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Old 03-19-2011, 11:28 AM
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Compressor conversion

Ok , I have gotten much help from you all on a prior conversion topic and I thank you all for that. I have located the electric engine that belonged to the compressor I acquired from our local boat builder , it has been rebuilt by them and given to me as well. I am still interested in converting to gas but I cant figure out what size gas engine I will need to turn the pulley on the compressor. The electric motor I have is a Dayton , Phase 1 , 5 hp with an rpm speed of 1740. The pulley on the compressor is 16in in diameter. Any help will be greatly appreciated , just cant wait to get this thing running , mainly for blasting purposes. Thanks all!!
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Old 03-20-2011, 12:58 PM
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The only help I can give is that electric motor are rated at continuous output, mechanical engines are rated at output. so that Dayton electric motor will run all day at 5hp, a 5hp gasoline engine will only put out 5 hp for a short period. You will need at least a 6.5 hp engine, more won't hurt. I'd keep the pulleys the same and try it. HF has a 6.5 hp recommended for compressors:
http://www.harborfreight.com/212-cc-...nia-68121.html

and

http://www.harborfreight.com/engines...ine-66015.html
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Old 03-20-2011, 02:14 PM
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Hi
I have a gas driven compressor, It has a Wisconsin engine 3 X 3-1/4" And the pulley size is 4-1/4" W/3 V Belts

The Compressor is a Curtis 4-1/2 X 2-5/16 Two Stage. The pulley size is 16-1/2" W/3 V Belts

I don't know the HP or RPMs of the engine yet, but should know sometime this week.
Rich
PS Going by the size of the engine, I would guess it's at least 12HP
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Old 03-20-2011, 03:24 PM
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If the pump is sized for a 5 HP motor (electric motors are never called engines) then you will need at least a 12 HP gas engine, 14 would be better, to realize the performance you would expect from the 5 HP electric, 6 1/2 would not be nearly big enough. The gas engine will also be rated at about 3600 RPM or roughly twice what the motor was running at so the pulley on the gas engine will need to be about half the size of the one on the electric motor. I know that is a big difference in apparent HP between the two different power sources but trust me on this one, if you use less than a 12 HP gas engine you will be disappointed with the performance and you will need to use an even smaller pulley on the engine shaft to run the pump even slower. The key here on the HP ratings is APPARENT HP and comparing a 5 HP engine to a 5 HP electric motor is comparing "Apples to Oranges"! You certainly can use a smaller gas engine but only at the expense of greatly reduced performance, in order to make a small gas engine work you would simply need a pulley small enough so that the engine could run at top speed with the pump loaded. The problem is that with only a 5 or 6, or even an 8 HP, gas engine is the pump would have to run so slowly it would not work well at all. 12 HP minimum, 14 would be much better and an 18 HP twin cylinder gas engine would not be too much.
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Old 03-21-2011, 11:09 AM
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Thank you all very much for your input and guidance.

Oldred , I have a new 13hp Honda engine sitting in the garage for sometime now , will that be enough to use in this case? I will not be using the compressor for more then blasting on occasion.
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Old 03-21-2011, 03:22 PM
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Yes that 13 HP Honda should work, you may not get the full potential from that pump but it should work pretty decent. There will be some trial and error involved on the pulleys to get the maximum from the engine so start out with a pulley at least half the size of the one on the electric motor and see if the engine can handle the load as the pump reaches maximum pressure, if the engine starts to strain and trying to slow down as pump shut off pressure is reached then you will need to slow the pump down a bit more with an even smaller pulley on the engine.
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Old 03-21-2011, 03:57 PM
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An electric motor puts out maximum torque when stalled, not turning. It will quickly over heat, and shut off, it it cannot start to turn.
An electric motor can put out more horsepower than its maximum rating for a short period of time.

We all know a stalled gas engine puts out nothing. It has to be running to produce torque. A gas engine is also limited to its maximum horsepower rating.
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Old 03-21-2011, 05:51 PM
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Exactly, and an even easier way to look at it when thinking in terms of a compressor is that the compressor demands max power at start-up where that idling engine is making it's least power, the electric motor OTOH makes it's rated power at that low start-up RPM. Still even at that a 5 HP electric motor will also handle a lot more load than a 5 HP gas engine even at higher rpm.
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Old 03-28-2011, 02:43 PM
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Quote:
Originally Posted by DanielC
An electric motor puts out maximum torque when stalled, not turning. It will quickly over heat, and shut off, it it cannot start to turn.
An electric motor can put out more horsepower than its maximum rating for a short period of time.
That depends on it's service factor... some will some won't
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