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Brushless motor sizing

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AndrewM

Sat Sept 26 2009, 12:32AM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

I'm building a boat and want to use a brushless motor for propulsion. (Yes, it has to be electric, and it has to be brushless.) I don't really have the ability to correlate my propulsion requirements with a torque-RPM curve, aka hand-waving. But perhaps someone can help:

To simplify things, I'm looking to keep the performance I get from an off-the-shelf electric trolling motor, which are brushed, 12V. Such a beastie pulls 20A at 1500 prop RPM. The problem is that the BLDC motors I'm finding are usually 24V, however I cannot change my system to higher voltage. If I find a 24V motor with "doubled" specs, will I get the performance I want at 12V?

I think I did a poor job explaining my question, but lets see what you all make of it.....

...

Sat Sept 26 2009, 05:58AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

A brushless motor is rated in volt-turns, which is the maximum (and more or less optimum) speed the motor can run at a given voltage. The mechanism for this limiting factor is the back emf produced in the windings as the magnets pass them--when the magnets are moving fast enough it will induce enough of an emf that you aren't putting any current through the motor anymore! That said, a 24v motor running at 12v will run about half speed. Assuming that the force-speed relationship for the prop is linear (which I believe it is for most simple props, at least when not moving), a motor turning at half speed will be putting out half the thrust it would at full speed, giving 1/4 the overall power. This also makes sense from a conversation of energy standpoint, you have half the voltage and half the current draw (keep in mind the current draw for any motor should be proportional to the torque the motor is exerting up until you stall the poor thing), you are putting in 1/4 as much power.

Have you tried looking at motors for RC airplanes? They come in multiples of 3.6v (7.2, 10.8, 14.4, and anything in between), and have good ESCs that interface to normal RC stuff painlessly. Some of the motors put out powers measured in horsepower, so you should be able to find as much power as your application needs

Steve Conner

Sat Sept 26 2009, 07:54PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

You need a 12v, 240 watt, 1500rpm motor. So, you look for a brushless motor with the right Kv constant to give 1500rpm at 12v, the right Ki constant to give 1/3hp at 1500rpm worth of torque when supplied with 20 amps, and last but not least a current rating of 20 amps. This spec should give you essentially the same performance as the trolling motor. It'll also be a similar size and weight.

I very much doubt a RC plane motor will manage 1/3 hp at 1500rpm without a reduction gearbox of some kind. They're small, high speed units, and their horsepower rating will be quoted at a high speed. You could use a smaller prop, but larger, slower-running props are more efficient, and with this talk of trolling motors, I got the idea that AM wants a real-sized boat to sail around in.

Bjørn

Sat Sept 26 2009, 09:48PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

This might run at the right rpm, my calculator died so someone else can check out the numbers:

AndrewM

Sun Sept 27 2009, 06:06PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

Good comments, thanks. I should clarify - I understand how to size a motor if I was spending someone else's money, but cost effective motor choices tend to come with an incomplete set of specs. So I'm trying to establish how, or if, I can reasonably correlate my design point (~250W shaft power at 1500 RPM) with a 24V (or other) motor running at 12V.

Bjorn's link was good, but also embodies the problem. Kv is given, but not Ki. The listed wattage (6500) appears to be a silly, as its clearly the product of the max rated voltage (48) and the stall current (120). The minimum voltage is listed as "20", however all of those hobby motors are 3-wire so there aren't hall effect sensors or anything to power, which makes me thing I can likely run it below the rate voltage without too much trouble. But how will it perform?!?! Sounds like the most efficient solution might be just to buy one and try it out.

Good to see Bjorn and Steve are still around, been a while!

Here is the project, if you're interested:

...

Sun Sept 27 2009, 08:47PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

The operating torque for a brushless motor should be more or less invariant of the voltage used to drive it, and is limited by the amount of current your speed controller can handle and how long before the wires in the motor melt. A brushless motor running from x rpm, x volts, drawing y amps, putting out z n-m of torque, vs the same motor running at w/2 rpm and x/2 volts, should still draw y amps and put out z n-m of torque, ignoring friction at the like (to the best of my knowledge). The only instance when this would break down is when there is significant IR losses, which can be accounted for by looking at the dc resistance of the motor. If you are running at 10a and have a 30r motor you will be dropping about 3v due to IR losses, so if you are really concerned you could say that a motor running at 48v (ie, 45v effective) would perform at the same current with half the speed as the same motor running at 28v (25v effective).

Sorry if that doesn't make sense, that is how I understand these beasts

Dennis Rogers

Sun Sept 27 2009, 10:52PM

Registered Member #1837 Joined: Tue Dec 02 2008, 02:20PM
Location: NYC
Posts: 65

I spend a lot of my time trying to make permanent magnet motors work correctly in specific applications and ... (previous poster) is right. Unless you have a field motor there's not a whole lot you can do about torque. If you want to run 24v motor at 12v expect to supply max amp rating of motor to get max torque (which at a guess I'd say will be half) at your reduced voltage.

My experience is with elevator door operators. To pass inspection they can't impart more than 30lbs of force at the door. To beat inspection I often have to slow the operator down to a painful speed (by reducing voltage) to pass inspection since there is no code for how fast they have to go. With a field motor you can have best of both worlds. Armature voltage for speed and field voltage for torque.

Dennis

AndrewM

Wed Oct 07 2009, 08:21AM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

I've read these posts 10 times and my stupid brain just cant wrap my head around it. I'm really close to grasping "..."s post, but, uh, not quite. Sorry, bear with me.

Let me illustrate my confusion.

Here

is a motor with the following specs:

Max power: 2700W
Continuous current: 60A cont, 78A burst
Voltage: 33-38V
kv: 245
Resistance: .03 ohms
Recommended ESC: 100A

So my design point then would be given by my *maximum* speed at which, I think, I want 500W of shaft power at 2000 rpm (for example).

Then I assumed that Power and Current vary linearly with RPM (reasonablish for first order, I think) and using the above specs I plotted power and current vs. RPM. I used the functions: I(rpm)=(V-rpm/kv)/R (where rpm/kv is my back EMF term) and P(rpm)=V*I(rpm).

So then, in order to make this particular motor work, I'd find "500W" on my power chart, and then read the RPM off the x axis. I'd need to then gear the motor such that, at 2000 shaft RPM, the motor spins at whatever the RPM associated with 500W is....

Right???

Steve Conner

Wed Oct 07 2009, 11:04AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

No. Current has nothing to do with RPM, it's a function of torque. And torque is determined by the load.

To be able to solve your problem, you need the torque vs. RPM curve of the propeller you'll be using.

With the data you have, all you can say is that with a Kv of 245, this motor will want about 9-10V to spin at 2000rpm. So it's quite acceptable for use with a 12V battery.

You can also say that, with a current rating of 60A continuous, it'll make up to about 500W at this RPM.

AndrewM

Wed Oct 07 2009, 03:38PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

[quote]To be able to solve your problem, you need the torque vs. RPM curve of the propeller you'll be using.[/quote]

The figure I gave was a "real" single point from my curve (500W @ 200rpm. Call this 2.4Nm of torque if you like)... I dont have a full curve but I have several data points.

Steve McConner wrote ...
With the data you have, all you can say is that with a Kv of 245, this motor will want about 9-10V to spin at 2000rpm. So it's quite acceptable for use with a 12V battery.

You can also say that, with a current rating of 60A continuous, it'll make up to about 500W at this RPM.

Here I lost you. Why is it appropriate to use the "rated current" to estimate output power at any RPM? I thought rated current was just a thermal dissipation concern?

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