Help with capacitor.....

[slim_boy_fat]

Hi guys, merry Christmas and a guid Hogmanay to all.

I'm having bother with an electric motor I was given - it's a 1HP 240V job and a big heavy barsteward [made by Bruce Peebles in the 1950s I reckon], measuring c. 10" in diameter. When I plugged it in, it ran fine, but the next time [soon after] it blew the fuse.

Replaced that and tried again, it hummed for a moment then blew the fuse again. New fuse, ran fine for three attempts then next time tripped the RCD .

I'm thinking the start capacitor is ******** but it has no label or printing on it to indicate size. From searching around the interweb it looks like it needs to be in the 130 - 200 mfd range but I'm wary of getting one which is the wrong size. I don't like electrics, because I don't trust what I can't see.......

Can anyone tell me which start capacitor I need , I don't want to spend a fortune because it a buckshee motor I don't really have a use for at the moment [folk tend to just give me random things, knowing I 'potter'...... ]

Any help appreciated.

 

[DAPPH]

First use a decent voltmeter to check the supply voltage without the motor switched on . Then take a reading with it switched on if possible ..just in case you're getting a massive drop in voltage that may indicate a short or mains supply problems

If you try and spin the motor in the direction of rotation as you switch the motor on and it runs then consider the capacitor is dead .

If it does not run then disconnect the motor , short out the capacitor wires with an insulated screwdriver or some ignition HT cable or a nice big resistor to stop you getting the capacitor discharge voltage ....which can burn holes in finger tips etc ( you should get a nice big fat spark ) then take the end caps off the motor and clean out the crud present .

.

To me it sounds like you have a short due to moisture ,spiders webs etc in the motor that run to earth . Try storing the cleaned out motor somewhere warm & dry for 72 hrs then try the restart again.
My ancient 30 yr old plus fan assisted stick welder suffers several trip outs before the current flow heat dries everything off & then it runs like a dream for hours at a time & stays OK for several months even if is not used .

If it still fails send it off for a full rewire and test as you might have some shorted out windings

 

[rtbcomp]

If it tripped the RCD you probably have a leak or a short to earth.

You may or may not get a spark by shorting the capacitor - it is connected to AC rather than DC so whether it is charged or not depends where in the mains cycle the power was when it was switched off.

 

[Pete.]

I would remove that capacitor, insulate the wire ends and see if the motor hums when powered (without tripping the mains) to see if it's just a shorted cap. It may or may not spin and if it does the direction might be one way or the other but it shouldn't blow the trip at least.

 

[DAPPH]

Thanks Pete & RTB

 

[brightspark]

I would remove that capacitor, insulate the wire ends and see if the motor hums when powered (without tripping the mains) to see if it's just a shorted cap. It may or may not spin and if it does the direction might be one way or the other but it shouldn't blow the trip at least.

that would only happen if the cap case was aluminium and bolted to the frame and it shorted to earth inside just unbolt it and insulate it from case

 

[slim_boy_fat]

Thanks for the replies so far lads, I appreciate them.

Can't get into the shed this morning as the door's frozen - and it's snowing again.......