't stuff a lot of gears in the space. I guess I need to gigure how much torque is required. Eric

Reply to
etpm

Use stronger magnets, consider having several poles.

Thicker or thinner does not effect max torque, only cross-section does. ten wires carrying 0.1A or one wire carrying 1A, the field is the same only the voltage is different (assuming 10 wires is 10 turns of 1 wire).

but maybe you really need a gearmotor?

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Reply to
Jasen Betts

Is Pittmon the Jamaican site Pittman? Can the back EMF be a problem because it might harm the controller or because of speed regulation. The controllers I have now probably won't spin a motor slow enough. But I have found chips for the purpose that use hall effect sensors for speed control.

Reply to
etpm

I considered steppers when I first thought about this but all the small ones I found had 15 degree steps which might mean kind of jerky motion at 30 rpm. I need a motor no more than about 1.125" diameter. Maybe I need to look at steppers again. Eric

Reply to
etpm

Yes, but since you're in the US you probably want to get Pittman -- mon.

Back EMF isn't a problem -- it's the ratio between the motor controller's power rail voltage and the voltage that the motor needs to be driven at your desired speed.

How much torque do you need? If it's more than the motor can deliver, you're screwed. If not, you may want to use a brushed servo motor.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!
Reply to
Tim Wescott

I take it this is a very small motor so it may not come into a factor like large motors. The larger motors can have their own cooling fan or an external fan. This is to keep down the heat. If the motor is turning too slow it will not cool. Place I worked burnt up several 50 HP motors and wondered why. I looked at them and they were being driven by a varitable speed drive. The nominal 1800 rpm was only running about

200 rpm. Not enough air flow for it to cool. Put a big fan on it to cool it and it was fine.
Reply to
Ralph Mowery

You could do microstepping perhaps, still that could be a challenge.

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Reply to
Jasen Betts

snipped-for-privacy@whidbey.com prodded the keyboard with:

Did you really mean fifteen degree steps ? A couple of the ones I have are 1.2 and 1.8 degrees per step.

--
Best Regards: 
                      Baron.
Reply to
Baron

Here's a small stepper that provides really small steps:

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Cost is $3.15 and includes a ULN2003 driver. You need to provide pulses to the ULN2003 circuit from an external circuit.

See the specs here:

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You won't get 30 rpm out of it - more like 6-7 RPM. It has pretty good torque so you can gear up the speed if you need to.

Ed

Reply to
ehsjr

Yeah, I meant 15 degrees. I'm talking about steppers that are about 1 inch diameter, which is the diameter constraint for the motor. I have many larger steppers with 1.8 degree steps. Eric

Reply to
etpm

Yes, figure out how much torque you need. I'd recommend a motor that's geared down by between 12:1 and 25:1, so the motor is turning at

2700-5500 (assuming I'm getting my math right). That should be a good speed for bearing and gearbox life and for keeping things quiet.

I know you said what sized space you needed to fit, but I'm too lazy to dig it out. Small motors with gearboxes generally get pretty long for the diameter, though, and at least on the gearboxes that I've worked with you can't trust the bearings with any significant side or axial loads. That means that if you use and off-the-shelf gear box you'll need to either use a coupling going into a bearing block (meaning even more length), or you'll need to make a bearing block right at the motor (meaning fiddly machining to get it right).

Most bare motors can't take much in the way of side or axial loads, either, though, so that's not as big of a deal as it might be. I remember one famous mechanism that I worked with, designed by a contract mechanical engineer who was really full of himself. It was supposed to be a fast mechanism for switching a lens array in and out; because he depended on the motor to support the axial load on a worm during rapid deceleration, it turned out to be that _and_ a motor-guts-puller-outer.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!
Reply to
Tim Wescott

IME a stepper motor has less torque for the size and mass than a DC brushed motor, much less DC brushless. Certainly less torque than a geared DC motor.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!
Reply to
Tim Wescott

That can, indeed, affect smaller motors. Life's complicated.

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com 

I'm looking for work -- see my website!
Reply to
Tim Wescott

The suggestion somebody had of a stepper may be worth looking into. 30RPM is real slow- many motors can't even run at such a low speed in the first place. There's too much drag from the magnetics to overcome, so unless you're holding them back with a big load, it's not a realistic speed for a small motor. Big motors with dozens of windings have no problem with stable low speeds. Examples would be a really old elevator motor.

Reply to
Cydrome Leader

Servo City. They have gear motors galore. Even fraction of RPM ranges. ENJOY!!

Reply to
Ron M.

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