Author Topic: Electrical Gurus... Is it possible to convert a starter motor into a generator?  (Read 21944 times)

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Offline Ricky_Racer

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That pretty much sums it up. Can it be done? If so, what must be done? Thanks!

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Offline GammaFlat

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From a theoretical standpoint, it depends - I knew that would raise a few smiles. 

As you're probably aware, an alternator (careful with terminology here, not a generator) and a generator require 3 things to make power, coiled wire, a magnet and relative motion.  By definition, electric motors, whether ac or dc (starters are dc) can become creators of moving electrons.  The complexity we're gonna get into real quick is what kind of field you have.  The field is usually the rotor portion of a generator or alternator - just think rotor/rotate.  Stator is the coil of wire we talked about earlier - think stator/stationary. 

The rotor is sometimes a permanent magnet and sometimes an electromagnet.  In the case of our alternators, it's an electromagnet (w/3 phases) and needs energy to make a magnetic field.  whew.... That's why we can't push start these babies with a completely dead battery or no battery at all.

Starters... hmm, I don't know how our starters are made but I suspect they have an "energized" rotor to create the field. 

Back to theory, DC motors make DC power when spun if they have a permanent magnet or have an energized rotor (if it has an electromagnet rotor).  Alternators make AC power which is why we need rectifiers to "rectify" the power to DC or stop it from alternating - so to speak. 

Are we all on board here? 

More to follow if anyone cares :)    .. and I'm sure TT can clarify well. 
« Last Edit: April 05, 2009, 08:17:53 PM by GammaFlat »
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Offline Spanner 1

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I think it's called a 'dynostarter"....also not efficient above a 50cc bike, if I remember correctly !
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Offline GammaFlat

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I think the theory is good.  Why have 2 similar devices hooked to your engine when you might be able to get by with 1.  I'm thinking you're into at least one bad issue right away.  A starter needs to be very high torque (turn a lot of electricity into physical energy) while an alternator or charging device has a much different "mission profile".  It wants to take a small amount of physical energy and convert it into electricity. 

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Offline Ricky_Racer

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Okay, let me extend this a little bit... My focus is on whether it is possible to deep-six the existing charging system and convert the existing starter (with full time drive) into a "generator."  Since I'm looking at a magneto ignition I'm not concerned about the peculiarities of the existing ignition system.  

I'm assuming this will also be a kick-only application so dual functionality isn't part of the question.

This is one of those "what if" threads. Hopefully it will lead someplace interesting. Thanks.  RR



« Last Edit: April 05, 2009, 08:46:37 PM by Ricky_Racer »
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Offline GammaFlat

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I'm sure there'll be plenty more to the story but these are my initial thoughts and I'm not an expert...
1) mechanical connection - starters need a real good solid (often momentarily connected gear driven) connection to your engine.  This can cause reliability issues if you try to convert a starter to an alternator.  Starters will suck friction energy off of your engine and may have reliability problems since their bearings and that physical connection are not engineered to spin so much.

2) Alternators are not engineered to make so much torque if converted to a motor.

Now we need TT.  What does it take to make an alternator behave differently under different conditions - be a chameleon.  My suspicion is that a large portion of the "starter" would not be required for being an alternator (or generator). 

Cars and motorcycles have been made for a long time without much being made of a dual purpose device like this.  I'll go back to the notion that the mission profile or "amount of energy being transferred" is so different between these devices (starter, alternator) and purposes (starting, supporting electrical system and charging battery) so different, that you'd have a hard time making a "nice marriage". 

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Offline 754

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RR, one of my pet projects is to mount a Nighthawk alt off the left side (looks like a CBX one)

A spec holding me back is figuring out ratio, I think it is overdriven.. will find out in next week i hope (see other bikes thread)

At any rate, looks like it will be gear or belt drive, its 1/2 the weight of the 750 parts or less

If I can find room I could run belt drive for it, and my magneto.. matching belt drives, both ends of crank. Plays hell on having my high pipe. >:(

But the big plus will be, with a belt idler & by loosening a bolt you can pull belt off and stick in your pocket to race... resulting in at least 3.2 lbs off your crank... :o.. and a lighter assembly the rest of the time. ..

Way better ground clearance on left side too.
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Online bryanj

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Electricaly a starter could be used as a Dynamo (DC Generator) but you need a totaly different type of regulator

Mechanicaly no it cant as a starter turns for a short time at high torque so the armature (rotating bit) runs on brass? bushes which are ok for short stints but a dynamo needs real bearings as it spins for a longer time
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Offline TwoTired

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The SOHC4 starter has a field coil and an armature.  They are normally wired in series, and both make magnetic fields.  The field pole pieces are stationary, and the armature poles reverse polarity as different commutator sections pass under the brushes.  The force from N-S attraction and like poles repelling is what induces armature rotation and creates torque.

Yes, the starter can make power, by breaking the series connection internally between field and armature, and using a separate output connection to each brush holder.  Then placing a voltage on the field coil(s) to create a magnetic field.  Spinning the armature in this magnetic field will induce a voltage in the armature that can be tapped off from the brushes.
I don't know how efficient this would be.  There is a brush to commutator phase relationship with the field coil poles pieces that I haven't analyzed.  I don't have an unattached starter motor to examine and measure.
Anyway the output would be DC so no rectifier would be needed.

Practical considerations:
The starter drive is normally through a one way clutch.  It keeps the starter motor armature from spinning unless power is applied to it in starter configuration.  It also keeps the engine from driving the motor armature to speeds where centrifugal force would make the armature winding fly out of capture.  With a starter loaded, it spins about 3200 RPM, It drives the crank at a 22 to 1 ratio.  If this ratio is maintained by direct coupling, the starter motor would turn 22 times faster than a 10,000RPM crank speed, (220,000).  The starter no load speed rating is 11,000 to 20,000 RPM.  I doubt it would survive 220,000 RPM.

Brush life was chosen for intermittent duty starting applications.  I expect constant armature spin operation would require frequent brush replacement.

Full field strength during starter operation saw current in the 120 AMP range at 12V, so the field wires in the stater are likely to be a pretty heavy gauge. 

Theoretically, the armature could deliver 120 AMPs of power with the field at full strength.  I don't know what the field resistance is for the starter motor.  You would have to select a field coil regulator that could handle the currents demanded by the starter field coil.

I learned from my bike mechanic buddy that the CBX uses a slipper clutch on it's alternator drive to keep from over stressing/over-winding the alternator.

I think I stayed up too late for this post.... Might have to edit it tomorrow...but now...must sleep....



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Offline Inigo Montoya

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The lovely thing about a dc motor is that it can be a dc generator as well. ANY dc motor can do this if it uses a permanent magnet. Now the question is how much. There are a lot of things that will affect this.
The # of wire turns on the armature.
The SIZE of the wire on the armature.
The timing of the brushes in relation to the magnets might but I cant remember for sure, I know it affect rotational speed for forward and reverse though.
Brush composition, brushes composed of more silver than carbon will allow more power flow.
Is the motor bushed of bearing? A bearing usually rolls easier than a bushing.

These all affect the amount of output. they also affect the motor when being used as a motor.
Bearings and brushes will affect life span.

I always liked rewinding motors and trying different brush cuts. You have to be careful not to crack the clear insulation off the wires though. Brush cuts can dramatically change the operation of a motor. Certain cuts will wear the commutator faster and the brush will always wear out sooner. While not much of an issue anymore, 60hertz would cause a lot of arcing on brushes and sometimes cutting the brush in half would help remove some of this arcing.

Motors are fun! ;D

Offline Ricky_Racer

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Now this is getting interesting ... Deep, but interesting.

754, I think you and I are on the same page. I'm looking at a future setup incorporating a rootes type blower driven from a toothed pulley on a flat cover plate on the dyno side, with an RC/Vertex magneto on the points side.

I had assumed that driving a standalone small alternator would be the key for running lights, but I was uncertain which alternator to use. The CBX-type might be perfect.

A friend posed the question about either mounting a small (Anybody know one?) alternator into the existing starter cavity or converting the stock starter into a generator. Thus this thread.

Since there is reasonably broad interest in reducing rotating crank weight by cutting down the existing rotor, I think this thread might be of interest to the community.

Plus, hell, we're all here to learn something new from some great teachers, anyway. Aren't we?   ;D  RR

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Online bryanj

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Here in Europe we have way smaller cars rhan you Yanks and a small generator would be relatively easy to find in a breakers, but small enough to go in the starter hole---probably not.

Big thing that killed the Dynamo in favour of the Alternator was current flow through the brushes---Big current=big brush area= higher drag and wear, Lucas made 60 amp Dynamo's for cars years ago but you had to check/clean/replace brushes regularly, then along came reliable diodes and you could build an alternator with tiny brushes carrying light current that lasted as long as bearings did, usually and as a bonus gave full power at lower revs.

A lot of 24 Hour bike racers fitted car generators run by a belt off the crank to provide the extra power needed for decent headlights so it has been done before
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Offline Inigo Montoya

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Brush composition does play a part but different compounds affect the wear still.

Offline Sporkfly

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What about running a pulley from the starter hole via the available gear to run a small alternator? Just throwing a silly idea around  ;)
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Offline Jonesy

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Wisconsin Engine (and I think Briggs & Stratton as well) built tractor engines that had a combined starter/generator that was belt driven via the crankshaft.
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Offline TwoTired

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Wisconsin Engine (and I think Briggs & Stratton as well) built tractor engines that had a combined starter/generator that was belt driven via the crankshaft.
What was the crankshaft RPM range?
« Last Edit: April 06, 2009, 04:51:03 PM by TwoTired »
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Offline Sam Green Racing

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Now this is getting interesting ... Deep, but interesting.

754, I think you and I are on the same page. I'm looking at a future setup incorporating a rootes type blower driven from a toothed pulley on a flat cover plate on the dyno side, with an RC/Vertex magneto on the points side.

I had assumed that driving a standalone small alternator would be the key for running lights, but I was uncertain which alternator to use. The CBX-type might be perfect.

A friend posed the question about either mounting a small (Anybody know one?) alternator into the existing starter cavity or converting the stock starter into a generator. Thus this thread.

Since there is reasonably broad interest in reducing rotating crank weight by cutting down the existing rotor, I think this thread might be of interest to the community.

Plus, hell, we're all here to learn something new from some great teachers, anyway. Aren't we?   ;D  RR



Hey Ricky, I remember a guy that tried to do something similar with a supercharger, his name was Mr Wankel. ;D ;D ;D ;D ;D ;D

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Offline many408

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I think it's called a 'dynostarter"....also not efficient above a 50cc bike, if I remember correctly !

MV used them on the 600 and 750 (really 789) America.

Offline Jonesy

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Wisconsin Engine (and I think Briggs & Stratton as well) built tractor engines that had a combined starter/generator that was belt driven via the crankshaft.
What was the crankshaft RPM range?

If it was a Briggs, it was probably 800-3600 RPM, give or take a few hundred RPM.

Here's a Wisconsin engine with the setup I'm talking about:


Come to think of it, the very first electric starter for automobiles was setup this way.
« Last Edit: April 06, 2009, 05:38:04 PM by Jonesy »
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Offline Inigo Montoya

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Thats an old timer!

Online bryanj

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Yamaha and i think Suzuki used them on 200cc ish two strokes but the brush wear was bad (and expensive)
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Offline 754

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RR, depending where your blower body ends up, you can probably fit it between crank & blower, or turn it outward for more room.

IT = CBX / Nighthawk style Alternator. Will try to get mesurements, they are short. I took, the rotor(2 parts). coil or stator,  outer cover and regulater, off the donor bike.

Cutting the rear part of starter cavity off case, will gain some room if needed.
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Offline Ricky_Racer

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Thanks, 754. I should have enough room under the huffer to mount the alternator on the blower mounting plate, perhaps doubling as an idler/adjuster. Alternatively, I've looked at hanging it off the front of the engine but that's a lot of road junk to be throwing at an alternator.

Have you looked at the Goldwing alternator? It seems to be a favorite among small frame units and they're readily available in the salvage yards.

I found this link that looks interesting, but I'm currently unsure of the prices or the size comparison with the CBX or Goldwing alternator:  http://www.racemettleltd.co.uk/alt.html.

I also ran across this: http://www.practicalmachinist.com/vb/blog.php?b=143  What do you think?  It's only 54mm x 78mm but is limited to 100W (about 8A) output.  What is the minimum amperage needed to light one of our bikes, assuming normal lights?  Assuming LED lights?

Could we live with 8A if the headlight isn't being used? If it takes about 5A to keep the battery charged, I'm not sure the tiny alternator is going to get the job done.  Opinions?  Thanks!  RR

« Last Edit: April 07, 2009, 11:27:27 AM by Ricky_Racer »
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Offline johnny_from_bel

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Theoreticaly motors and generators are the same. I used old dynamos as DC engines. I could get them for next to nothing from the scrap heap. Works just fine.

A starter motor is made to crank out as much has 700W for like 10 seconds or so. It is not build for contignous use. Also it is a series engine. Its output will vary a lot with load. Dynamos are of the shunt type.