Small Compressor Stall Stopper

[Ken I]

I didn’t know whether to file this under tips, boo boos or safety – you decide.

It starts as a tip on home compressors…

Small single phase compressors rely on the starter switch to drain the header pipe volume between the discharge port and the check valve on the tank.

(That’s the blast of escaping air you hear when it reaches cut off pressure or you switch it off by depressing the switch knob.)

This is so when the motor restarts it does not have to do so against system pressure – it will otherwise stall and burn out.

That’s also why the handbook will tell you to always switch off at the pressure switch not the mains outlet.

We have a local problem with power failures and brownouts (low voltage).

If you have a power failure while the compressor pressure switch is in run mode, the header pipe will be at system pressure and it will stall when the power comes back on.

Similarly during a normal restart under brownout, the pressure resistance builds faster than the motor can "spool up" and it stalls.

Perhaps the first rule is to never leave the equipment running unattended – but the road to hell is paved with good intentions.

After my second rewind I decided to improve things

By adding a small pressure vessel (a used soda stream CO2 canister) to increase the “startup” volume, the motor gets more time to (just a couple of seconds) to spool up – remember the starting torque of these things is appalling - typically only 10% of its rated torque.

Also by having a very small air leak in this system it allows this starting volume to drain down in about 20 seconds – that way (as long as the power failure is longer than 10-15 seconds) it will restart without stalling. This leak is between the pressure switch and check valve and so only "leaks" while the compressor is running and does not drain down the receiver.

The boo boo came from my first “pressure vessel” which was an old spray paint can to which I had soldered a barbed air fitting.
It worked well for a year but being very thin it corroded and failed :-

It blew up whilst I was working nearby – helluva bang but no harm done.

Regards,

Ken

I should know better.

 

[Cogsy]

It blew up whilst I was working nearby – helluva bang but no harm done.

You were close by when this happened and no harm done? I'd guess there'd at least be battle damage to the underwear :fan:.

Thanks for the tip, I might modify mine as well.

 

[gus]

Your aerosol serves as an unloading bottle to buy time before the discharge pressure goes up too high and stall starting. I used 2'' O.D. x 6'' L x 1/8'' T.steam pipe with screwed caps as unloading bottle.
I have used this option to do away with the expensive unloading solenoid valve. Same solenoid is normally open on starting for 3 secs. Or a normally closed solenoid valve can be used with a timer to open and closed after 3 seconds.
I have built and shipped hundreds of Single Phase and three phase air compressors with no motor stalling or power trips during my time with Ingersoll-Rand,Singapore Type 30 Plant.

It is unsafe practice to use an aerosol can to serve as unloading bottle.

 

[Ken I]

It is unsafe practice to use an aerosol can to serve as unloading bottle.

No kidding ?

That was part of the point of the post - it was meant as a quick and dirty fix to see if the idea worked - it did and I stupidly left it like that.

I am a certified engineer and used to test and sign off on serious pressure vessels - hence the "I should know better" comment.

Maybe you meant the CO2 bottle - which is certainly not to code - but I am satisfied that its able to handle way more than the receiver.

While on that subject, how many HMEM members have had their compressors inspected to local codes ? periodic tests and inspections etc.

Ingersol Rand - say no more - fine compressors - I'm talking those cheap hobbyist type with the pressure switch that simply empties the header pipe - there is no start up time delay between motor start and the compressor delivering.

Your idea of a piece of capped tubing is a good one though.

Regards,

 

[mu38&Bg#]

Interesting. I purchased a small compressor last year and it seems to leak at startup until a significant pressure is built up in the head pipe. That doesn't help it start when it's cold, however. The oil in the crankcase is so thick it will drain the starting capacitor and trip a breaker before it gets going fast enough to make decent torque. I have to find a suitable synthetic oil.

Greg