# Magneto Functionality

Discussion in 'General Electronics Chat' started by mossman, Mar 10, 2015.

1. ### mossman Thread Starter Member

Aug 26, 2010
131
3
I'm trying to understand the functionality of a motorcycle alternator/generator in order to calculate RPM. My Honda CRF250X has an 8-pole stator and I measured a 111 Hz AC signal at my headlight, which is fed directly off the alternator. Using the equation N(RPM) = 120f/P, I get 1,665 RPM, which is within the manufacturer spec for idle RPM (1,700 +/- 100). There are a few people that are disagreeing with my calculation, so I want to determine who is correct. A technician that works on motorcycle stators told me that the headlight uses three poles, the rectifier/regulator uses three poles, and the ignition control module (ICM) uses two poles. However, I have a couple problems with this statement. First, in order to get a contiguous AC signal, the poles have to be spaced evenly around the stator. Since the stator on my motorcycle has 8 evenly spaced poles, it is not possible to have three evenly space poles around the circumference of the stator. Therefore, I believe the headlight would have to be fed from 2 poles, 4 poles, or all eight poles. To get my 1,665 number, I used 8 poles for P in my equation, but I am not certain if this is correct. Second, it seems to me that all eight poles, or at least four, would be utilized to generate the voltage for the ICM since the ICM needs a higher voltage than both the headlight and the regulator/rectifier (battery charging circuit). I plan on measuring RPM again, only this time by probing the CKP sensor, which puts off a pulse once per crankshaft revolution. Once I determine I have the correct RPM, I will then need to know the number of magnets on the flywheel/rotor at which time I can get a definitive answer on the number of poles used. In summary, the questions I am trying to answer are as follows:

1) Is the equation RPM = 120f/P valid for all AC generators?
2) Is the number of magnets on the rotor typically equal to the number of poles in the stator?
3) Is the number of magnets and poles typically in multiples of two or are there other combinations?
4) Does each pole generate half an AC cycle?
5) Does increasing the number of magnets increase the frequency for a given RPM?
6) Does increasing the number of poles increase both the frequency and voltage output for a given RPM?

Here's an example of an aftermarket stator for my bike. I'm not sure if it differs at all from the stock stator. Notice two of the poles use a smaller gauge wire and more windings. I suppose these are the poles used for the ICM?

I suppose if the rotor contains eight magnets (four N and four S), then there would be a contiguous sine wave output for the lighting circuit regardless of where the poles are positioned. If this is correct, then regardless of the number of poles, there will be eight pulses per one revolution (each pole providing a half AC cycle each time a magnet passes over it). If the headlight coil is wound around three consecutive poles, then the direction of the windings would alternate (one pole CW and two CCW, or two CCW and one CW) in order to get a contiguous AC output. And the output voltage would be 3 times the voltage produced by a single pole. I think I understand it now.

Last edited: Mar 10, 2015

Jul 18, 2013
10,515
2,369
AC generation takes 8 poles to produce 4 electrical cycles.
Max.

3. ### mossman Thread Starter Member

Aug 26, 2010
131
3
Correct. Each pole produces one-half cycle so it takes two poles to make one full AC cycle. This is why the RPM formula is 120 f / P. 120 is 2 x 60. 2 to get a full cycle and 60 to convert from rotations per second to rotations per minute. You're actually dividing the poles by 2, but the 2 moves to the numerator.

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,779
932
stator poles dont matter in RPM calc. you want the number of Magnetic poles.

5. ### mossman Thread Starter Member

Aug 26, 2010
131
3
If you took an AC generator with four equally spaced magnets and four equally spaced poles then increased the stator poles to eight, would you not double the frequency of the output signal for a fixed RPM? With four magnets and four poles, you would get a half sinusoid for every 1/4 rotation, whereas if you had four magnets and eight poles, you would get a half sinusoid every 1/8 rotation, which is double the frequency, correct?

Jul 18, 2013
10,515
2,369
It is usually expressed in electrical rotations/mechanical.
IOW, on a 8 pole motor there is 4 electrical rotations/mechanical.
Max.

7. ### mossman Thread Starter Member

Aug 26, 2010
131
3
...meaning four full sinusoidal periods, correct? IOW, each pole produces 1/2 of a sinusoid.

Jul 18, 2013
10,515
2,369
Yes.
Max.

9. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,779
932
regardless of the number of stators, each coil will be exposed to a magnetic pole twice during a single revolution. If there are more than one pair of magnetic poles then the exposure will be increased ( two, four, six, etc) for a single shaft revolution. Other stators will undergo the same inductive process, but offset in the time domain from neighboring stators. The frequency will not differ if magnetic pole number and RPM stay the same, whether you have two stator coils or 100.

10. ### mossman Thread Starter Member

Aug 26, 2010
131
3
I understand. Next thing I'm having difficulty with is determining why one of the coils would be center-tapped. There is one coil (meaning continuous wire) wrapped around five poles, with a center tap (in the middle of the third pole presumably) that feeds and estimated 15V to both the 12V headlight and the regulator/rectifier box that charges a 12V battery. Below is the schematic (my library didn't have an incandescent lamp symbol, so I used a diode for the headlight). There would be about 30V total across the entire coil, with 15V being fed to the reg/rect and 15V to the headlight. With the bottom of the coil with the same ground as the battery, is it possible to rectify and regulate the voltage from the top of the coil so that 12V is delivered to the battery?

Last edited: Mar 12, 2015
11. ### mossman Thread Starter Member

Aug 26, 2010
131
3
It appears that they simply used one coil to cut down on the number of wires leading into/out of the stator housing. Makes sense. Only the yellow and blue wires enter/exit the stator housing (the green wire is external and the coil is grounded inside the housing). Still don't understand if the regulator can output 12V with respect to ground when the AC input is acquired from the top half of the coil.

Last edited: Mar 12, 2015
12. ### mossman Thread Starter Member

Aug 26, 2010
131
3
Here is a revised schematic. I don't see how it is possible to charge the battery without the charging current flowing through the entire coil. The point is to only use the top half of the coil.

13. ### mossman Thread Starter Member

Aug 26, 2010
131
3
I may have drawn the innards of the regulator/rectifier incorrectly (not sure if there is a full wave bridge), but all of the outside connections are accurate. I believe what I have is a "half wave single phase stator".

Last edited: Mar 12, 2015
14. ### mossman Thread Starter Member

Aug 26, 2010
131
3
This much I'm sure of. What is in the regulator rectifier is what I am confused about:

15. ### mossman Thread Starter Member

Aug 26, 2010
131
3
Found this schematic while searching around. Looks like what I have, only I still don't understand it completely. Why not keep the lighting and charging circuits separate and have one more wire coming out of the stator? I understand using SCRs allows you to rectify the AC and only use some of the positive cycle, but it appears that the battery charging current will still flow through the entire coil (labeled "magnetic generator"), which doesn't seem correct (or maybe it is correct because charging a battery doesn't draw much current). BTW, the "load" in my case is the electric starter.

Last edited: Mar 13, 2015