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Old 05-29-2016, 12:30 AM   #1
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Default Locked rotor condition

Hey dudes I got a table saw out of someone's trash a while ago and every once in a while it acts like a locked rotor, sometimes no spin sometimes extremely slow spin and others no resistance at all. I haven't done any testing to it yet.
I would like to fix it just for the experience and fun of the project.
Any ideas?


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Old 05-29-2016, 12:39 AM   #2
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Problem with the start winding or centrifugal switch?
Is it an induction motor or a universal motor (AC/DC) with brushes?
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Old 05-29-2016, 12:41 AM   #3
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It's just a single phase motor.
It's a craftsman from Sears


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Old 05-29-2016, 12:54 AM   #4
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120v 7.8a single phase induction


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Old 05-29-2016, 02:45 AM   #5
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120v 7.8a single phase induction


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Easy enough..

Check the starting switch to make sure they are not sticky or not operating properly..

If the starting switch is in good shape then take a quick look at starting windings to make sure they are not burnt. ( common with run winding especally hard cutting.can overload the motor easy..)

Also the starting cap(s) can go bad too..
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Old 05-29-2016, 10:57 AM   #6
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Intermittent starting trouble on a single phase induction motor is almost always the start switch.

It's inside the motor and often all it needs is a shot of WD40.
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Old 05-29-2016, 11:29 AM   #7
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Seems like the circular saws I've seen have all been universal. Where are they hiding capacitors for induction motors?
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Old 05-29-2016, 11:31 AM   #8
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I worked for Sears selling power tools in the 70s while going to school, it was a good part time job for a student. For years, Sears had used Century Motors brand labeled as Craftsman, those old motors lasted for decades, some of my 50s vintage Craftsman bench tools are still running. Then in the 70s they switch to Emerson. Those Emerson motors were notorious for not lasting long, but tended to make it just past the warranty. Their favorite failure mode was for the starting switch to get gummed up and fail to open or re-close consistently. When it fails to open, the motor starts, but doesn't develop enough torque to finish accelerating. If it does work to start, but fails to re-close after you turn it off, it doesn't even start the next time, it just sits there and hums or shakes. If you are handy, you could disassemble the start switch and give it a good thorough cleaning with some electrical contact cleaner, it's worth a try. The start switch is behind the end bell on the non-drive end of the motor.

I learned a lot about single phase AC motors while working there because although we were not supposed to fix things, they would often tell us not to return tools to the factory for refurbishment (because they would be over stocked on returns or something). They wanted us to destroy them to remove them from the market, but I would take them home to dissect them so that I could learn. Good times...
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Old 05-29-2016, 11:38 AM   #9
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Seems like the circular saws I've seen have all been universal. Where are they hiding capacitors for induction motors?
Portable circular saws, yes they are universal but the bench power saws (he said it was a table saw) were all CSCR or Split Phase, depending on whether they were direct drive or belt drive. They used Split Phase on the direct drive so the arbor could get as close to the table as possible, which was cheaper to make and sell, but still not as close as belt drive, which allowed for deeper cuts.
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