Alternators and gas powered charging systems

[johnrobholmes]

I am going to build a hybrid motorbike in the soon future and am wanting to go a bit further in my design. I am using a CRF50 motor and need an alternator so that I can charge a 12v battery for a lighting and charging system.

I want to use a slotless RC motor. Since these do not have a field coil I will run into the problem of having a widely varying output voltage. Does this basically stick me with using a traditional self contained alternator system, unless I want a DC-DC converter to take care of this?

 

[fechter]

You can use some kind of dc-dc converter to take care of this. It might be hard to find something off the shelf that works well though. If you recall I found some 12v switching power supplies that will work anywhere from 24v to 240v. I susupect the voltage range of your slotless RC motor may be a bit low for good results with a SMPS. A simple buck regulator will work as long as it has the right input voltage range to match the motor.

Do you know the engine's most efficient RPM?

Do you know what kind of voltage the motor will generate at this RPM? (use the voltage constant)

How much power are you trying to get out of this?

 

[johnrobholmes]

I would like a few hundred watts. 300w would be nice.

Turns out the CRF50 engine stator can be modded for standard alternator type output. This is the engine I will be using. A single phase coil is wrapped around 5 of the 6 slots and rectifier used for pulsed DC. Then they throw on a linear regulator to bring the output to 14.4v. The stator is only good for 90w though. The trick is to get it wound for at least 15v at idle, and then make sure the regulator can take the voltage at full throttle too.

Seems a bit easier to accomplish now that I have some sleep. I will call a few places and see if the 90w limit is because of the stator or the regulators used. If it is the regulator I will use a DC to DC converter with a few caps to smooth out the voltage on the input side.

 

[johnrobholmes]

I just got off the phone with a stator winding company, and they stated that 90w was the max that I could get from the stock stator. No way to get more power from the stator, and the tech couldn't tell me whether it was the stator or the regulator that was holding it back. All he could say was that the company couldn't get more than 90w out no matter what they did.


So, I am basically looking at adding another alternator or figuring out why this stator won't put out more than 90w. That is only 6.25a at 14.4v, and the stators are of decent size. A lot bigger than a hobby motor of 90w I can tell you!

 

[fechter]

It's probably limited by the winding resistance and the magnet strength. Those things typically use Alinico or ceramic magnets. You should be able to get that kind of power out of a Kollmorgen motor used as an alternator. I would avoid an automobile type alternator, they are big, heavy, and not very efficient.

If you used a Kollmorgen or similar RC motor as a generator, you might be able to just run the output through a 3 phase rectifier into a dc-dc converter.

Another crude but proven approach is to use a triac or solid state relay across the alternator coil as a shunt regulator. I've done this on two motorcycles with good results. The down side is it will be very inefficient at light loads. If you only run the engine full blast or off, you can avoid the inefficient range.

Yet another approach is to use some kind of servo to operate the throttle on the engine to maintain the proper voltage.

 

[johnrobholmes]

My hand will be twisting the throttle. This will be a hybrid

My thoughts were that it was the magnets holding it back mostly. That was my first idea for increasing power output. I can't keep an eye on the stator temps though, since it is crammed into the flywheel. Maybe there is a little passage for a sensor wire to pass somewhere.


If I just can't get it working with the stock parts I may slap a coreless RC motor on the chain.

 

[PaulM]

If I just can't get it working with the stock parts I may slap a coreless RC motor on the chain.

I assume you are using the CRF50 engine because you already have it? A few hundred watts won't be working it very hard lowering your efficiency. I would try to avoid going through the chain for efficiency and noise reasons. For the amount of power you are looking for, I would be tempted to sell the CRF engine and pick up a four-stroke weed eater engine. It would be much lighter, smaller, and likely more efficient as you should be closer to the "powerband" than on the CRF motor. I'm not sure how the shaft is attached on a weed eater, but it might make it easier to couple your RC motor directly.

 

[johnrobholmes]

I am using the crf50 motor because it is the motor for my scooter, which will have an electric drive system too.

 

[bluesrocks]

Sounds interesting. Is this engine running an alternator and driving the bike at the same time? Got pics?

You mention a 12v alternator and 12v battery for lights... What's the motor's battery bank voltage? Is there another on board charger for that too, or is your powered assisted bike a single 12volt system (14.4v)?

I'm not well versed in tech elect stuff, but it seems unusual having more than one battery system on one bike. Conversely, if it's just one single 12v system, wouldn't it be better to use more volts (i.e. 36-96v) to help avoid line/connection losses?

But lets say you went 48v, then you'd lack 12v lighting power. Perhaps incorporate a voltage transformer, or maybe a new bank of some nicad/lipo batteries (and recharge at home)? How about LED lights for the bike, they probably require a fraction of the current and last forever, thus becoming a popular lighting option for big rigs.

 

[johnrobholmes]

No pics yet, I am gathering parts currently. The motor is being packaged up this week and once I get done with another motorcycle project I will start on the frame for this one.

Since I have no electric start on this bike, my intention is to have a single bank of batteries although I was considering a 12v cell for lighting purposes and voltage smoothing. 48v is most likely, but with the heavy nature of the bike (150lbs dry) I may go with 72v. It will be MUCH cheaper to find switching converters for 48v. The problem will be the charging system, as most off the shelf systems use 11 to 15v dc input. I do have some 200w DC switchers available though, and am hoping that I can daisy chain them for additional wattage.

The plan is to have a jackshaft with parallel input from both the electric motor and the 50cc engine. The alternator on the engine may be able to produce enough power to charge the system, but if it does not I will have to put another alternator on there somewhere. I would prefer it to be behind the clutch so that I can get power any time the engine is running.

LEDs are the plan.

 

[jag]

I might be missing the big picture of what you are trying to build, but I was assuming a hybrid with ICE and electric motor of about equal power (a few kW), and 48 or 72V battery pack.

Wouldn't it work just getting a regen controller, apply regen to charge 72V battery pack, and then as Fechter pointed out use a standard powerbrick to supply 12V systems (and a small 12V battery if desired.)

The challenging (but interesting) part is to design and implement a control scheme for applying electric motor power or regen so to make for a smooth but powerful ride. Someone here at the university was planning to make a thesis project out of that for a hybrid car. A crude first approach would be just manual application of regen and electric motor power until you get an idea for how different mixtures of power feel.

 

[bluesrocks]

Wouldn't it work just getting a regen controller, apply regen to charge 72V battery pack, and then as Fechter pointed out use a standard powerbrick to supply 12V systems (and a small 12V battery if desired.)

To clarify. Are you saying that instead of

"The plan is to have a jackshaft with parallel input from both the electric motor and the 50cc engine. The alternator on the engine may be able to produce enough power to charge the system, but if it does not I will have to put another alternator on there somewhere."

you suggest

"Wouldn't it work just getting a regen controller, apply regen to charge 72V battery pack, and then as Fechter pointed out use a standard powerbrick to supply 12V systems (and a small 12V battery if desired.)"

That is to say, instead of adding an alternator to charge the battery bank, use the electric motor as a generator while the gas engine is running to charge the battery bank.
Is that correct? Forgive me if I'm missing something.

What's a standard powerbrick?

 

[jag]

Wouldn't it work just getting a regen controller, apply regen to charge 72V battery pack, and then as Fechter pointed out use a standard powerbrick to supply 12V systems (and a small 12V battery if desired.)

To clarify. Are you saying that instead of

"The plan is to have a jackshaft with parallel input from both the electric motor and the 50cc engine. The alternator on the engine may be able to produce enough power to charge the system, but if it does not I will have to put another alternator on there somewhere."

you suggest

"Wouldn't it work just getting a regen controller, apply regen to charge 72V battery pack, and then as Fechter pointed out use a standard powerbrick to supply 12V systems (and a small 12V battery if desired.)"

That is to say, instead of adding an alternator to charge the battery bank, use the electric motor as a generator while the gas engine is running to charge the battery bank.
Is that correct? Forgive me if I'm missing something.

That was my thought, but not necessarily John's (the orig poster) plan.

What's a standard powerbrick?

The typical 120V AC to x V DC switched power supply which comes with much consumer electronics. Many of these work fine on DC, and sometimes keep working much below the stated input voltage. So in say a 72V battery system for an electric motorcycle a cheap way to charge a standard 12V lead acid battery and/or run MC/automobile 12V accessories is to buy a suitable powerbrick. Alternator voltage is typically 14-14.5V. Toshiba Qosmio (sp) laptop supplies deliver 15V at 5-6A. Can be bought on ebay for about $10. Drop voltage with one diode and you have a 14.3V supply.