Frequency to RPM conversion factor

[frackers]

I'm just making a new controller and logger for the mill and thought I'd put a tacho on it. I've implemented a frequency counter on the micro that doers all the work but I'd just like to confirm what it will see from monitoring one phase before the rectifiers.

Standard 10 foot Hugh axial with 12 magnets per disk and 9 coils. I think I get 4 full cycles per revolution on each phase (or is it 12? or even 36)  so if I take frequency in Hz, divide by 4 and multiply by 60 I should get RPM.

Code: [Select]

rpm = freq / 4 * 60
Does that look right or have I got something upside-down...

[prasadbodas2000]

I think that since you have 6 pole pairs (12 poles/mags per disk) you will get 6 full sine wave cycles per revolution of the disk. so frequency will be 50Hz if RPM is 500...(my simple thing to remember is a 3000 RPM single pole pair machine works at 50 Hz supply frequency)

[kitestrings]

frackers,

Others can give you a more technical answer, but I believe the formula is:

rpm = freq (hz) x 60 / (# poles/2)

For a 12-pole alternator is is pretty easy, you just move the deciimal point;  50 hz = 500 rpm.  Now I've been known to forget this formula, and something that I find helpful is to start from known relationships.  A standard 60 hz motor (2-poles) is 3600 rpm, a 4-pole motor is 1800 rpm, a 6-pole motor would be 1200 rpm, etc.  You can back into the formula from this starting point.

If you're on a 50 hz system, common speeds will be different (2-pole, 3000 rpm), but the formula is the same.

Hope this helps,

~kitestrings

[frackers]

Many thanks for putting me straight - hopefully I'll be testing new software this evening!!

[BrianSmith]

Was the formula right?  I had it in my head you would see a cycle for every magnet (no divide by two).  It seems like the voltage would go up (or down depending on flux direction) when the magnetic field cut one side of the coil and then go down(or up) when it cuts the other side of the coil. 

Kinda Confused in Alabama... 

[Flux]

Yes the formula is right but I, like some of the other posters, prefer just to remember that 2 pole is 3000 rpm at 50 Hz or 3600 rpm at 60 Hz.

A complete cycle starts at zero goes positive returns to zero goes negative and returns to zero. this means passing under a north and south pole.

Most literature uses pole pairs and this makes good sense as a unit magnetic pole is a dream concept. Whether you work on pole pairs or just divide by two comes out the same in the end.

Things are fairly straightforward for conventional alternators but you start having to think with inductor machines which are often unipolar and the flux does not reverse, just working between maximum and zero. The theory still works but magnet count and pole count can be different but let's not cause confusion here, you probably won't meet one of those.

Flux

[BrianSmith]

Thanks Flux, that makes sense to me.  Much appreciated.

Less Confused in Alabama