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Old 03-09-2006, 01:51 PM
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a little question

hi all

i have a 150 litre compressor that ran off a 3hp 240 volt single phase motor. my problem is that the motor has blown and i was wondering how i would go about converting it to petrol engine. a new motor over here in the uk would cost more than buying a new compressor. any help would be greatly appreciated

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Old 03-09-2006, 02:30 PM
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You would need at least a 6-7 HP gas (petrol) engine to replace the 3 HP electric motor but you would also need a governor to control engine RPM along with a "kick down" device of some kind to idle the engine during the phase when the compressor is up to operating pressure and power is not needed until the pressure drops to a point where the motor needs to kick back in. This is quite complicated and expensive to get all the correct parts and get them working together so I would think a replacement electric motor would be the better choice if possible.
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Old 03-09-2006, 06:42 PM
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Before you buy anything, you need to check (3) things that are common problems with single phase motors.

Single phase motors have (2) windings. One for starting and one for running.

There is a centrifugal switch that selects which winding will be used for (starting or running). These switches frequently fail and are easily replaced.

Also, these motors have a capacitor that controls the phase shift (trust me) to determine which direction the motor will turn.

Remove the belts. When you power the motor, if it hums but does not run, then spin it by hand. If it runs, one of the above components are bad.

And, normally these motors have a overload protector that can fail.

Check out these items, they are all inexpensive to replace.

If the windings are burned out, rewinding the motor is usually not cost effective unless the motor is not a commonly available unit.

vicrod
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Old 03-10-2006, 05:59 AM
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What you are up against here is there is no means of unloading the compressor. As long as the motor is running, the comp will continue to build air, The cost of adding in an electric clutch and the control for it would more than pay for a new compressor
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Old 03-10-2006, 06:58 AM
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61, I was asleep at the wheel on that one. You are right, that would be the biggest obstacle to converting to a gas engine, making the idle control part look easy.
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Old 03-10-2006, 09:23 AM
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Was just thinking and you could use a centrifugal clutch like ones used on go carts and pocket bikes to unload the compressor. And then you would also need some pressure operated actuator that would pull the throttle as pressure in the tank dropped.

The actual setup would be simple, but the idle control could be costly along with the clutch

John
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Old 03-10-2006, 10:05 AM
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look for old genorators they have a magnet sutup to keep the engine at a desierd rpm dont know how it works but look into it
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Old 03-10-2006, 12:13 PM
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As 61 pointed out simply idling the engine will not work you would have to have an unloader device or else the engine would still be trying to compress air only at a reduced speed. I suppose the clutch setup might work but it too could be complicated and not very reliable since it would tend to wear quickly due to the heavy start up load and high number of on/off cycles. Vicrod has offered excellent advice and these steps should definitely be taken before resorting to anything drastic.
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Old 03-10-2006, 04:15 PM
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thanks

many thanks for your help gents. i think i will go the expensive way and buy a new electric motor for it found a place on ebay wgere i can get one for 100.00 . just wondered if it would beeasy to convert to petrol as a friend has an old briggs and stratton 4 hp engine lying about doing nothing and it was free!! .
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Old 03-10-2006, 04:29 PM
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New electric motor That 4 HP briggs would not have been big enough anyway since it takes nearly twice the power rating from a gas engine to replace an electric, may seem odd but it's true. Besides the operating cost of the briggs would have been painful
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Old 03-10-2006, 08:53 PM
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Well all I can say about the clutch set up is that it is still going strong and the gas compressor I use has no problems. It uses that clutch and a belt drive set up. When in use the engine will idle with the clutch disengaged. And when the pressure drop below about 110 psi the pressure switch pulls the cable to full throttle. engine speeds up clutch engages and compresses air till the switch is closed again then the motor idles.

This compressor was used full time for about 5 years (that was about 20 years ago or so) And for about the last 15 it has been on a trailer and is used once in a while to blow out lines in fall and other jobs like shingling and framing when there is no power or a need for large amounts or air. This is the history as I have been told. I was only around to confirm the last 15 about.

Last edited by projectjohn; 03-10-2006 at 09:02 PM.
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Old 03-11-2006, 05:19 AM
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John, I never said the clutch setup would not work, I just pointed out that it can get a little complicated to build when you consider the fact that the engine also must have a "kick down" device rigged up to idle it down for the clutch to disengage. A pump unloader valve works much better and that is why it is the industry standard for gas powered compressors. However I bet that clutch makes the engine easier to start on a cold day since it would completely disengage the pump and remove all the added drag that even an unloaded pump will produce.
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Old 03-11-2006, 02:23 PM
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The compressor was built from some old air tank ( or some other tank)They used an air actuator that as the pressure dropped it would pull (or push don't remember right now) on the throttle. I don't know if my dad or uncle built the "kick down" but they followed the kiss method. It is very easy to start that compressor easier than starting the lawn mower.

It is amazing what some one can build when money is tight and they have some time. But now that compressor doesn't see much use any more.
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