Test Coil Voltage

[egreen]

If I test the voltage of a single coil using one magnet rotor and get 0.6v AC at 60 RPM will I get 2 times that voltage (1.2v AC) when the second rotor is in place at the same RPM?  The coil is 0.5 inch thick and the each magnet is 0.5 inch thick (1 inch thick for 2 rotors).

Thanks

[Flux]

Who knows!

It is likely that you will roughly see a doubling of voltage with the second magnet rotor, but the flux path of a single rotor is not defined and the voltage you get will depend critically on the gap between the magnets and coil.

It is unlikely that you will see much flux at the far side of a 1/2" coil with 1/2" thick magnets so as you increase the distance from magnets to coil you link fewer turns.

I certainly wouldn't base any final construction on data from a single rotor and apply it to a dual rotor. I think it highly likely that you will end up with a cut in speed that is not very close to what you want and although it may work well enough you may miss the optimum by a fair bit.

Flux

[wooferhound]

I would not measure the AC voltage coming from the coil. When I built my Genny and did the coil testing, I ran the output of the coil through a Bridge Rectifier with a 1500mfd capacitor and a 1000ohm resister across it's output. This will much better represent the voltage that will be going to the battery. Not to mention that many voltmeters do not accurately measure AC voltage unless the frequency is around 55 hz.

[egreen]

You make some very good points Flux.  I have both magnet rotors ready to go but I don't have the jacking bolts ready yet so after I made my first coil last night I was just really excited to see what kind of voltage I was getting by using one rotor.  I'll get the 2 rotors mounted and take it from there.

Thanks

[egreen]

I had a problem getting a consistent AC voltage reading when I used a digital multimeter.  However, my analog multimeter seems to work well on the 3v AC setting.

I have a bridge rectifier that I can hook up.  However, since I am only getting 0.6v AC can I even hook up the bridge?  I thought that I needed at least 1.4v (consumption of the diodes).

I understand that the resistor is there to introduce a load (like a battery would), but what is the capacitor for?  I'm guessing that I won't need a capacitor when I'm all finished and have the batteries hooked up.  Or will I?

[Flux]

I will be interested to see Woof's comments but this is just my thoughts on this.

The method is not really suitable for very low voltages where the diode drop is a significant part. I would not be inclined to try it with coils giving less than several volts.

There can be errors from cheap multimeters on low voltage ac but at least they should compensate for the diode drop by scale shape or at least using different types of diode. A silicon bridge will read virtually nothing up to about 1.5V peak.

The capacitor will make it read peak volts, which is about cut in volts from a single phase supply ( all you can get from one coil). As long as you realise that it is peak and don't include the rms to peak factor again then it is ok but you will still need to figure the star factor for 3 phase. A bleed resistor across the capacitor will prevent it reading the highest peak especially with hand cranking but the load must be relatively negligible or you will no longer measure peak. There is no sense in trying to add any form of load as that changes the whole situation.

This would be a convenient way for checking a single phase stator for cut in but I would be reluctant to suggest it as a way of determining things from a single test coil with fairly low volts. For a 3 phase stator feeding a rectifier the capacitor is not needed. Like all other things there are many ways to do a given job, all will work if you work within their limitations and you know what you are doing. Trouble comes when people try to combine parts of different methods.

Flux

[electrondady1]

 i have had good results from testing with just one rotor .

it's easy to simply place a piece of cardboard over the rotor and spin away.

it's not far from being 1/2.

 

[egreen]

I assembled the alternator with both rotors with the test coil (42 turns) and the voltage changed from 0.6vAC (one rotor) to 1.0vAc. Unfortunately I calculated that I need one coil to produce 1.7vAC.  Guess I'll have to add more turns.

A few questions:

1. In order to add more turns I'm going to have to make the inside of the coil smaller (smaller than the area of the magnet).  Will this hurt the voltage output?
   
2. Will it hurt the coil output if the magnet passes the top and bottom of the coil instead of just the legs?
   
   

The magnets that I'm using are N42 (2" X 1/2" X 1/2") on 8" diameter rotors (16 magnets per rotor w/ 12 coils).

[Flux]

# In order to add more turns I'm going to have to make the inside of the coil smaller (smaller than the area of the magnet).  Will this hurt the voltage output? "

No you will still gain extra volts but you may have to add an extra turn or two over what you calculate from the voltage ratios as some turns will not link all the magnet flux.

 "Will it hurt the coil output if the magnet passes the top and bottom of the coil instead of just the legs?"

Again it will not hurt but you are linking even less flux with some turns. I am not sure why you need to restrict the radial dimension, it doesn't matter so much at the coil centre, but I can't see why there is any need to do this on the outside.

If you are almost there then just squeeze the coil in the circumferential direction and make the hole a bit triangular to gain a bit more space at the centre.

From your results with the second rotor I suspect your stator is a bit on the thin side and the other option would be to widen the air gap and wind the coils thicker to gain extra winding space. You would again need more turns that calculated from the voltage ratio as you will lower the gap flux. Probably a bit of both may prove to be the best way. Squeeze the hole a bit triangular to gain space at the centre and wind the coils a bit thicker.

Are you sure about your 1.7v?

If this is 16 pole you have 4 coils. At 1.7v per coil this gives 4 x 1.7v phase = 6.8v

If star connected the line voltage will be 6.8 x 1.73 = 11.76 v

Dc volts will be 11.76 x 1.4 = 16.4

This might be ok for a fully charged battery, allowing for diode drop, but you don't really want to get best performance into a fully charged battery. The chance of stalling is much less at 12v with high line and rectifier losses but generally people go for too low a cut in anyway so I think  aiming for 1.3 v per coil would be nearer the mark.

If you go for the 1.7v then aim to get it with the minimum safe air gap, then you have the option to increase the gap and you are safer mechanically.

Flux

[egreen]

I was going to make the stator 1/2" thick.  But now that I need more turns I'm going to make the stator 3/4" thick, this way I won't have to make the center of the coil much smaller.  Is a 3/4" stator okay if the total magnet width is 1" (1/2" on each rotor)?  I think that I'll try for 1.5vAC per coil, so I'll have to go from 42 turns to about 64 turns.  This should result in (1.5 * 4 * 1.73 * 1.4) - 1.4 = 13.1vDC.

Thanks for the infomation on the coil size and shape.  Turns out that the jacking bolts are only 3/8" from the bottom of the magnets (8" rotors don't leave much space).  Because of that I'll have to move the coils out a little bit which will cause the bottom of the coil to pass over the magnet face.  That's why I was asking about the top and bottoms of the coils in my previous post.  I'll keep the top of the coil out of the path of the magnets.

[Flux]

You are right the 8" rotors with 2" long magnets will cramp things badly at the centre, it is worse with 1" wide magnets.

The 3/4 " thick stator will be ok. With the narrow 1/2" wide magnets it is about as thick as I would go with the air gap at 1" the flux will be starting to fall below 600mT but it will still be ok.  The ideal air gap for wide 1/2" thick magnets is a bit under 1" and with decent mechanical clearances that lets you use about 5/8 thick stator. Narrow magnets tend to drop flux density quicker with increase of gap.

As long as you can get the voltage then all will be well. There comes a time with a given wire size that squeezing more turns in is offset by having to increase stator thickness and you end up not managing to get the result but in your case you should manage it.

Flux

[egreen]

I'm not sure why it took so many turns (72) but I finally achieved 1.5vAC.

Since I'm short on space on the inside of the stator is it okay if I make the coil connections on the outside of the stator?  The "How To's" that I read on otherpower.com show the coil connections made on the inside of the stator.

[Flux]

Yes, I always do it on the outside.

Flux