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Already told you how to check. And 13amp on 3hp often trip.
- Thread starter dobbslc
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Bladevane
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Once you have disconnected it from the compressor if it spins easily by hand but still won't start it could be the centrifugal switch that is U/S or stuck. Wrap a thick bit of string round the shaft a few times, pull quickly to get some speed in the motor and switch on the power. If it is the switch the motor should run to speed. If you then kill the power listen for a click as the motor slows to a halt. If no click the switch is deffo the problem. If it does not run at all there is a deeper electrical problem which might be something simple like a cable disconnected in the terminal box.
eSCHEn
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Well that's good news in that it's not seized Rob 
At least a motor is easy enough to swap out (or get rewound) should it come to that. As you probably already know, 3HP is right on the limit of what 13A can supply so it wouldn't take much to make it draw that wee bit more and pop the fuse. Do you have a higher capacity supply you could try it on? Maybe not needed given you've got a megger though (I'm no sparky)
Edit: bladevane's got even better info than mine
At least a motor is easy enough to swap out (or get rewound) should it come to that. As you probably already know, 3HP is right on the limit of what 13A can supply so it wouldn't take much to make it draw that wee bit more and pop the fuse. Do you have a higher capacity supply you could try it on? Maybe not needed given you've got a megger though (I'm no sparky)
Edit: bladevane's got even better info than mine
dobbslc
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Thanks guys, I'm feeling a bit more optimistic about this now.
I'll grab a 16amp commando plug and socket this week and rig that up, but I think there's a fault somewhere as the motor just twitches a bit before poping the fuse.
I've already tried spinning the pulley as I start it but the same thing happens.
If I get it running I'll then have to see if it fits under my bench, it's a bit bigger that I thought.....
If it does all get sorted I'll have a compressor that should run everything I need.
I'll grab a 16amp commando plug and socket this week and rig that up, but I think there's a fault somewhere as the motor just twitches a bit before poping the fuse.
I've already tried spinning the pulley as I start it but the same thing happens.
If I get it running I'll then have to see if it fits under my bench, it's a bit bigger that I thought.....
If it does all get sorted I'll have a compressor that should run everything I need.
eSCHEn
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Good stuff Rob. Positive attitude helps as even if it is fubared you're at least in a good(ish) mood
What do the brushes look like? If they were worn badly then you wouldn't get the power and I think it would draw more as well.
At a push you can use the cooker connection point for a higher rated supply but be careful if you do as it could draw a lot of amps before it trips...
What do the brushes look like? If they were worn badly then you wouldn't get the power and I think it would draw more as well.
At a push you can use the cooker connection point for a higher rated supply but be careful if you do as it could draw a lot of amps before it trips...
I'd say the twitching before blowing the fuse might give some hope that it's just a capacitor problem.
If you search for "capacitor start capacitor run" then you'll find a basic schematic for the motor which might let you work out what to test. At rest the switch should be closed, the windings should be probably be a few ohms and the caps should be be open-circuit, though they may look short initially while your meter charges them up.
You may be able to connect just the run capacitor and get it to start with a bit of string, etc. If that works OK, then it's a problem with either the start cap or the switch. If the wiring permits, you could swap the caps over for a test too.
richie00boy
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AC motors don't have brushes.
eSCHEn
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Not all AC motors are brushless Richie. A lot of universal motors are used as they're cheaper and they definitely have them.
Dead right. However, the OP did say he'd 'disconnected both capacitors', which assuming they're start/run caps does rather imply it's an induction motor rather than a universal in this particular case.
eSCHEn
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Good point Wilbur. I blame not realising that on the fact that I'm an incompetent moron
That sounds like cruel rejection to those of us who are both...
eSCHEn
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What a very kind way to put it
I am indebted sir x
dobbslc
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Ok I've managed to sneak out the house for a quick fettle!
I've tested the motor coils and caps and found.
Start coil = 1.6-1.8 ohms.
Run coil = 1.6- 1.8 ohms.
Switch looks and works ok.
20meg ohms + to earth on everything.
Run cap (200 uf) went slowly from high to low meg ohms. I think
Start cap (50 uf)went almost straight to 0.5 meg ohms.
I found a very loose spade terminal on the neutral supply cable the goes to the start cap....
That's all I've got time for tonight.
I've tested the motor coils and caps and found.
Start coil = 1.6-1.8 ohms.
Run coil = 1.6- 1.8 ohms.
Switch looks and works ok.
20meg ohms + to earth on everything.
Run cap (200 uf) went slowly from high to low meg ohms.
I thinkStart cap (50 uf)went almost straight to 0.5 meg ohms.
I found a very loose spade terminal on the neutral supply cable the goes to the start cap....
That's all I've got time for tonight.
Maker
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A capacitors resistance changes with the supply frequency* (or something like that), so there was probably little point measuring themRun cap (200 uf) went slowly from high to low meg ohms.
Start cap (50 uf)went almost straight to 0.5 meg ohms.
.It sounds like it's this loose spade terminal, fingers crossed
.*If you're interested, you can work out the resistance of a capacitor by the formula;
Ω=(2*pi*f*C)^-1
Where f is the frequency (50 when it's plugged in but the multimeter will he DC so I don't know what's happening here
) and C is the capacitance in farads.A capacitors resistance changes with the supply frequency* (or something like that), so there was probably little point measuring them
That last bit is only sort-of true. If you measure with a meter (on ohms) then the meter charges the cap up with the meter's current source. At first, while the cap's discharged, lots of current flows (and the voltage across the cap is low) so the meter indicates that you have a low resistance. Gradually the meter charges the cap up, so the current falls, the voltage rises and eventually the meter should think that it's seeing an open circuit.
The time this takes to happen depends on the relationship between the meter current source (which you don't generally know and might change as it re-ranges anyway) and the capacitance of the cap.
So you're right that it's nigh-on impossible to *measure* the capacitance of a capacitor using a meter on ohms, but you can often get an idea if it's working a bit. Like you say, a real capacitance measuring device would use some kind of alternating current.
eSCHEn
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I like your explanations Wilbur - they make sense!
