Best way to get high current from dirtbike magneto? 3

[tabletop]

Hi, I have struggled with this problem for many years now and Im almost ready to conceed defeat...
I want to generate aprox 12 amps @ 12VDC average from my dirtbikes for a powerful headlight (3 50W halogens). At least 5 Amps at idle. I can easily generate say 10 amps at 12V even at idle by rewiring the stators to suit but the problem is regulating the voltage when the RPM goes up. At around max RPM I end up with over 150Volts. Other than it being lethal voltage/current, no bulb will work over such a broad voltage range. I have got around it in the past by using bypass power mosfets around a (special hi input voltage) 7812 voltage reg. Works, but the 7812 is not durable enough.
btw, yes I know there are aftermarket solutions but they are very expensive and still quite low output. Also I enjoy the whole DIY challenge. I dont want to run a battery so a (self regulating)alternator is out of the question.
There must be a better way. I have been thinking of ways to regulate by way of limiting the induction with rpm increase.
I have designed a very simple mechanism that increases the gap between the stator coils and the magnets in proportion to RPM rise (using centrifugal force). Can anyone tell me (before I build it) will increasing the gap with RPM rise lower the *VOLTAGE*? Im pretty sure it will lower the current, but I can increase wire size and add winds to compensate. Will this work, or is there a better way still?
Thanks.

 

[waross]

I have seen zener diodes used on bikes to regulate the voltage for battery charging. The current rating for the maximum current plus a safety factor. Voltage rating for the voltage you want.

 

[TheBlacksmith]

Current is determined by voltage and resistance, so increasing the air gap as you propose will reduce the voltage, but then for a fixed resistance (the headlight), the current will start to drop. I think the zener diode, shunting excess voltage to a heavy duty (ballast) resister as pointed out by waross would give better regulation.

 

[jtkirb]

Most crude regulators on old bikes used a zener type device with a heat sink. Do the calcs because you could be unloading quite a bit of power through the diode.

JTK

 

[Skogsgurra]

Remember that I have seen a triac being used on Enduro bikes to regulate the voltage. Have no idea how that worked. The zener will cause a lot of losses at high speeds. Not very efficient at all.

A better approach is probably to rectify the voltage and feed it to a switcher.

Gunnar Englund

http://www.gke.org

 

[sreid]

I would think that a simple non-isolated buck type switching regulator would be the way to go.

 

[jtkirb]

As far as wasted energy at high speeds- this is a motorcycle magneto. You don't care about a wasted watt here or there.

Feed a switcher? yikes, talk about complicated. He wanted simple.

JTK

 

[sreid]

I don't care about a wasted Watt but getting rid of 138 Volts x 12 amps = 1600 Watts is undoubtedly the reason his linear regulator is failing. And you're killing two Horse Power.

 

[Skogsgurra]

jtkirb,

Did you really do your math?

Ever heard about NS Simple Switcher? It is really simple - at least compared to the problem of getting rid of 1.5 kW dissipation in a bike. Don't forget that the heat sink gets covered with dirt - not an easy thing to handle...

Gunnar Englund

http://www.gke.org

 

[tabletop]

Thanks for the replies folks!
I dont think there are zener diodes with zener voltages around 12V that will happily see a voltage swing from 12 to 150 volts. Same goes with switching regs. Please, Im happy to be proven wrong.
Yes, efficiency is not important. The energy will be wasted anyhow, such a shame I cant use another bulb as a ballast!

 

[waross]

That's not a bad idea.
Better ilumination and remember that any current that goes throught the lamps does not have to go through the zener. The normal voltage range is more like 13 to 14 volts. (For battary charging) The zener limits the voltage to 13 or 14 volts, that's the point. It loads the dynamo up so that increased speed causes increased current at the same voltage. The increased current causes an increased internal voltage drop in the dynamo. The excess energy is mostly wasted as heat in the dynamo windings. That's why the system is only used on small dynamos. Size the diode for full current so you don't over load the diode if the lamp fails or is removed.
Also, remember that the dynamo is probably generating AC that is then rectified. We are probably looking at an impedance that increases with frequency.
Any comments or suggestions on that point?

And on a completely different note;
In my bicycle years, I had a bicycle dynamo that had a very simple mechanical regulator. The magnet could slide on the shaft. a small arm (Radius maybe 5/8" or so) was pinned to the shaft so that it could move in an arc. There was a short link from the arm to the magnet. The magnet was held in the center of the windings by a spring. The reaction arm was as close to parallel to shaft as it could be. Centrifugal force would tend to move the reaction arm perpendicular with the shaft, restrained by the spring. As the magnet moved out of the center of the coils, the voltage was regulated.
It's not very high tech or electrical, but it might be the answer to your question. It may not balance perfectly but who would notice on a dirt bike at high RPMs. I would ballast it with a lamp and try it at different RPMs and measure the voltage. Adjust the spring force and/or the weight of the reaction arm as required until you get acceptable regulation. If you get it close, then throw the zener in the circuit for fine tuning.
Respectfully

 

[itsmoked]

Let me float this...

Hook up the three lamps.
Hook up a fourth that would drag things down to say 9 or 10V when they are all on.

Hook up a voltage controlled relay that closes contacts to the fourth lamp. Let it vibrate away controlling the power to the fourth bulb.

Keep in mind that until 15 years ago all vehicle alternators were controlled by vibrating relays.

A step up would be the same thing based on a solid state relay or a transistor.

Any "waste" energy goes to providing more light.

Comments?

 

[tabletop]

waross, your bicycle dynamo reg is exectly what I have designed- regulating by reducing the generation process. I like it. But will be a last resort, prefer electrical method first.

You said: "remember that any current that goes throught the lamps does not have to go through the zener." How so? Are you suggesting I could shunt the zener into the gate of a power transistor (or relay coil)which in turn feeds a dummy/ballast bulb? Will this still regulate? My understanding of electronics is limited. Excuse the dodgy drawing:

+ O------------------
| | |
| | | Z= Zener
Z / | T= Transistor
|------T | BB=Ballast Bulb
\ H H= Headlight
| |
BB |
| |
- o------------------

1. Will this regulate?
2. If so, I'll still need a zener that will go to 150V, damn.

itsmoked; hmmm, voltage controlled relay, I like it too. Never thought of that. Just have to be careful not to get contacts wet/dirty...not so easy on a dirtbike tho.

 

[Skogsgurra]

What about this then?

A The magneto delivers a voltage that is proportional to speed.

B The frequency is also proportional to speed.

C The lamp is purely resistive.

If you chose a low voltage lamp and put an inductor in series, the current through the lamp will be determined mostly by the reactance of the inductor. Not solely, but mostly.

So, when speed goes up so does voltage - and frequency. If the lamp voltage were nil, the current would stay constant since V = k1 x frequency and X = k2 x frequency whence I = V/X = k1/k2 = constant. Now, the lamp voltage is not zero so you get a certain change in light flow. But a lot less than you would have without the inductor.


Regarding previous posting: What do you mean with "zener going to 150 V"? That is not what happens. The zener goes to whatever voltage it is designed for (say 12 V) whereupon it starts conducting. Current is determined by external voltage and resistance. Dissipation in zener is determined by current through it. But the zener diode per se never goes to 150 V.

Gunnar Englund

http://www.gke.org

 

[tabletop]

skogsgurra, I have to admit you are pushing me to the boundries of my electronics knowledge so I could be wrong but, placing an inductor in series with the lamp will reduce the current flowing as the frequency/RPM increases as you say. However I dont believe it will reduce the *voltage*. It's the high voltage that kills the bulbs.
Please, tell me Im wrong and it will in fact work as you say as this would be an EASY solution. If so, could someone please take a stab at a value for that inductor and I will begin experimenting!
Im guessing frequency would be roughly 1200Hz idle to 10000Hz peak and my voltage swing is from 12 volts to 150 volts. I want to regulate to 12 volts.

btw, what I meant was, I believe a zener diode that has a zener voltage (breakdown voltage) of 12 volts would not be able to withstand voltages as high as 150 volts. No?

 

[waross]

skogsgurra
Good point. I think some us are assuming that the dynamo includes rectifiers so that the final current is DC, but it may be AC. My old bicycle dynamo was AC. With rectifier diodes, the inductor you suggest could still be used in series with the windings ahead of the rectifier.
I would imagine that the induction of the windings acts just as you suggest to somewhat limit the current at high RPM.
I think it's worth a try.

itsmoked
That's a good theory but there is a problem with scale. We may have a voltage swing of 10 to 1 or more.
With three lights in service the ballast light would give us a load ratio of 4 to 3. Probably not enough load. How about sections of the element out of an electric clothes drier for a ballast load?
In support of your theory, I once had a bike that used a variation of your theory. The head light was always on. It must have been used as a ballast, because whenever the headlight burned out, the alternator would overpower the regulator and start blowing out the other lamps on the bike.
Riding conservatively the other lamps may last 1/2 hour or so. At high RPM's the other lamps would be gone in seconds.
Re the vibrating relay.
You would be switching the load current rather than the field current and your ballast load may have to be several times that of the working load. Before altenators, generators used the same vibrating rely control of the generator field but the field current was several times higher. Yes, I'd try it if I had the parts to hand. If I had to buy parts,however, it may not be my first choice.

tabletop
Zener diodes are available in ratings up to at least 300 watts. For a bike regulator they are connected in parallel with the lights.

- 0----------------
I I
I I Z=Zener
Z H H=Headlight
I I
I I
+ 0-----------------

yours

 

[Skogsgurra]

You are wrong there tabletop. Voltage never kills anything - current does. Reducing current makes the load live longer and probably a little bit happier.

Yes waross. I think so too (that the generator's inherent inductance does what my external coil would do). Which leads to this question for tabletop: Do you measure the voltage without having the lamp(s) on? If that is so, you are reading the no-load voltage and that is usually very high compared to loaded in these little machines.

There is another question that I think need to be addressed: The frequency in an unusually high range. How did you measure 1200 Hz idling - and 10 kHz peak? Power frequencies are usually in a much lower range - I would say 25 - 500 Hz.


Gunnar Englund

http://www.gke.org

 

[waross]

Frequency is per second, RPM per minute, 20 Hz and 167Hz.
Are probably correct. That would correspond to a two pole alternator turning 1200 RPM and 10,000 RPM respectively.
With more than two poles, the frequency will be multiplied by the number of pole PAIRS. Consider also the gearing reatio between the motor and the alternator.
yours

 

[itsmoked]

I don't want to believe that the "loaded" voltage swings to 150V...
I think that is unloaded, hence the dynamic loaded voltage is much smaller. 8V to 2? V

warross; granted the alternator field is probably smaller though I would expect that if you wind the magneto correctly so that it has just enough windings to run 3 bulbs (12.5A @12V) that another +4A load should be enough to control the voltage. I thought regulator field currents hovered around 4A. Not that I'm a fan of mechanical relays. I would use a BJT or a FET strictly in switching mode.

Hey tabletop what is the "LOADED", with three bulbs, voltage swing of your present set up?

 

[tabletop]

Ok, clearing some things up. Yes the magneto generates AC although Im more than happy to rectify if need be.
I have only measured unloaded to get max of 150 volts (on my last bike). Unfortunately I cant measure loaded because the bike I have now doesnt even have a stator coil for headlights etc. Only one for the ignition. Easy enough to include more coils though. I can tell you though from past attempts, that at idle the lamps would burn slightly dull, just open the throttle and they'd be bright, abit more and...POP.
Yes y'all were spot on, that should have been aprox 20Hz to 167Hz (got my seconds and minutes confused)
I guess Im more after a concept of how best to achieve this as I often change bikes and may want to apply it to another. It seems than the zener in parallel with the lamps is the easiest concept.
waross, I cant find zeners at digikey or rs-components above 50 or so watts. There are 'transient voltage suppressors' in 500watts+ is this what you mean?

http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=85307&Row=228408&Site=US

Thanks guys, I feel like Im finally getting somewhere.


skogsgurra- "You are wrong there tabletop. Voltage never kills anything - current does. " ...but I can put this same lamp across a 12v car battery with loads of amps and it wont die?