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Generators & Electric Motors General Discussion Antique Generators and Old Electric Motors: Questions and answers about restoring and showing old power generation systems.

Generators & Electric Motors General Discussion

1800 rpm Onan with 3600 rpm generator


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  #1  
Old 07-08-2013, 01:35:57 AM
Graycenphil Graycenphil is offline
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Default 1800 rpm Onan with 3600 rpm generator

My Onan MDJE generator head died a few months ago, and I think I will run another generator head off of it, with a belt drive. 3600 rpm units seem much more plentiful, so here's my question:

Will putting a small pulley on the Onan shaft, and another pullley twice the size on the generator work right? I think that will give me the right rpm; will the governor operate properly, keeping the volts and hertz where they should be?

Thanks.
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Old 07-08-2013, 02:36:54 AM
Power Power is online now
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Default Re: 1800 rpm Onan with 3600 rpm generator

You got it reversed! Big one goes on engine, but it is a tapered shaft, don't know how you can mount pulley unless you machine engine shaft and forever destroy it.
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Old 07-08-2013, 03:05:14 AM
Roland Hayes Roland Hayes is offline
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Default Re: 1800 rpm Onan with 3600 rpm generator

The taper will probably be a standard 5 degree, its easy to bore a taper into a pully on a lathe and with a shallow 5 degree taper there is no need for a key.
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Old 07-08-2013, 11:00:15 AM
Graycenphil Graycenphil is offline
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Default Re: 1800 rpm Onan with 3600 rpm generator

Thanks. That's why I ask questions here. Is it as simple as using one pulley twice the diameter as the other, or is there a more complicated calculation?

I had thought about using the generator shaft, which isn't tapered, and putting the pulley on the end of it, but I like the idea of milling the pulley better.
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Old 07-08-2013, 11:31:30 AM
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Onan Dan Onan Dan is offline
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Default Re: 1800 rpm Onan with 3600 rpm generator

Got into that last year tried to run a 3600 generator head off my 1800rpm onan bge engine. had to reconfigure the shaft from a tapered head to a 3/4inch shaft by 4 inches long on the onan engine. well long story short used the big pulley 6in dia on the onan eng and the 3 in pulley on the gen head just the opposite your doing. here whats happened one pulley or belt will not run the setup. you will need three or 4 row gang pulleys to stop the friction . any way I wasted my time and money . luck onan dan
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Old 07-08-2013, 11:36:38 AM
Roland Hayes Roland Hayes is offline
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Default Re: 1800 rpm Onan with 3600 rpm generator

As you are using pulleys, there is no need for the engine to run at any particular exact speed, the alternator of course must in order to get the correct Hz frequency, my suggestion is to run the engine at 2200 rpm which is not that much higher than 1800 but at that speed the engine will be up into the fattest part of its torque curve and make more power, it might even be more fuel efficient at 2200 than at 1800. All you need to do to prevent belt slip is to use big diameter pulleys, use a 10" dia on the engine and a 6" dia on the alternator and they should be twin groove with cogged V belts.

This is a generator that I built a few years ago, on this one the engine could be adjusted to run at between 750 and 1200 rpm, the 3000 rpm Knylor alternator has permanent magnet excitation and was chosen because it had good voltage control over a wide rpm range, for low power such as over night security lighting to reduce noise and fuel consumption the engine speed would be reduced and for maximum output the speed increased. the correct UK 50Hz occurred at about 1000 engine rpm
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Last edited by Roland Hayes; 07-08-2013 at 12:45:30 PM. Reason: Added pictures
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Old 07-08-2013, 11:59:04 AM
JerryLee JerryLee is offline
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Default Re: 1800 rpm Onan with 3600 rpm generator

Is it as simple as using one pulley twice the diameter as the other, or is there a more complicated calculation?

Seems to me that the circumference is the key dimension. If you run the engine at 1800 RPM and you want the generator to be at 3600 RPM, the circumference must be half on the generator as it is on the engine. You can find an easy conversion formula on the internet. Just do a search on "circumference formula" and see what you get.

Try to find a wide poly-V belt. Any decent machine shop can make both pulleys for you without breaking the bank.
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Old 07-08-2013, 02:05:46 PM
Jim Rankin Jim Rankin is offline
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Default Re: 1800 rpm Onan with 3600 rpm generator

The best type belt drive is a cogged belt so no slippage and less tension required, so less side load on the engine. Not sure what kind of main bearing your engine has, but belt driving something vs direct driving it puts a very different load on the crankshaft bearings. Even though the cogged belt will need less initial tension than a V-belt, it still pulls the shaft sideways because it must transmit torque through a 90* turn. Mount the pulley as close to the engine as possible to lessen the strain on the crankshaft.

A ball or roller bearing can stand the side load a lot better than a babbit or aluminum sleeve bearing, so the 2 bearing alternator is readily able to stand it, but the engine may not do so well long term if it doesn't also have that kind of bearing.

If you use a cogged belt, you simply count teeth and pick 2 pulleys that give you the ratio you need and a load capacity which exceeds your generator's capacity. An internal combustion engine as a driver requires a higher "duty" factor (multiplier really) than a similar capacity electric motor because of the pulses of torque as each cylinder fires.

If you use v belts or micro vee belts, you select the pulleys based on rpm, and load and end up with 2 that have a "pitch diameter" ratio that result in the correct speed. Pitch diameter is related to outside diameter of the pulleys, but it's always a little less than the OD. So you can get close by using OD, but look up the PD and you can get it closer.
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Old 07-08-2013, 02:10:52 PM
len k len k is offline
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Default Re: 1800 rpm Onan with 3600 rpm generator

My recomendation is to go with a cog belt not v-belts. V-belts have ~1% slip and will need more tension to transmit the torque. This tension will cause a little more wear on the shaft support bearings. There are standard formulas for sizing pully diameter to hp transmitted, making diameters large enough for a given belt size. An industrial supply house or belt manufacture's website should have that info. I wouldn't worry too much about pully "pitch diameters", you will have to use standard sides and can tweek engine rpm to maintain gen rpm. Pullys are not that cheap, maybe another used genhead??

typo.....I wouldn't worry too much about pully "pitch diameters", you will have to use standard sizes
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Old 07-08-2013, 10:35:28 PM
ArodaPowerCo ArodaPowerCo is offline
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Default Re: 1800 rpm Onan with 3600 rpm generator

I would be very careful belt driving a generator head with an Onan twin. From personal experience, Onan opposed twins do not like to have side-loaded crankshafts. The bearings are the least of your worries. Theses engines have two throws on the crank so that both pistons are at TDC or BDC at the same time. This creates a weakness due to the crank being shaped like a "Z". Too much belt tension for too long and the crankshaft WILL break. I used to maintain a fleet of stump grinders, and I couldn't keep Onans on them for longer than 1000 hours. Kohler CH25 V-twins (only one crank throw) would in many cases exceed 4000 hours (with an overhaul or two) before they were no longer rebuildable.

To alleviate the need for lots of belt tension with your Onan, use very large diameter two-groove pulleys to increase the belt gripping surface area. Like 12" on the engine and 6" or so on the gen head. This will significantly reduce the tension needed to acheive satisfactory belt friction, and if you use cast iron pulleys, add a lot of weight. Lots of rotating mass will buffer motor starting loads better (like a well pump), and reduce governor droop.

Not trying to discourage your idea, just be warned that you need to be careful when belt driving with these engines. Properly set up, your idea will work just fine for a very long time.

Eric

Edit: For some reason was thinking you were running an opposed twin. Just re-read and saw "MDJB". Similar holds true for inline twins due to no bearing support between cylinders.
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