brushless alternator.

[SMOKEY44211]

I have fabricated a brushless alternator using a 12volt stator and rectifier/regulator assembly scavenged from a 20HP air cooled gasoline engine. One stator is rated for 15 amps the other fo 30 amps. The magnets rotate around the perimetor of the stator. The stator has 18 poles and is wound for single phase. Output voltage on the 15amp stator is 23 volts AC 14.2DC at 500rpm. I would like to substitute rare earth magnets in place of the ceramic ones on the original application. The dilema is that they are not available in the radius that I am working with. Question 1. would a flat face rare earth magnet perform well in this application? I would like to increase the amperage output. Question 2. would it be better rewind the stator with more wire on the poles ot run it through a step down transformer to achieve that objective? Question 3. is there anything to be gained by rewiring the poles for two or three phase output? Finally what device or method do these style alternators use to regulate the voltage?--------Phil

 

[itsmoked]

I cannot answer most of these questions though someone here surely will. I will just point out that the stator structure is designed to carry a certain amount of magnetic flux. Amping up everything may not get you what you want. Increasing the frequency,(turning it faster), would allow a greater flux to be handled and would make the alternator "higher output". Stronger magnets and different 'electrical' changes by themselves can only get you so far,(i.e. probably not far).

You do need to consider the rotor structure as you speed it up.

You might consider using a standard automotive alternator. Typical junkyard fare puts out more than a kilowatt at 15V, 80A. The rotor is specifically designed to spin very fast. (20,000RPM)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[OperaHouse]

Those questions could be better answered by some of the people at

http://www.otherpower.com

message board. They build these all the time.

 

[waross]

Hi SMOKEY44211
This is one of those "It depends".
We have no idea of the design parameters of your equipment.
A few factors to consider.
Changing magnets;
If the radius is wrong you will have excessive air gaps that will negate any advantage of stronger magnets.
You need to consider not just the face of the magnet passing the stator but also the back face where the magnet contacts the iron supporting structure. The magnetic circuit does not like air gaps, be they behind or in front of the magnets.
If your magnetic circuit is near saturation, and it may be designed that way to economize on materials, you won't gain much from stronger magnets.
As far as rewinding, reconnecting, three phase, etc. I hesitate to make suggestions without seeing your setup.
I have seen a lot of alternators on small engines intended for battery charging that take a lot of design shortcuts. A recommendation that is valid for all conventional industrial alternators would not be appropriate for some of the small engine alternators I have seen.
Tell us a little about your application, what do you want to power?
If you give us the make and model of the 20HP engine, someone may Google up some drawings and be able to give you accurate information rather than guesses.
yours

 

[SMOKEY44211]

Many thanks to all that have responded so far. The stators were intended for use in a Koehler command pro 20HP. The 15 amp version is the same one used on Onan 16HP. and 18 HP. models. I believe the Briggs+Stratton vanguard 20 HP. model uses it also and I am sure there may be others that I am not aware of. The 15 amp version measures 5.540" diameter .400" thick lamination stack. There appears to be 14 turns of .056" wire on each pole. The 30 amp version (I have only found it on the Koehler products) has the same diameter as the 15 amp. measures .567" thick, and has 19 turns of .056" wire. There is a curiosity on that one as there is a single strand of the continuous wire that weaves in and out of 9 of the poles (does not wrap around the poles) which is 1/2 the stator and continues back in the same weave pattern and exits out.( I may be able to get a clear enough picture to post. Difficult to describe). Rectifier/regulator assemblies run about $30-$40 for the 15 amp versions. The 30 amp one lists out around $180. I have found one intended for a 45amp Harley-Davidson for around $100. The magnets that I have are 1" square and .125" thick. I could get thicker ones and grind in the radius needed. This appears to be a very time consuming proccess to avoid heating the magnets and if the gain is only marginal I'd preffer not to. The project that I am working on is an automotive 4 cylinder with minimal accessories. Electric fuel pump (8 amps) and ignition system (3 amps) make up the constant draw items. Illumination system, (10 amps) heater fan, (10-15 amps)and wipers (10 amps) make up the bulk of transient draw. I think I can squeek by with 30 amps but 45-50 amp capability would add a margine of insurance. My reason for attempting to use a brushless style alternator is that the space normally allocated for original equipment one is needed for an accessory item that is key to the success of the overall project.-------Phil

 

[waross]

You can find a manual here

http://www.kohlerengines.com/service/manuals/manuals_results.jsp

Be aware that the magnets do not act independantly.
The magnets and the supporting flywheel are an engineered unit. The flywheel serves as the return path for the flux.
You may consider using a section of the Koehler flywheel with magnets attached instead of buying new magnets.
That will save you from having to determine the magnetic properties of the material on which you are proposing to mount the magnets, and designing the new magnetic circuit.
Output; You can increase the output by rewinding the poles with heavier wire. When you do this, you will not be able to get as many turns of wire on the pole. As a result you will have to be turning at a higher RPM before you get a usefull output.
Your regulator appears to be a shunt regulator. You will probably destroy it if you rework your alternator for more output.
You can stack two stators to increase output, but then you have issues mounting a double row of magnets.
Check thread237-145132 for some of the issues involved in building and regulating alternators.
There is probably much less work involved in finding a way to mount an automotive type alternator than in designing and building a flywheel alternator. The result will also be much more predictable.
yours