Author Topic: How can an AC motor run fast?  (Read 1337 times)

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Offline rea5245

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How can an AC motor run fast?
« on: July 20, 2015, 04:26:05 pm »
Hi,

I have a store-bought digital flip clock that runs off a 60 Hz AC motor (there's a manufacturer's sticker on it that confirms it's "60Hz"). It's old - a few decades - so I could understand it having problems. But the problem I'm seeing is that it runs fast. It advances the time by one minute every 55 seconds or so.

Using a Kill-a-Watt meter, I've confirmed my line frequency is 59.9 Hz. How is it possible that an AC motor would run fast? If I understand AC motors correctly, even a noisy power line shouldn't cause such a thing.

Thank you,
   Bob
 

Online IanB

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Re: How can an AC motor run fast?
« Reply #1 on: July 20, 2015, 04:39:35 pm »
Did you ever see it keeping correct time? Was there some event when it stopped working properly?

Such clocks would use a synchronous motor that is locked to the AC frequency. If the clock is not keeping time it seems that somehow the motor could be faulty.
I'm not an EE--what am I doing here?
 

Offline AG6QR

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Re: How can an AC motor run fast?
« Reply #2 on: July 20, 2015, 05:03:21 pm »
There is a gear train between the motor and the things it is driving.  Is there any chance someone "repaired" it at some time in the past and swapped a gear for one containing a different number of teeth?

Not all AC powered clocks use synchronous motors, though that's a very common way to do it.  AC synchronous motors give off an audible hum at the mains frequency.  Can you hear the hum in this clock?  Are you sure it's got an AC synchronous motor?

Is its speed of running consistent?  Or is it somewhat erratic?
 

Online Tomorokoshi

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Re: How can an AC motor run fast?
« Reply #3 on: July 20, 2015, 05:06:17 pm »
The speed of a synchronous motor is:

Ns (RPM) = 120 * f / p

f = line frequency
p = number of poles

http://en.wikipedia.org/wiki/Synchronous_motor

If a pole pair shorts together, then p is decreased, which would increase the speed.

You might be able to hear a beat frequency as the rotor passes by the shorted poles.
 

Offline G7PSK

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Re: How can an AC motor run fast?
« Reply #4 on: July 20, 2015, 08:35:11 pm »
Most clock motors only have one coil so any pole shorting will be of the mechanical variety more likely that a pole or two is missing from the motor, a finger broken off in the past but check for iron dust in the motor as that is what would short the poles.
 

Offline Fank1

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Re: How can an AC motor run fast?
« Reply #5 on: July 20, 2015, 11:33:33 pm »
Clock motors usually have a "shading coil" it's just one loop of copper wire.
Check to see if it has become open.
 


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