Compressed air motor to brushless generator ?

[picclock]

Hi

Odd sort of off topic requirement. I live in the UK which makes things trickier. I'm looking to source a 2HP (or more) compressed air motor to drive a brushless generator. I figure to get about 1kW out of the arrangement - more or less. The issues I'm having are sourcing the parts for this with limited funding. I figure the best way to get a generator is to buy (beg or borrow) a second hand petrol type (2KWor above to get the efficiency) and scrap the motor. Alternately, Machine Mart do some alternators but I don't think they are brushless.

However sourcing an air motor seems to be much more of a challenge, and a lot more expensive. Does anyone know of a UK or European supplier of such things, or any alternatives. I'm assuming that the alternators run at 3000 rpm, but it should be OK to use pulleys to get a match to the motor output, providing the ratio isn't too high. I can devise a feedback system to monitor and adjust the airflow to keep the setup turning at the correct rate.

Any info on suppliers of such or secondhand sources would be most welcome.

Many Thanks

picclock



 

[John Rudd]

Picclock,

I recently sold two generators on Ebay....both needed their rotors rewinding  ( not a daunting problem at all..)
....So buying a 'spares or repair' may be the cheapest way to get an alternator although they may not be brushless.........Some of the small two stroke powered units are probably brushless

As for the wind powered motor how about a compressed air power tool of somesorts 

[picclock]

Hi John

Thats why in some ways I figure buying a new alternator from Machine Mart might be a better solution - certainly less mucking about. I have looked at the compressed air tools but they seem to be very high revving and low on power. Saw a Gast air motor in USA for only $211 - impossible in the UK. That one said it needed 0.94L of oil (http://gopro.pro/gast_air_motor_4am-frv-13c.htm) - Why does it need it and how often would it need replenishing ? I have no idea about this stuff.

Thanks for your reply

Best Regards

picclock

[John Rudd]

Gast air motors are pretty common place in driving hydraulic pumps, we use them at work...

They need the oil for lubrication of the vanes and bearings....

You would need to gear the motor down to make the torque useful...

[picclock]

Hi John

Thanks for the info  . My understanding is that most of these air motors develop maximum power / efficiency at about half speed. I'm just going to connect it to the alternator with pulleys and a belt so the operating speed quite flexible like that. Re the air motor oil, presumably that is going to get mixed into the exhaust which sounds yukky.

Don't know if you have ever used the Clarke alternators (http://www.sitebox.ltd.uk/clarke-22kva-3000rpm-alternator-oclarke_8233005). Do you know if they have a preferred direction of rotation ? Can't seem to find any technical info on them regarding efficiency curves either, guessing that at about half power is likely to be fairly efficient.

Have found an outfit in the UK called Rotair (Motair? http://www.rotair.co.uk) which uses a vaneless (lobe ?) system for which they claim great things (well they would !!). Great thing is they are in Essex, and although its a bit of a trip a visit may be on the cards. Price they have given is 845+vat - but that's more than I can afford.

Best Regards

Picclock

Had another slightly madder thought - I can get a 4 stroke petrol generator (http://www.ebay.co.uk/itm/2-8KVA-4-Stroke-petrol-Generator-NEW-/220999930177) and connect compressed air to the motor inlet having junked all the petrol stuff. I would think the efficiency would be pretty bad though and it may not autostart.  Its actually cheaper to buy and ship this than just buy an alternator  .

[AdeV]

I'm curious to know why you'd want to do this?

Compressed air is "expensive" - you need a motor of some description to create it, then you need a strong tank to store it, and you're going to hook up a low efficiency air motor (which are a bit like Harleys - great at turning input energy into noise & not a lot else...)? Why have the compressed air in the middle, surely it'd be easier to simply take the motor off the compressor & use that to run the generator direct?

On the other hand, if this is some mad project, then....   

[kwackers]

Aren't alternators always brushless? (Coils on the casing)

[picclock]

Hi Adev

Mad  project fits the bill - but interesting if it works. If it does I'll post details, that way I won't have to hang my head in shame if it fails  .

Had another idea  - must be the weather - If I squirt compressed air into my cheapo generator 4 stroke it will only power every other stroke, but if I change the cam reduction gear to make it 1:1 then it should have power every stroke - and it may be OK with the shorter valve opening times as the gas volumes will be lower but twice as often. 

I'll have to think on it some more but it sounds like a sort of plan .. .

Other thing is, if it is going to cost me £200 for a delivered Clarke alternator and this petrol generator is £175 including IC engine then it seems like a no brainer, even if I can't get it to work with 4 stroke engine I am no worse off - just some time spent - which the government hasn't seen fit to tax yet !

Have some other inquiries out for air motors so I'll finally decide when I know the costs involved.

Best Regards

picclock

 Hi kwackers - alternators normally have brushes to excite the rotating coil. Often mounted on the end of the alternator. They only carry low current, and by varying the current you can control the alternator output. Brushless ones are sneaky, they control the current in the centre winding by inducing it from another coil on the shaft.

[John Rudd]

Aren't alternators always brushless? (Coils on the casing)

Ermm no....The output usually comes for the stator, the static part, and the rotor is the excited magnetic field...The excitation voltage from the AVR is applied by brushes to the slip rings...

[mattinker]

Picclock,

I'm curious as to why you want to drive an alternator with compressed air, it's going to take an awful lot of air! Do you have a big engine driven compressor? If so it would be far more efficient to run the generator from the compressor engine!

Regards, Matthew