compressor unloaders

I took a call from a customer who is having compressor trouble on a train. The train gets its power from the third rail. The third rail is inside the track next to the right rail. The rail is 600vac three phase. It’s a six cylinder Carrier compressor motor driven. I don’t know the model number. When the compressor is first started the train is not moving and the foot is in full contact with the third rail. The compressor is started unloaded to get the motor up to speed. When the compressor reaches full compression the A/C system works great. NOW FOR THE PROBLEM: When the train is moving the third rail has gaps in it and the train loses power for about 5 seconds. All other power devises are 24vdc and use batteries to keep running. When the compressor loses power there is a 14 second delay before the motor re-energizes. The motor re-energizes and the compressor is still loaded. The motor reaches LRA and trips the breaker. There can’t be a longer delay between power. There is a 5-14 second delay only. Is a capacitor bank possible? Is there such a thing as electrical unloader? Smaller compressors inline maybe? I heard that the compressors need over ten minutes to unload on there own. Any help will be great.
This is my first time. Please let me know if more info is need

Welcome FrostTech,

That's an interesting application and question for your first post.

If I understand your problem the compressor is still fully loaded when the power drops off during that time delay. Then when the power is restored the compressor while operating at full capacity resorts to FLA requirements.

I may be wrong, but from what I remember of the Carrier compressor (they are actually manufactured by Carlyle Compressor, which is a subsidiary of Carrier or something like that) these compressor capacity control is controlled by suction pressure.

Therefore these would not react to your problem. In fact this type of unloading actually makes the problem worse as the suction pressure would increase when the compressor stalls out.

I want to say I have seen some other type of unloaders for these compressors but it has been a long time since I have looked for information.

Here are few links to review....

Try this link and see if this looks familiar.
http://www.carlylecompressor.com/cor...TI1251,00.html

http://www.carlylecompressor.com/Fil...lveInstall.pdf


and try this link - look on page 23. You may be able to covert the capacity control from hydraulic to pneumatic or something along those lines.

http://www.carlylecompressor.com/Fil...h_09rh-2xa.pdf

I hope that helps you get started on a solution.

Welcome to the Forum. As US Iceman said that is a interesting problem.
I would think that your problem stems from the fact that the Oil Pressure does not fall off during the short time you are without power, leaving the machine fully loaded. Or it could be that the normal short period when the compressor is building Oil Pressure is missing due to the rapid cycling.
The only cure for the problem that comes immediately to mind is a UPS System. Expensive, but I would think it would solve the problem.

Thanks for the Help US Iceman. I will try to keep the thread up dated. I'm not sure if it's possible.

If the thread closes after the end of the month, just start a new thread to advise us of your progress. My thoughts on your problem are: If you can get the compressor to reduce it's capacity instantly when the power supply is lost, then when power is reapplied, the compressor should start unloaded.

What I do not know, is what will happen when the power is re-established as the compressor is already rotating. This to me is the biggest unknown...

How does one 'third rail at 600vac' convert to 3 phase?

a good heavy fly weel! Go back to the good ol days, a good heavy fly weel will keep spinning good n fast for that short period in effect unloading the motor at restart, aside from some make up speed!

Frank, I called the customer and he reassured me that it's 600vac 3 phase. He doesn't know how they get 3 phases off one rail. He said he would contact the Electrical Engineer at the Transit Depot. I never even thought of it, "just one rail". Good call!