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Old 06-11-2011, 05:42 PM
Tim66 Tim66 is offline
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Default How to determine polarity of a motor start capacitor

To start, I have very little knowledge about electronics as I am a software engineer but am willing to get my hands dirty with electronics if I'm certain of what I am doing.

So anyway, a friend gave me a 14" bandsaw that had a problem getting started. After turning on the switch, the motor just hummed and wouldn't get up to speed. You had to physically spin the top wheel of the bandsaw to get it going (not the safest method). I pulled the motor out and saw that the motor start capacitor had leaked (the guy that gave me the saw suspected this was the problem).

So, I bought a new capacitor (McMaster - Part Number 7245K16), but I can't figure out the polarity of the new capacitor. The old one has a black screw on one terminal and a gold one on the other (though the black screw is where the capacitor appeared to leak so it could just be black from what ever leaked out).

New Capacitor

The only thing I can see on the new capacitor are two terminals with a circle somewhat closer to one terminal than the other that appears to have a "left-handed" checkmark in it. I'm not sure if that's some sort of electrical symbol marking the negative post or not.

So, can anyone tell me which terminal is negative (if any)?

Thanks.
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Old 06-11-2011, 06:05 PM
Hagen Hagen is offline
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Motor starting capacitors ar non-polarized, AC devices. It doesn't matter which way you connect the wires.
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Old 06-12-2011, 05:37 PM
Tim66 Tim66 is offline
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Thanks, I really wasn't looking forward to having it blow up in my face!
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Old 06-12-2011, 06:06 PM
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#12 #12 is offline
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It can still (almost) blow up in your face. If the motor is stalled and can not start, the dry electrolytic capacitor that you have will overheat and release copius amounts of white smoke. They don't actually explode, but if you see the white smoke, it is ruined and will have to be replaced when you buy a new motor or clear the obstruction.
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