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Mains operated synchronous motor clock running super fast

HectorLooi
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HectorLooi Distinguished Member

HectorLooi

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This is so weird. 

It's 4:10am, I got up to use the bathroom and noticed my ancient Smith's electric table clock showed 12:25.

Then I noticed the second hand running super fast. I used my phone stopwatch to time it and it was taking 30 seconds to complete one revolution.

How can that be??? 🤪

Synchronous motors running speed are dependent on the mains frequency. I looked at my bedside radio clock, which also depends on the AC frequency for timing, and that's showing the correct time???

Then I start to worry. Is something in my electrical system that is arcing and causing spurious mains signals?

I unplug my Smith's clock and plug it into a different wall outlet and it's running normally. Really weird.

I'm going back to sleep.... brain not working....

Goodnight!

HectorLooi Distinguished Member

HectorLooi

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I found that the clock runs fast when I plug it into the socket that has a built in surge protector.

I took apart the surge protector and found that it only has 2 components inside, a gas discharge tube and a metal oxide varistor wired in series across the L and N line. Something must have failed and clamped the voltage above a certain value and affected the motor.

The clock runs fine when plugged directly to the wall socket.

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