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Motor start, motor run capacitors

Posted By: Tesla

Motor start, motor run capacitors - 03/24/15 02:09 PM

I've picked up a second hand Gast 'ring compressor' - the kind used to blow air for pneumatic systems at banks and the big box stores -- and the occasional pet store with plenty of fish tanks.

It's wired for 240VAC one-phase... it's a nominal 2.5 hp motor... start cap -- run cap -- and thermal protection via solid state inside the pecker-head.

But the capacitors are all mixed up/ missing.

I don't normally work this side of street... and was hoping that the ECN crew would/ could guess a first approxmation for suitable values.

The internal wiring megs out fine.

Near as I can tell, the motor was either wired Monday morning by a hung over trooper or it was pumped at 120 VAC by a prior owner -- or some such.

I don't have a motor test bench -- no decade boxes. I'd rather not throw every capacitor in the world at this little project.

Posted By: gfretwell

Re: Motor start, motor run capacitors - 03/24/15 05:09 PM

I would start by looking for the motor specifics on the net, maybe poke around in the Grainger catalog.
They may just tell you what you need and source it for you.

This sounds like what you have but Gast does not make the motor so You are going to have to sort that out somewhere else.
Posted By: sparkyinak

Re: Motor start, motor run capacitors - 03/25/15 05:07 AM

Hmmm. Didn't Tesla invent the induction motor?.... dunno
Posted By: Tesla

Re: Motor start, motor run capacitors - 03/25/15 05:15 AM

The Gast nameplate reads: R6125-2

It's an R6 series unit, 1-phase, 2.5 hp with a 2 port.

2.5 hp 1.86kW 215cfm 45in/H2O pressure 56in/H2O vacuum

This is not a high torque machine, though the rotor does have a fare amount of heft.

Posted By: gfretwell

Re: Motor start, motor run capacitors - 03/25/15 05:52 AM

I think I would fire off a message to Gast support. I had no joy searching the documentation online.
Posted By: Tesla

Re: Motor start, motor run capacitors - 03/30/15 10:24 PM

And the answer is:

All field windings check out...

All installed capacitors check out... values and condition

Factory error: the connector from the run cap to the centrifugal switch was flopped over the connector from the start cap -- and vice versa.

The actual internal design of the Emerson motor is NOT shown on the Internet nor in the literature.

Pretty amazing, no?

The way this Emerson motor works:

At start the switch is in position one and the 50mFd start cap is in the circuit to the start windings.

Upon reaching the magic speed, the centrifugal switch shifts to position two and the 50mFd cap is cut out entirely.

However, in position two, the run capacitor (10mFd) drops in to the circuit.

The centrifugal switch is obviously in mass production and is to be expected in all recent motors built. (Forty-years +)

None of litureture ever references this dual connection centrfugal switch... a rotary version of "normally closed" "normally open" logic.

BTW, taking the Gast/ Emerson motor apart was quite a challenge. Fortunately I had a hammer and a temper.

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