convert electric to petrol??

[gerrymoore]

Hi all...is it feasable to convert a new 3hp electric compressor to run with a 5.5 hp petrol engine?? I live in Spain and the power here won't run a compressor and plasma cutter together....shopping around it's cheaper to buy an electric one and convert it rather than buy a petrol powered one in the first place.

I'm fairly competent as an engineer and trained in electronics...anyone see a reason why not??

The only problem I can see is getting the petrol engine to tickover when the tank is full and then power up again when needed!

thanks.......

 

[Uncle Joe]

Theres a couple of ways to do this. The easiest is to get hold of an adjustable pressure release valve, and set it to blow off at the max pressure that you will be using, say 6 bar. Doing it this way means that there is no need to allow the motor to run at idle.

Two words of caution though. 1: Use a vee belt drive, and 2: Dont forget that a petrol engine revs higher than an electric, so the gearing between drive unit and compressor needs changing to stop the compressor block over revving.

 

[gerrymoore]

Thanks for the swift reply.....note taken on all your points...nothing I can't handle :-))

 

[rtbcomp]

Theres a couple of ways to do this. The easiest is to get hold of an adjustable pressure release valve, and set it to blow off at the max pressure that you will be using, say 6 bar. Doing it this way means that there is no need to allow the motor to run at idle.
.....

As a matter of interest what's the other way(s)?

 

[Uncle Joe]

For example, use some kind of clutch between drive unit and compressor block that is operated by air pressure. When the unit comes up to pressure, the clutch releases and air stops being pumped.

I've seen both cone clutches and tension wheels used for this in the past.

 

[eil]

Aka ice cream vans, vehicle air con.
Another way would be to get the older type compressors with a suction valve unloader fitted that would be my personal choice.

 

[rtbcomp]

A Google of " Suction valve unloader" turned this page up http://www.tpub.com/content/automotiveaccessories/TM-5-6640-212-14/TM-5-6640-212-140167.htm

I may have one of these, something very similar was fitted to a Westinghouse compressor I've got. I assumed it was a safety relief valve, but it is quite big. I'll see if I can find it.

This would make sense as I believe the compressor was fitted to a vehicle of some sort.

How does it work?

 

[eil]

It works by setting your relief valve / air Governor to the required pressure, when the pressure is reached it releases pressure from the discharge side of compressor to atmosphere thats how it works on the small petrol type compressors, but instead of dumping that pressure you pipe it up to the suction valve unloader which in turn pushes down on the suction or inlet valve of the compressor allowing it to free-wheel so to speak.
The advantages of the unloader system of capacity control is :-
Better cylinder cooling of the compressor
Better motor cooling on electric types as its free-wheeling on low amps
Stops too frequent stop starts which use more power and create more heat

There is an example of that system on Archie`s pic on AC25 she`s ere thread

 

[rtbcomp]

I see, so it has to have a mechanical connection with the inlet valve. That thing of mine just sat in the outlet so it probably is a safety valve, I still haven't found it, but it did show up a couple of days ago so it can't be far away.

So, the thing with the tap like knob sets the pressure and the two bits near to the filter connected by the tube opens the inlet valves?

 

[eil]

It does have a mechanical connection by means of a plunger acting on the inlet valve, obviously it was a costlier method of unloading but if the compressor doesn't have that provision tapped into the head you would have to unload by other means. I just like that system as you can put your hand on the cylinder as it does a good job of cooling down the cylinders.

 

[Justme]

Why not just use the cutter in between cycles on the compressor so both are not running at the same time?


Justme

 

[gerrymoore]

Thanks for all the input guys!! Yes I suppose I could get the compressor full and then swap to the cutter...haven't actually got either of them yet...still deciding which ones to buy and how to get them over here.........got my eye on a chinese 200 litre job for just £250....doesn't sound bad with 12 months warranty - but a long way to take it back :-((

I had another idea.....not sure how the electric motor is switched off in the first place, I assume some sort of relay is tripped when a certain pressure is reached?? I could use this relay to trip a solenoid connected to the carb on the motor to make it idle......does this sound feasable??

 

[rtbcomp]

The motor is switched on & off by a pressure switch, if the solenoid opened the throttle when energised you could connect it in place of the electric motor.

You would still need to disengage the pump though, maybe a centrifugal clutch would work, or dump excess air.

 

[eil]

Gerry, the engines don't rev up and down on these compressors its set to come up to a certain rpm and thats it they have their own Governor to keep that rpm, so to convert to petrol you need your engine, a suitable pulley so you do not over rev your compressor and you need a continuous run unloader.
like this http://www.cdivalve.com/products/continuous/continuous.htm

Or if you so wished you could fit a T piece in your discharge pipe before the non return valve on the tank and install a solenoid valve to open when your pressure switch cuts off, you would need to fit a relief valve in case the solenoid would fail for safety.

 

[gerrymoore]

thanks again for the input....the motor I have in mind is a 5.5hp industrial type that should have the governor rev limiter built in. I like the look of the continous run unloader...just hope the're not too expensive!!