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Joined: Jul 2007
Posts: 1,335
I'm dealing with a motor where the run start and run capacitors were ripped off. One of the leads is labeled T9 at the run capacitor. Could it be by chance a T6? In my research, I came across one diagram online and it too was labeled T9 in Lieu of T6.
Is that typical?

"Live Awesome!" - Kevin Carosa
Joined: Jul 2004
Posts: 9,935
Likes: 34
I have seen the black wire going to the cap labeled J9.

Greg Fretwell
Joined: Jun 2004
Posts: 1,273
Your best bet is to go to the manufacturer's Internet site and hunt down their favorite wiring schemes.

Emerson Electric has an entire page dedicated to every variant of their designs.

The thing to remember with one-phase motors that are cap-start and cap-run is that the cap-run is in the design not to improve power factor at all -- but to suppress harmonics. (torque, noise and vibration)

There are two basic schemes that have been employed.

1) Is the 'either--or' scheme: The motor starts up on the start cap ONLY; then when the motor spools up - - the centrifugal switch cuts out the start cap and then engages the run cap.

2) Is the 'both-- then one' scheme: The motor starts up with BOTH caps in the start winding circuit. Then, when the motor spools up, the start cap is cut out.

The run-cap is always dinky -- 7.5 mfd 10 mfd 12.5 mfd are typical values -- and can be back calculated when missing JUST BY THEIR (footprint) SIZE. Go to ANY capacitor distributor/ manufacturer Internet page and pull off the dimensions. The key one will be the LENGTH. The NEMA players have at most three values in a given length.

Because the run-cap stays in the START circuit, always, it's not only dinky -- it's not critical to the motor, which would run if the start circuit (the run cap) were wire nutted out of the motor running circuit. It's there merely to improve torque and suppress harmonics. Its dinky rating is there to CHOKE back the current flow into the start windings down to modest levels -- lest the wiring be burnt up straight away.

So, for test purposes -- and NO LOADS -- you can power up such a motor with the leads to the run cap nutted off/ taped off in isolation.

A properly working motor would do its thing whether it was design #1 or design #2.

[Keeping in mind that one can conceive of schemes that route critical conductors for the start cap through the run cap location. When this is done, they should become obvious as one can either tug on them physically (direct and short taps) -- and such a link would be a dead givaway that it's a 'jumper.']

The start cap is sized based upon current more than any other factor. So a low voltage motor at low horse power may need the exact same cap as motor with twice the HP at twice the voltage.

All caps are rated by voltage limits -- which are NOT based upon the RMS we're all used to. The figures used are the PEAK voltage availble from AC power... plus some insurance.

Run caps will be marked with a value and a % tolerance on the case.

Start caps will be marked without a % tolerance -- instead they will show a capacitance RANGE right on the body.

So it's a snap to spot one versus the other even if you knew nothing else.

The dinky values used by run caps does not make their caps fantastically smaller than a start cap... though they will always be smaller; say half-off. ( not 93% off )

Like the run caps, the start caps have a limited range -- for a given voltage -- for a given size -- NEMA standards and all.

Because they're only in the circuit for the starting... even being off a tad in the start cap will not be a crippler... though it's always best to get the factory's spec installed. If the motor is older, your best bet is to pick the medium value -- for the cap size that best fits the 'foot print' still evident on the motor.


You've not listed your tool suite, so I don't know if you can meg out the (motor) circuits. I'd never want to put an abused motor back into service until it was megged.

You might even be looking at a ruined motor that has been salvaged for its caps.

That may mean sluggish bearings, a rotten centrifugal switch, etc.

BTW, if you've got the ability to spin the motor -- even chuck it in your drill motor -- then you can dope out bearings -- and if you can get it up to speed -- safely -- you can test for the centrifugal switch, too.

The second you are dealing with a 'salvaged' motor you need to take it through a fair amount of testing -- at low (instrument testing level) power (no load) -- lest you let the smoke out.

As a practical matter, older motors have only scrap economic value.

Why? They can't compete with the latest units which are so much more efficient that they pay for themselves in less than two-years in active service.

If you're just a hobbyist -- then it's a whole different equation.

I'm not in Alaska, but hereabouts, if it's not factory perfect -- we don't install it. We're not set up as a motor repair shop. Even defective caps are a 'tell' that the motor is at the end of its design life -- as far as we see it.

Last edited by Tesla; 04/24/15 03:19 PM.

Joined: Jul 2007
Posts: 1,335
I got it all figured out. Not to mention brand names but in big lettering across the data plate read "Brand such and such" hunted down the company through six name changes. Called technical support ant left messages. Could not find anything on the internet. As I catch someone finally in tech support that answers their phone, "we don't do the capacitors, our customers do" which I thought was weird but I called the equipment manufacturer who's used the motor. I didn't have an account with them so they wouldn't give me a time of the day. Got hold of a local rep which is extremely rare being on an island in the middle of nowhere. We contacted the factory and came up with nothing. After an exhaustive research on the internet, I came up with zilch

Going through all my notes, starting from scratch, I called the motor company back and we talked a little bit. I told him the motor came from his company in big blocked letters across the data plate. The guy on the other end goes, "But we don't make motors, never did." I told him I have a photograph of the data plate I'd gladly send to them and as I am looking at the same pic as I did over and over again, in small print in the lower corner of the data plate, "motor designed and built by Brand X." Well I felt like an idiot. At least I was polite to the guy on the phone.

So I contacted Brand X, gave them the motor model number. Less then 5 minutes, I had the diagram on my computer, 10 minutes later, the Caps were reconnected... I feel like an idiot.

crazy blush blink

"Live Awesome!" - Kevin Carosa
Joined: Dec 2002
Posts: 206
Never feel like an idiot when you find the right answer! Besides, the tech rep of the company who badged it ought to have known where they outsourced their motors.

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