Electric motor knowledge required!

I wonder if one of you electrical geniuses can help?
A while back I acquired a small portable compressor. It is a 240v,
single phase, brushless, capacitor start machine. Marketed by SIP under the model name of Airmate WM39 it is probably between fifteen and twenty years old.
When switched on the motor 'hummed' but would not run. If helped with a flick it tried to turn but never more than about half a rev.
A new capacitor of the same spec. made no difference.
A peek inside the end cover showed no sign of burning or damage.
The cylinder, piston and con-rod were removed to lighten load. Like this it would run - with the help of a flick to get it going - for about twenty seconds; when it would slow and stop. The casing after these attempts was warm but not hot. No nasty smells were made!
A complete strip down revealed no physical damage whatever. All was clean, bright, and un-burnt. There were no marks on the rotor.
There are two pairs of wires from the stator; one pair to the switch and the other pair to the capacitor. None of these are down to earth. There is a satisfactory earth wire in place. Across the switch wires there is a 6 ohm resistance, and across the pair to the capacitor there is 10 ohms.
Please Teacher - what is wrong with my little machine?
Mike :(
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Mike Whittome

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On Fri, 12 Dec 2003 00:48:58 +0000, Mike Whittome

This is pretty strange - the behaviour and the connections don't add up. A capacitor start motor with power applied to the main winding should run normally once it's given a flick to get it going. The internal wiring should be:-
supply power directly to the main winding supply power to the series connection of :- starting switch capacitor starting winding. Since these last three items are all in series they can be connected in any order - it doesn't make any difference.
What doesn't add up is the resistors across the switch and the capacitor. If they really are 6 and 10 ohms this is incorrect and they should be removed.
A common fault on capacitor start motor is faulty or oxidised contacts on the starting switch.
Jim
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Jim,
Thanks for your interest.
I have rushed out and measured the resistance across the end of the two pairs of wires coming from the stator more carefully, and show below all the six possible combinations ........
-------------------------A S green pair to switch -> T -------------------------B A T O --------------------------X R red pair to capacitor -> --------------------------Y
Across
AB 4.8 Ohms AX 7.0 Ohms AY 10.8 Ohms XY 3.9 Ohms BY 10.0 Ohms BX 6.3 Ohms
Reading my post and your response again I may have not expressed myself well. From the stator windings are two pairs, red and green. It is the at the end of these that I have been taking the readings. The stator is sitting on the bench with nothing else connected to it.
I think we can rule out the switch contacts as I tried with a direct mains connection to the green pair, bypassing the switch. This made no difference to the symptoms.
Mike
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On Fri, 12 Dec 2003 11:55:15 +0000, Mike Whittome

I'm afraid it doesn't look hopeful. Firstly the capacitor start motor connection sequence I listed is correct and unambiguous. This means that with the bare stator not connected to anything else 4 wires should mean separate run and start windings with NO interconnection. If this is what you have then your readings indicate an interwinding short.
A common arrangement is 3 wires with one of the wires common to both start and run winding. A fourth wire (improbably) could be used for a two speed or a dual voltage setup and this would show all windings interconnected. However these are pretty rare beasts and I can see no reason why one should be used in your compressor.
In a spirit of desperation you could try momentarily connecting the power supply (suitably fused) to each of the possible pairs and trying to flick start it. Success should identify the run winding. With this identified, experiment with the capacitor connection to try to find one that develops starting torque.
Jim
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Jim,
Your continued interest is appreciated.
If I understand you correctly then, there is roughly something like the diagram I have attempted below. There should be no connection between the two circuits? Which I appear to have! :(
A 22g switch _____________________-_________ live | __________________-_________ neutral | | B 22g | | presumably the | | IIII III IIII III IIII III IIII III STATOR IIII III IIII III IIII III X 27g | |_____________ | | | === capacitor |_______________| Y 27g
Examining the connections of the leads to the windings - after a bit of careful prying apart - we have .......
Each of the two red leads from the capacitor join to a single 27g wire from the windings.
Each of the two green main leads from the double pole switch join to one 22g wire and one 27g wire from the windings. Would it be these 27g wires which are giving the cross connection.
Mike
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On Fri, 12 Dec 2003 16:20:07 +0000, Mike Whittome


The plot thickens! You described it as a capacitor start motor which would have a centrifugally operated single pole switch in series with the starting winding. The circuit you have drawn is not for a capacitor start motor but looks like a misconnected capacitor RUN machine fitted with a double pole manually operated on/off switch.
If this is the case, Connect one 27g wire to one 22g wire and also to Neutral. Connect the remaining 27g wire to one capacitor terminal. Connect the other capacitor terminal to the remaining 22g wire and also to Live.
Apply power and it should start and run normally
An unsolved puzzle is that capacitor run motors have pretty poor starting torque and would not normally be used to run a compressor!
Jim
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Jim,

A little knowledge is a dangerous thing - yes you are quite correct.

I will try this tomorrow.

I live in hope. !!!

Indeed an unsolved puzzle. Also, I know the history of the machine from new. The previous owner certainly would not have fiddled with the connections so we may yet have a further mystery on our hands. Whilst these small compressors are not terribly expensive these days I am loath to dump it without a struggle - particularly as there is no visible damage.
Will advise developments
Mike
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If it's 15 to 20 years old and the electrical wizards get your rmotor going, it's worth being 100% sure that there's no internal corrosion to the air resevoir. These compressors do leave a lot of water in the resevoir and unless it's been 'blown down' by removing the drain plug when the pressure has dropped down to about 20psi, then the water sits in the bottom corroding away. I always leave the drain plug out when not in use in a possibly vain attempt to dry out the resevoir. I hate to think of the consequence of a corrosion failure to the resevoir when running about 100psi a few feet from me in the workshop. Comments very welcome from experts in this field which might provide some informed input as well as reassurance !!! Should they be regualrly hydraulically tested or will that do more harm than good?
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Dick,
Point taken - thanks.
Mike.
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Mike Whittome

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