Relationship of rotor poles to # of magnets

[Woodie]

I am new to this forum and have been 'lurking' , reading the posts trying to understand the general approaches to these little motors. I own one Himax outrunner and it runs very well, but being a natural 'tinkerer', I want to build my own. Have ordered a GBv 1.1 kit to practice with, I should have soon. Have also begun 'harvesting' rotors, bearings, shafts, etc from hard drives, cdroms and diskette drives which I hope to be able to use in building my own creations. BTW, I have a small lathe which should help with flux rings, front bells, mounts, etc.

My question is what is the relationship of rotor poles to # of magnets in one of these outrunners. I have two identical 27mm rotors with 12 poles, would I have to increase the number of magnets from what is used with a 9 pole rotor to use these rotors?

TIA

Woodie

[swatson144]

welcome woodie.

If you haven't already please looky the /motor theory/flux ring material thread. Looks like you'll need it :)

A quick calc for poles etc
http://www.bavaria-direct.co.za/models/files/Winding_Scheme_Calculator.htm

a chart
http://fast-results.com/lnl/nutpol_e.htm

What you are looking at is an LRK and searching that in the forum or google will probably make you have more questions. Though at this time GB is mostly 9 arm cdrom, they do have some 12 arm 25mm stators for sale, so that lets a foot in the door :D

What thickness are the 27mm and what donated them?

[Woodie]

Swatson,

Thanks for the links, looks like I could go to a 16 magnet rotor with ABCABCABC windings.

The stators came from Wester Digital Cariar 11300 Hard Drives. Stators are actually 27.5mm OD, 15mm ID, 3mm thickness.

However, the larger id reduces the winding area on the teeth. I did an experimental wind on one tooth and can get about 12 (maybe 14) winds with the wire that comes in the GB v1.1 kit.

Woodie

[LBMiller5]

Woodie,

There are a couple rules of thumb. For a standard 3-Phase motor, the number of Stator poles must be a multiple of 3, IE 3,6,9,12,15 etc., and the number of magnets must be an even number, but not equal to the number of poles in the stator, or a multiple of the number of poles in the stator. For example, if you have a 6-pole stator, you cannot have 6 magnets or 12 magnets. Likewise, in a motor with a 9-pole stator, you could not have 9 or 18 magnets.

Here is a brief explanation as to how our little 3-phase motors work: Each time the speed controller sends a pulse to one of the phase coil windings, 3 of the 9 poles are energized, and the rotor rotates 1 magnet spacing. The more magnets you have in the rotor, the less the rotor turns from each speed controller pulse. This has the effect of having a gear reduction unit on the front of the motor, and increases the torque while decreasing the rotational speed. If you have fewer magnets, the motor spins faster with less torque.

If you have ever seen the inside of a simple brushed motor, like a direct drive Mabuchi 300, or a typical R/C car motor, you will notice that there are 3 poles on the inside of the motor, and 2 magnets in the can. (These are not Out-runner type motors but they work the same. In a standard motor, we hold the can still, and let the insides spin. In an Out-runner type motor, we hold the stator still, and let the can spin.) In this type of motor, it takes 3 pulses from the "speed controller"/commutator (one complete cycle) to cause the motor to rotate 1 full turn. With 4 magnets, it would take 6 pulses from the speed controller (2 complete cycles) to make the motor rotate one full turn. With 6 magnets, it would take 9 pulses from the speed controller (3 complete cycles) to make the motor rotate one full turn and so on.

The reduction factor in a motor is equal to half the number of magnets in the rotor, so our typical CD-ROM motor with a 9 pole stator and 12 magnets has the equivalent of a 6 to 1 gear reduction. That is why an out-runner motor will turn the same size prop as a Direct Drive GWS 350 motor running through a 6 to 1 gearbox.

LRK motors have even more torque because they are wound on a 12-pole stator in 3 phases with 4 poles per phase, and they typically have 14 magnets. When the speed controller sends out a pulse to one of the phases, 4 poles are energized, so you have 33% more torque available on every pulse than you would with a 9-pole stator, plus the 2 extra magnets make the LRK motor have a 7 to 1 gear reduction factor. Because of these 2 factors, an LRK motor wound with the same amount of wire per pole, and receiving the same amount of current, will produce (4/3)x(7/6) or 1.333 x 1.166 = 1.556 times as much torque than a 9-pole stator running with 12 magnets, however it will run only (6/7) times as fast or 86% of the speed of the 9-pole motor. That is what makes the LRK motors so powerfull, for the same wind, you get 55.5% more torque, with only a 14.3% drop in speed. Granted you do not get this power for free, and you will have to increase the voltage or current to power the extra 3 poles in the motor. The point is that you can!

Well I think that is enough for now. I am sure someone else will want to add their comments as well.

Till next time..........

Lucien

[instain]

Each time the speed controller sends a pulse to one of the phase coil windings, 3 of the 9 poles are energized

Just a minor correction... A six-step controller (what we use) hooked to a Wye connected motor energizes two phases at a time, which would energize 6 of the 9 coils - three "forward" and three "reverse." Freewheeling current flows through the other three coils. In a Delta motor, current is driven through all three phases. It gets a little tricky to make such a general statement without showing a waveform.

Andrew

[LBMiller5]

You are correct, there are 2 phases energized at each point in time. Sometimes you start down a path and forget the basics! THe theory is still the same, and much easier to understand as presented.

Lucien