Treadmill motor generator questions?

Badfish740

I've got three permanent magnet DC treadmill motors laying around. These seem to be big with the DIY energy crowd for building wind turbines, but that's not what I want to use them for. I'm wondering how much power I could get out of them when driven by either a 3.5 or 6.5 HP Predator (Harbor Freight) engine. This guy strapped a similar motor to an old (and probably worn out) 3HP lawn edger and with a proper pulley setup is getting 12V/16A:

http://mdpub.com/generator/

Is there any reliable way to predict how many amps I could get out of a motor based on RPM/HP/VDC ratings? For example, I know that if I connected the motor to a 3.5 or 6.5HP engine, I could get 48V simply by fitting the proper pulleys to make sure that the motor is turning at 2240 RPM, but how many amps? What I'm trying to figure out is whether or not I could run a 48V electric outboard drawing 27A via a small DC generator mounted elsewhere in the boat, or at least use the generator to supplement a 48V battery bank so that the boat could run all day.

I've seen "cheater" setups like this before on electric only lakes. Basically what guys do is bring a very small generator with them like the really quiet inverter models people take camping. They run their batteries down and once they're out in the middle of the lake or they beach somewhere for lunch they run the generator top up the batteries or on sophisticated systems, they run the generator while underway so that the motor is pulling power from the generator with the batteries acting as a buffer. The thing is, they're generating AC power from the generator and having to use a power converter to charge the batteries/run the motor. Seems pretty inefficient, and that just generating DC from the get-go would be much better. I'm hoping that some of the electrical wizards here can help me out.

BB.

Why not just use a car alternator? You can get 50-100+ amps out of one of those... And they have a regulator already?

10 amps at 13.5 volts on a 3hp gasoline engine is not very efficient (in theory, a 2 hp engine should be able to output around 1 hp from an alternator or 50 amps @ 14.5 volts).

-Bill

Badfish740

BB. wrote: »

Why not just use a car alternator? You can get 50-100+ amps out of one of those... And they have a regulator already?

10 amps at 13.5 volts on a 3hp gasoline engine is not very efficient (in theory, a 2 hp engine should be able to output around 1 hp from an alternator or 50 amps @ 14.5 volts).

As far as amperage that's the obvious choice but I need 48V and would like to be able to run the generator while the motor is running. I've read there is a way to do this by placing diodes in the system in order to isolate the 12V parallel charging circuit from the 48V series circuit powering the motor, but I don't know where to put them.

BB.

You could setup for charging 12 volts at a time on a 48 volt battery bank. But there is no way to setup a single 12 volt charger to charge all batteries in a 48 volt bank at the same time.

Here is an older but nice discussion on another board that may give you some ideas:

https://groups.google.com/forum/#!topic/alt.energy.homepower/-3xW7GkALtA

-Bill

mike95490

Treadmill motors, driven as a generator, at high RPM's will not last.
The rule of thumb for engines (at sea level) is 6hp = 3Kw solid output. Some motors will give more power for shorter time.
You can use an alternator with external voltage regulation and get 55V after replacing the diodes, and could expect 50A, at it's normal RPM

Badfish740

mike95490 wrote: »

You can use an alternator with external voltage regulation and get 55V after replacing the diodes, and could expect 50A, at it's normal RPM

Thanks, this sounds like the best option I've found yet-any chance you could point me to a thread, etc...where this is laid out step by step? I really don't have any experience modifying automotive alternators.

waynefromnscanada

mike95490 wrote: »

You can use an alternator with external voltage regulation and get 55V after replacing the diodes, and could expect 50A, at it's normal RPM

Probably wouldn't even have to replace the diodes. Many have used them to produce 100 VDC with their existing diodes. Worth a try, and if you later have to replace them, well, so be it. My mini-micro hydro uses the standard Delco alternator(except for having the rotor coil replaced with a Perm Magnet and a rewound stator for low RPM, uses the original diodes and on initial startup, with the sudden inrush of water there is a surge usually over 70 VDC and no probs. It normally outputs 30 to 40 volts 24/7, depending on where on the curve the MPPT controller decides to pin it until the next sweep.

Badfish740

waynefromnscanada wrote: »

Probably wouldn't even have to replace the diodes. Many have used them to produce 100 VDC with their existing diodes. Worth a try, and if you later have to replace them, well, so be it. My mini-micro hydro uses the standard Delco alternator(except for having the rotor coil replaced with a Perm Magnet and a rewound stator for low RPM, uses the original diodes and on initial startup, with the sudden inrush of water there is a surge usually over 70 VDC and no probs. It normally outputs 30 to 40 volts 24/7, depending on where on the curve the MPPT controller decides to pin it until the next sweep.

Thanks Wayne-as you can see my tag still says "New Guy" so I need all of the help I can get. If I don't have to replace/modify anything, even better, but what would I need to do beyond coupling the alternator to the engine? I only need 48V and enough amperage to run the motor (27A) plus some extra to keep the batteries topped up.

waynefromnscanada

Badfish740 wrote: »

Thanks Wayne-as you can see my tag still says "New Guy" so I need all of the help I can get. If I don't have to replace/modify anything, even better, but what would I need to do beyond coupling the alternator to the engine? I only need 48V and enough amperage to run the motor (27A) plus some extra to keep the batteries topped up.

You would have to modify/ replace the voltage regular, or eliminate it and control it by changing engine RPM. One thing though - - - what is this about needing 27 amps to run the motor? And what for are the batteries?

Badfish740

waynefromnscanada wrote: »

You would have to modify/ replace the voltage regular, or eliminate it and control it by changing engine RPM. One thing though - - - what is this about needing 27 amps to run the motor? And what for are the batteries?

Think of it as a kind of hybrid electric boat. I have a 1000W 48V motor that will be coupled to the driveshaft of a small outboard leg. It draws 27A at 48V under load. The boat will be equipped with a 48V bank of batteries, but I'd like to be able to do the following:

Launch (with the batteries fully charged), fire up the engine/48V alternator, and run the motor primarily off of the power coming from the generator. This would allow me to do 6 knots for the two mile trip across the lake without depleting my batteries. The batteries would then be utilized for "silent running" when cruising the shores watching for wildlife, trolling while fishing, or when the rangers are within earshot (no gas outboards allowed on the lake so I'm bending the rules here). At this point the engine/alternator could be run to top up the batteries while we're sitting on shore eating lunch, or we could use it to power the outboard on the way back.

I don't have the soldering ability nor the circuit building knowledge to build my own regulator, but controlling the voltage via RPM could be a possibility, since it would be easy enough to dial the correct RPM in with pulleys, the question is how smooth would it be? Assuming I got it dialed in at 52V or whatever the appropriate charging voltage would be, how much fluctuation could the batteries/motor tolerate without damage? I did find this, which is interesting:

*EDIT*
Nevermind-I thought I had found a 48V alternator for a Dodge truck, but it turns out it was an inaccurate description of an item for sale on Ebay.

mike95490

"Gas Outboards" generally means not pumping exhaust gas and oil through the water. Same with "inboard" engines. But if it's a noise issue, you are screwed.