Electric Motor 101

[kenny_dilger]

I am looking for sites that have tutorial information on how electric motors work. Actually.... I understand the basic principles, what I am really after is how different aspects effect perfmance on a motor, particularly brushless motors.

I.E... How the number of polls, windings, winding thickness, magent strength, etc... affect performance. I am going to begin experiementing with CD-ROM motors and wanted a little background.

Thanks,
Kenny

[uglystick]

Tutorial my eye. That's a science degree. Good Luck :-)

[Andy W]

You may want to ask in Modeling Science - the more technical guys tend to have answers to these types of questions..
I'll move this over there, with a redirect..
..a

[vintage1]

I can give you a few snippets, but not teh whole story - because boy even teh PHD physics and engineering grads are still argung over that.

First off, the torque is from the current, times the number of windings, tomes teh magnetic field strength.

Power is torque times RPM. Or is it RPM squared? I forget :D

losses are of two general types - iroon loswees - which basicallyare teh frictional and eddy current losses lumped into one bag. Thse tend to represent a foxed 'current loss' - the so called 'idle current' that you need to get the motor to spin. I am faitry sure these are fairly independent of the load, or RPM of the motor, and you need to be drawing a fair bit more than te idle ciurrent to get decent efficiecy..

...but the more current you draw, the more the copper losses 0- resistive losses in teh motor 0 build up.

What this gives you is that efficiency is related to current directly for a given motor. Too little current and its all iron loss. Too much and its all copper losses. Somewhere in the middle is 'best efficiency'.

On to magnets. Quirte simply, the higher the magnetic flux, the more efficient the motor is, and the less winds it needs to get a specific torque for a given current. And teh more winds it needs to get a specific RPM out...

which brings me on teo that alst point . Windings.

Windings are almost completely irrelevant to efficeincy, or power. They are more of a gear ratio. Put more turns of thinner wire on, and provided you up the voltage to run the motor at the same RPM, it will deliver the same power at the same efficiency. That's because less winds lowers the torque, but because the wire is thicker, the copper loss goes down, so it can take more current. Puitting the torque back where it was. Think of windings as a way to tune a given motor for the specific voltage it's used at.


Theoretically, there is no upper limit to teh wpwer you van get out of a motor. Since most o the losses are copper losses, you can go in incraseng he voltage, and RPM, at teh same current and get more and more oit of it. What happens of cousrse is that we rin into pysical and electronc problems - the motr flies apart, the brushes bounce, or the ESC can't keep up with the commutation speed.

Outrunners with multi-pole slow RPM type direct drive props run into the commutation problem earlier, and also the larger diameter casings do not cope with high RPM as well as inrunners, so they are bnever quite as good on power to weight as inrunners , but the latter almost always have to be geared except spinning very small props or in EDF applications.

Fine tuning motors is done by varyng the magnets, winds, type and shape of iron used and the advance on the commutator or ESC. This gets very subtle - so much so that they starts to be too complex to apply, and its ususally easier to try a variety of things out and measure to see which is best.

But teh basics ofgood desin are awlays

BIG strong magnets.
Close magnetic path using decent iron or coreless designs.
Accurate and strong construction for low friction at high RPM.
Good heat transfer from windings to the outside air.
Lightweight case.

Look at a Razor. Guess who has been doing it a while.

Windings - play around till you are getting close to the commutation or RPM limits on whatever pack you want to use. Then gear down to appropraite prop RPM, If direct drive, it's a compromise beween RPM limits on the PROP efficency, against lack of it on the motor.

Or to put it anotherway, the prop is more efficient running slower, and the motor faster. Somewhere in the middle - usually 8-20k - is the best overall compromise.

[kenny_dilger]

Thanks for the info! It looks like it will take a good bit of experimentation with a watt meter and a tach. A good place to start I guess is the CD-ROM construction motors thread and then experiement from there to figure out exactly what I am after.

[nicnicholson]

Excellent reply, vintage, that one should go where all the CD rom newbies can read it.

I think your fingers were getting close to their commutation limit there as you were typing, however. Decrease the number of windings? <grin>

thanks!

[vintage1]

darned right. Actually I started typing it, and then the in-laws turned up, so it got finished much later.