Convert the generator of a motorbike to a starter - generator

Thread Starter

KurtVD

Joined Nov 2, 2019
10
Hello,
I don’t know if it’s possible, I hope together we can find out if and why. It’s a small displacement motorcycle (100cc), and it has successfully been done before (links further down), but my research so far seems to indicate that my generator doesn’t have enough power, and not just by a little, but like by a lot not enough: someone calculated for me that i need about a 1000 Watts (since the alternator sits on the crankshaft, there’s no gearing, unlike with a traditional starter) and that my generator won’t be stronger than a 30 Watt motor.
How did he get those numbers? I don’t exactly know how he got the 1000 Watts needed to crank the engine, but it doesn’t seem outlandish. The 30 Watts at low rpm are an estimate we made, knowing that the generator is barely able to power the 12V30Watts headlamp at idle (it’s wired directly to the alternator and the bulb is just on, very very dimly, at idle. It needs almost about 1500 rpm to shine at full brightness).
Here’s the part I totally don’t understand: the generator can’t possibly be 33 times too weak than it theoretically needed to be, I guess I must take the voltage into account before I can compare these number, but how?
Then, I have found out that there have been a few Japanese motorbikes that used such a starter generator, in the 1970s. Those were small displacement bikes just like mine (100-200), and apparently while the starters weren’t the best (strongest) starters, they were reliable.
I know that these bikes were using generator-starters with electromagnets, while mine has a permanent magnet, but I guess that’s not a disadvantage for this project? Also, the alternator in the pictures had been converted to 3-phases, what difference does it make for the conversion? Because obviously, I could just as easily get an unmodified one. Other than that, I can’t see what so special about the generator-starters in the diagrams, that it couldn’t be done again, relatively easy. But maybe that’s because I don’t enough understanding, so I hope someone can help me out.
Here are the pictures of the alternator i want to convert:
https://postimg.cc/gallery/f7siwac0/And here’s a link to a page with diagrams for the starter-generators of some Japanese bikes
http://www.dansmc.com/genstarter.htm
 

shortbus

Joined Sep 30, 2009
7,091
It is much easier to convert a generator(dynamo) to a motor than making an alternator to a motor. This is because of both the starter motor and generator haveing brushes, where as your alternator has no brushes.
 

Thread Starter

KurtVD

Joined Nov 2, 2019
10
It is much easier to convert a generator(dynamo) to a motor than making an alternator to a motor. This is because of both the starter motor and generator haveing brushes, where as your alternator has no brushes.
I see, I didn’t know that. As I’ve said, this means I would be working with this same generator in an unmodified state. I suppose (I haven’t seen the unmodified one, the modification was done by my mechanic) that means it would have brushes too.
So if we assume I have a 2-phase generator of the same size, what does that mean to answer my questions?
 

Thread Starter

KurtVD

Joined Nov 2, 2019
10
So I have been informed (by short bus) that it is much easier to convert a generator into a motor-generator than an alternator. Which means that I would use -for my project- the same model of alternator as shown in the pictures, but before its conversion, when it was still a 2-phase generator.
 

shortbus

Joined Sep 30, 2009
7,091
The starter-generator is a ccommon thing on older tractors and some riding lawn mowers, it uses full battery power when starting and is regulated to a lower power when running and generating to recharge the battery.
 

shortbus

Joined Sep 30, 2009
7,091
Which means that I would use -for my project- the same model of alternator as shown in the pictures, but before its conversion, when it was still a 2-phase generator.
Was your alternator ever a generator? Can't tell from the picture, but many or most small bikes like that have a magneto type electrical system. Using the magnets on the flywheel/rotor to both make electricity to charge the battery and to generate the spark. You can tell if that is what you have by being able to kick start the motor with no battery connected.
 

Thread Starter

KurtVD

Joined Nov 2, 2019
10
but many or most small bikes like that have a magneto type electrical system. Using the magnets on the flywheel/rotor to both make electricity to charge the battery and to generate the spark. You can tell if that is what you have by being able to kick start the motor with no battery connected.
Yes, in this case it must have a magneto type electrical system, and not a generator. I didn’t know that there’s a difference, I’ve just begun learning about different types of electric motors (and now generators). I do know that my mechanic has converted it from 2 phases to 3, because she told me so. I don’t what difference that makes however. Also, she said she could install up to 8 coils instead of the 2 that are installed at present. But here too, I have no idea if that could make the conversion any easier.
 

Thread Starter

KurtVD

Joined Nov 2, 2019
10
The starter-generator is a ccommon thing on older tractors and some riding lawn mowers, it uses full battery power when starting and is regulated to a lower power when running and generating to recharge the battery.
Interesting, I didn’t know. However, for my purposes, I’d like to focus on the starter-generators that were used in the motorbikes I mentioned in the OP. What would I have to do to convert mine into something similar, about equally as powerful? Or is there a reason why this can’t be done?
 

Thread Starter

KurtVD

Joined Nov 2, 2019
10
Think she's pulling your leg. Unless she added another set of coils.
Unlikely, there’s really no incentive for her to do that - it’s much more likely that I’ve misunderstood something, since this is a whole ‘new world ‘ for me. She did mention that she can install up to 8 coils instead of the 2, but I haven’t asked to what effect or purpose.
Because it's not a generator it's an alternator, and has no brushes.
But aren’t there brushless motors? And there are also permanent magnet motors, so would it be possible to convert this into a motor-alternator by using a controller or something (I think it’s called motor drive)?
 

Alec_t

Joined Sep 17, 2013
10,470
But aren’t there brushless motors?
Indeed there are. Three coils about the same physical size as your present one could form the basis of a BLDC motor, but there are a couple of snags: (1) there isn't room for them within the confines of the flywheel and (2) they would almost certainly not be able to carry enough current to provide the necessary torque to overcome cylinder compression.
 

Thread Starter

KurtVD

Joined Nov 2, 2019
10
1) there isn't room for them within the confines of the flywheel
Apparently, this workshop modifies this type of alternator by installing up to 8 coils, but obviously smaller ones. Does it have to be exactly 3, or is 3 just the minimum?
they would almost certainly not be able to carry enough current to provide the necessary torque to overcome cylinder compression.
Well, this brings me back to my question in the OP: How did Yamaha & Suzuki manage to get enough torque out of their generators to start engines reliably with up to 175cc (mine is only 100)? From what I’ve read, a brushless motor would rather be slightly more efficient than the design they have used, so I’m trying to make sense out if it...
 

Alec_t

Joined Sep 17, 2013
10,470
How did Yamaha & Suzuki manage to get enough torque out of their generators to start engines reliably with up to 175cc (mine is only 100)?
If you look at a typical starter-generator, e.g. as per the post #1 link, I think you'll find it's a lot bigger than would fit inside your flywheel. The one shown there has a lot more coils and poles than 3 (which is a minimum).
There are various trade-offs for a given available space, e.g. : more coils = smaller coils, thicker wire = higher current but fewer turns.
 

Thread Starter

KurtVD

Joined Nov 2, 2019
10
If you look at a typical starter-generator, e.g. as per the post #1 link, I think you'll find it's a lot bigger than would fit inside your flywheel. The one shown there has a lot more coils and poles than 3 (which is a minimum).
There are various trade-offs for a given available space, e.g. : more coils = smaller coils, thicker wire = higher current but fewer turns.
I’ve researched it as good as I could, and it seems that the diameter of these existing starter-generators is about the same as my alternator, maybe a little bigger, but not by much, however they are twice as deep inside (or long?). But they are 2-cylinder engines with almost double the displacement, so I guess they also need more power to crank than my single cylinder. Although, on the other hand, they are 2-cycle engines, so maybe that makes a difference (i.e. less cranking power needed), but I don’t know that.

All this to say that while I also believe that it won’t have enough power, I don’t think it’s by a lot.

So hypothetically speaking, or if I just wish to try it anyway, even knowing that it won’t work at the end, what are the ‘big steps’ I would have to do? Would it be best to start by installing 8 coils? Are there coils available that can carry more current (at equal size) that I could use? And concerning the controller/motor driver: I think a VFD (driver) could work in this setup, is that correct? Are there VFDs available in a small form factor, for this voltage and current requirements?
 

Alec_t

Joined Sep 17, 2013
10,470
what are the ‘big steps’ I would have to do?
I think the first step would be measuring just how much torque is needed to turn the engine over, how much torque your single coil generates if you put 3A through it (given that 3A seems to be its normal current when powering a lamp at full brightness), and finding the minimum shaft rpm to get the engine to fire.
Are there coils available that can carry more current (at equal size) that I could use?
I've no idea. You'd have to research that, e.g. by checking on motorbike forums and at parts suppliers. But simply carrying more current isn't enough. The number of turns on the coil is also important for generating a sufficiently strong magnetic field. And how would you fit extra coils of the existing size in?
I think a VFD (driver) could work in this setup, is that correct?
In principle, yes. An ESC, such as used in remote-control models, might do the trick. But until you do the measurements you won't know the current requirements.
 

shortbus

Joined Sep 30, 2009
7,091
Guess I'm just not up with the times and old, what seems to be so hard about kick starting a 100cc moped?

I’ve researched it as good as I could, and it seems that the diameter of these existing starter-generators is about the same as my alternator, maybe a little bigger, but not by much,
The other problem comes from the alternator of your engine being directly built on to the crankshaft, and a generator equipped engine has a gear drive, the gearing will help produce more torque to turn over the motor.
 

Thread Starter

KurtVD

Joined Nov 2, 2019
10
The other problem comes from the alternator of your engine being directly built on to the crankshaft, and a generator equipped engine has a gear drive, the gearing will help produce more torque to turn over the motor.
I literally mentioned that in one of my posts, maybe even my opening post. It’s true for a traditional starter motor. But if you opened the second link in my OP, you would know that this isn’t the case with these examples.
I guess I have to thank you that you take your time to participate, but if you don’t read what I wrote beforehand, and I just have to repeat the same things all over again, it’s a bit tedious. I suppose you’re doing that because you think it’s a dumb idea, as you state. Fair enough, i can understand that. But then I don’t get why you even answer at all?
 

shortbus

Joined Sep 30, 2009
7,091
if you don’t read what I wrote beforehand, and I just have to repeat the same things all over again, it’s a bit tedious.
Exactly, I'm not judging if it is a bad or good idea, just trying to get it through to you that it would, using what you have now to do it. Trying to get you to understand the difference between an alternator and a generator/motor. The link in your first post to dansmc says it all, and you can't or don't want to accept it.
 

Thread Starter

KurtVD

Joined Nov 2, 2019
10
In principle, yes. An ESC, such as used in remote-control models, might do the trick. But until you do the measurements you won't know the current requirements.
I guess this won’t be easy at all, finding a fitting controller.
The number of turns on the coil is also important for generating a sufficiently strong magnetic field. And how would you fit extra coils of the existing size in?
That was a misunderstanding, extra coils would have to be smaller. So obviously there would be a trade-off, I don’t know what that would mean, more but smaller coils.
 
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