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lchien52
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Bronze
Re: Which is more accurate?
lchien52   5/3/2013 10:50:57 AM
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the quartz analog clocks rely on an inexpensive quartz crystal which is probably+/-100 ppm, typically probably 20 ppm.

The AC synchronous motor clocks rely on the power company. I think most of the major AC power grids litteraly count the number of cycles in an hour and speed up or slow down slightly every hour to make the long term stability way better even than the short term stability. So that there are exactly 5,184,000 cycles in a day (60 Hz US standard).

 

 

Rob Spiegel
User Rank
Blogger
Re: The things we get used to
Rob Spiegel   1/31/2012 3:02:21 PM
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Yes, Bpark, in the story it was a two-stroke. A dirt bike if I remember right. That would make sense. So the engine doesn't care what direction it turns. That must be quite surprising when it happens.

bpark1000
User Rank
Iron
Re: The things we get used to
bpark1000   1/30/2012 9:32:36 PM
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2 stroke engines with reed valve intakes can run in either direction.  One of my friends would start up model airplane engines, and they would sometimes get going the wrong way.  He would throw a rag into the propeller to stop the engine so it could be restarted.  So the bike engine running backwards is no mystery, if it's 2 stroke.

jwillsey
User Rank
Iron
Re: The things we get used to
jwillsey   1/30/2012 6:30:15 PM
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I assumed the DC adapter was for another appliance plugged into the same outlet as the clock.

I cannot imagine the adapter causing the grinding sound. My first thought is of a poor service connection. Plugging in the adapter caused an intermittant connect-disconnect (sparking) of the 110v service to the outlet. If you caught this motor just right, it could reverse it. You may also hear it as chatter or grinding in the clock. Not a safe condition.

bpark1000
User Rank
Iron
Re: The things we get used to
bpark1000   1/30/2012 12:34:59 AM
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DC adaptor?  I thought the synchronous motor was run directly from the AC line.

Jack Rupert, PE
User Rank
Platinum
Re: The things we get used to
Jack Rupert, PE   1/28/2012 2:22:38 PM
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That sounds like an extremely plausible explanation bpark1000.  What do you think caused to correct itself when the DC adapter was plugged in?

Rob Spiegel
User Rank
Blogger
Re: The things we get used to
Rob Spiegel   1/12/2012 12:24:22 PM
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That makes sense, Bpark. We had an entry in the Made by Monkeys section by the owner of a small motorcycle. He told the story of his bike starting backwards. Same concept, I guess.

William K.
User Rank
Platinum
Time ticks in reverse: clock runs backwards
William K.   1/11/2012 11:12:34 PM
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I had an analog alarm clock that developed a similar tendancy to run the wrong way. What I found is that it had a synchronous motor that was equally comfortable starting and running in either direction, and it used a rachet mechanism to force it to only run in the desired direction. When the grease on the rachet mechanism thickened a bit it kept the rachet dog from engaging the wheel, and so the motor could now rotate in either direction. 

bpark1000
User Rank
Iron
Re: The things we get used to
bpark1000   1/11/2012 5:46:09 PM
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There is no mystery at all why the clock ran backwards.  There is a type of motor where the rotor is a permanent magnet inside an AC pole structure  (like a "standard" synchronous motor).  If the rotor is light enough, its vibration amplitude at 60 Hz will exceed a pole pitch.  Such a synchronous motor will spontaneously start in a random direction.  A ratchet mechanism forces it to reverse if it starts in the wrong direction.  The drop damaged this mechanism, allowing the motor to start backwards.

Ratsky
User Rank
Platinum
Re: Which is more accurate?
Ratsky   1/11/2012 3:32:04 PM
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You mean you don't just keep everything on GMT <vbg>? I only have about 6 or 7  "conventional" clocks (microwave, oven, coffe maker, cordless phones, ancient clock radio and an old VCR, etc.)  and I just take my cellphone (the Verizon one, the AT&T Blackberry is way off) around with me.

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