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15 hp RPC struggles to start 7.5 hp 3 phase air compressor - add cap?

It started fine with the tank empty. (There is no pressure gauge installed on the tank at present.) However, with the tank partly full the motor pulley spun quite slowly as if overloaded, and never came up to speed before the output breaker on my RPC blew.
Connect a gauge and see what tank pressure is when the breaker opens.
Look into reducing diameter of motor pulley.
 
Not discussed yet: what's the size of the incoming service? Given you can run the compressor with the aux. idlers on line it's *probably* good enough. Worth checking the manufactured leg voltage with and without the aux. idlers, *and* checking the utility legs as well. Compressors - hard load to start, your idler is a bit undersized for this one to start.

1) larger idler.
2) single phase motor
3) slow start with a VFD - I've seen that work well
4) fiddle with the unloader situation.

Optimizing the decision tree means what's your goal - get the air or finess the problem and learn new stuff on the way.
 
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Not discussed yet: what's the size of the incoming service? Given you can run the compressor with the aux. idlers on line it's *probably* good enough. Worth checking the manufactured leg voltage with and without the aux. idlers, *and* checking the utility legs as well. Compressors - hard load to start, your idler is a bit undersized for this one to start.

1) larger idler.
2) single phase motor
3) slow start with a VFD - I've seen that work well
4) fiddle with the unloader situation.

Optimizing the decision tree means what's your goal - get the air or finess the problem and learn new stuff on the way.

My shop has a 200 amp panel.
My RPC has 3 voltmeters indicating line-to-line voltages:

boxMeters.jpg


Regardless of whether the extra machines are running or not (not counting transients during their startups) the meters show just the same as the above picture.

Regarding your list, nos. 1 and 2 are non-starters. No. 3 would probably work but buying a 7.5hp VFD would cost more than my whole compressor. I am contemplating fiddling with the unloader, not trying to modify the factory unloader but rather to add a valve to dump air from the discharge tubes to atmosphere for an adjustable time on startup.

You forgot 5) which is of course sell the damn thing and move on..

metalmagpie

PS Jim Rozen, I remember your name from the old rec.crafts.metalworking days.
 
Regardless of whether the extra machines are running or not (not counting transients during their startups) the meters show just the same as the above picture.
The transients during the startup of the compressor are what's important. Do all three legs sag significantly? Just two? How large is the sag?
 
Regarding your list, nos. 1 and 2 are non-starters. No. 3 would probably work but buying a 7.5hp VFD would cost more than my whole compressor. I am contemplating fiddling with the unloader, not trying to modify the factory unloader but rather to add a valve to dump air from the discharge tubes to atmosphere for an adjustable time on startup.

double check, but I learned the hard way, unloaders work opposite this. takes air pressure to engage them. dump air when running, but not to atmosphere from tank or you have HUGE air leak.
 
double check, but I learned the hard way, unloaders work opposite this. takes air pressure to engage them. dump air when running, but not to atmosphere from tank or you have HUGE air leak.
Not to atmosphere from tank. Between the air pump and the check valve going into the tank are the discharge tubes. I suppose you could also call them exhaust tubes. It is these tubes that are opened to the atmosphere by the factory unloader, and it is these tubes from which I would do a supplementary unload with a solenoid valve. Not the tank itself, no no no.

Some unloaders are air operated. For example, some intake valves have little air pistons which push down on the valve disk. This allows air to be drawn into the cylinder but then it prevents the piston from compressing the air. But not all unloaders work this way.

metalmagpie
 
The transients during the startup of the compressor are what's important. Do all three legs sag significantly? Just two? How large is the sag?

My electrically savvy buddy wants me to set up a camera on a tripod and record those meters during startup. I haven't done this because to me it seems that the generated leg sagging is the only realistic scenario, so setting up to confirm that is likely to be wasted work.
 
The other, unlikely observable would be the utility leg sagging for some reason. Have somebody put the compressor on line while you watch that. A null experiment is still an experiment - you can absolutely rule that out then.
 
My electrically savvy buddy wants me to set up a camera on a tripod and record those meters during startup. I haven't done this because to me it seems that the generated leg sagging is the only realistic scenario, so setting up to confirm that is likely to be wasted work.
If you have lights in your shop, you have effectively already done the experiment. If the air compressor is drawing down the line, you should be seeing the lights dim, or some may even blink off.

A drop in voltage sufficient to stall the compressor would be significant, and should be easily visible in the lights. If you have not noticed it, that is probably not the issue.

Plus, the thing started with some extra idlers. That means the service etc can hold up the compressor starting load, PLUS the idle draw of extra motors.

The known weak spot is the generated leg, and the "extra idlers" test pretty well confirms it.
 
The compressor motor is drawing more current after the rebuild, if the compressor wiring is marginal or there is a long distance you may be getting voltage sag at the motor. Another option would be to reduce the motor pulley size.
 
lighting will be on a separate branch circuit(s). Yeah I know it's a long shot but just look at the darn meter when the thing trips on. Rule that out.
 
I haven’t seen anyone yet ask if it’s possible the compressor motor is drawing more amps than it should?
 
I haven’t seen anyone yet ask if it’s possible the compressor motor is drawing more amps than it should?
You kidding? Everything for this guy is "wasted work" (his words). Can't check voltage, balance, amperage, nothing.. But he wants to add caps..............:bawling: Basically he wants someone on the Inet to tell him his issue because he is too lazy to troubleshoot the issue. I'd say let him waller round a bit.
 
lighting will be on a separate branch circuit(s). Yeah I know it's a long shot but just look at the darn meter when the thing trips on. Rule that out.
Easy enough.

But if the utility is drooping, the lights will show it, different branch circuit or not.

Basically, since the setup runs the compressor OK with added "idlers" in the form of machines "idling", that is a very positive indication that there is no issue aside from a sagging generated leg.

That was stated in post #1 of this thread, and should just about eliminate any other cause aside from the RPC not quite having the ability to hold the 3rd leg up enough to start the motor.

We were not told how big those motors were. It would be easy enough to just put an extra motor on the system, of a suitable power to do what those machines did. Depends if there is a such a motor available on the spot, or obtainable by the OP.

While not zero cost, it is likely far cheaper than buying a whole replacement RPC. Probably cheaper than putting a single phase motor on the compressor. And it is known to work.
 
I had a similar issue trying to use a VFD to start a 10hp quincy. farted around with the unloader valve forever, no joy.

Installed an electric air solenoid valve on the unloader and delay timer set long enough to get motor up to speed plus a few seconds. works a charm now for many years.

I too ran into a similar problem when I upgraded to a 10hp IR compressor. My 30hp rated Phase-a-matic RPC would not provide enough current inrush power for the 4-6 seconds that it takes for the compressor to come up to speed. The problem is worse in the winter when the compressor oil is colder/thicker. A heater on the compressor helps a lot.

Like others, my solution was to install an electric unloader valve onto the compressor head and run it off of a timer. Mine is set for 8 seconds. It’s worked fine for 3+ years now.

Wiring to the RPC from the load center is oversized (2/0) as is the wiring for the 3rd leg from the RPC to the transfer switch.

Compressor is fine; if I run it off of my 400hp generator it spins up almost instantly.
 
Easy enough.

But if the utility is drooping, the lights will show it, different branch circuit or not.

Basically, since the setup runs the compressor OK with added "idlers" in the form of machines "idling", that is a very positive indication that there is no issue aside from a sagging generated leg.

That was stated in post #1 of this thread, and should just about eliminate any other cause aside from the RPC not quite having the ability to hold the 3rd leg up enough to start the motor.

We were not told how big those motors were. It would be easy enough to just put an extra motor on the system, of a suitable power to do what those machines did. Depends if there is a such a motor available on the spot, or obtainable by the OP.

While not zero cost, it is likely far cheaper than buying a whole replacement RPC. Probably cheaper than putting a single phase motor on the compressor. And it is known to work.

The secondary machines were a 6.5hp Nardini lathe and a 3hp Scotchman ironworker.
When I get back to that project I will try it with just the ironworker's 3hp boost.

metalmagpie
 
No. 3 would probably work but buying a 7.5hp VFD would cost more than my whole compressor.
The Chinese-made VFD's on Ebay (and other sources) are cheap in price, and surprisingly good in service. They build and sell so many VFD's, the Chinese have gotten really good at it.

As a pricing example: a 7.5kw (10 hp) can be had for $160-$190 delivered.

No affiliation, just a satisfied user.

ToolCat
 








 
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