Atlas copco

I think the 20HP motor was the give away!

Oh yes :) We want these as part of our generator project, wondered if anyone here knew anything about them!

We're not looking for household size however, as we need to use the engine's power efficiently (ideally at 3000rpm). I realise they are very big, but a 20HP induction motor is about the size we have been looking for - and it isn't often they come along at this price.

Thing is, I don't know if there will be any problems. I've only worked with more modern 3 phase motors, where you can order new bearings and parts easily, and many people are willing to work on them. I also know very little about compressors - would standard piston rings work? Do they just stop working, or do they lose efficiency when they get worn? Things like that.

I've found several references to Atlas Copco piston compressors, and people say they go on and on without problems, but maybe not ones of this age.

Andrew

Hi Andrew

Still new to this newsgroup think as such have just posted my original reply somewhere else!

As part of my job I work with Copco compressors on a regular basis, however these are the new screw types not the older piston types as shown in your pictures. The compressors are excellent and will last for ages as long as they are well maintained. Spares should be available either from the UK or from Belgium. I have contact in Copco if you either post or contact me OT with the part numbers I can see what I can do.

With regards to using the motors, beware, Copco may have used custom motors (as they do now) which have a completely different torque curve to cope with the requirements from the compressor. I presume that this is a fixed speed machine, which may also indicate that the motor is non standard. What I man by that is that standard induction motors are 4 pole machines with a synchronous speed of 1500rpm. Approx 2% slip gives a real shaft speed of approx 1470 rpm.

The poles can be changed to give a different fixed speed i.e. a 2 pole machine will have a synchronous speed of 3000 rpm. Which is what you have mentioned you would like to run the motors at. If they are not designed for this you will have problems.

Secondly you mention about running the motor as a generator. This can be done the way that we do this in my company is to allow the load to drive the motor thus regenerating power. To control this we use Inverters rated for

100% duty to regenerate the 20HP you mention. As I am sure you are aware, simply spinning an AC motor will provide no volts because no current is flowing in the stator. We use Inverters both to control the switching and also the regeneration. If this is the method you propose then again the motors may not be suitable because this method exposes the motor windings to approx 800Vdc P to P so the insulation may not be rated for this, modern induction motors have insulation rated for this cant remember what category it is.

Hope this helps in some way and I have not insulted your intelligence.

Hi Rich,

A while ago a chap was hawking a scheme around the rallys for running (small) 3 phase motors as generators by loading them with capacitors. I tried it at home and it worked. I then started searching through various electrical text books to try and find out why.

Apparently what you have is an "asynchronous induction generator". Run at negative slip (ie faster than synchronous speed) an exiting current is induced in the rotor and the device can deliver power into a suitably reactive load. Don't know if it would be practical with larger machines as I imagine there are a lot of reactive KVA circulating doing nothing, so a current overload (probably of caps) would be possible before even delivering any useful output! I would be interested in your comments, and Arthur could probably chip in something on suitable caps.

This is what we intend to do. :) I shall post any experiences I have when something has been done, I know Andrew will also.

Ed

Nick's last words:

Bzzzztt!! phutt!! aaaagghhh!! :-))

Kind regards,

Peter

Peter Forbes Prepair Ltd Luton, UK email: snipped-for-privacy@easynet.co.uk home: snipped-for-privacy@easynet.co.uk

Probably be a candidate for the Darwin awards;-)

This bit on asynchronous induction generators used in wind power installations might be of interest:-

A useful site on many such matters:

-- Die Gotterspammerung - Junkmail of the Gods