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Sherlock Ohms

What's the Time?

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Iron
Re: WHAT TIME IS IT
5/10/2013 2:08:24 AM
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The grid might run 59 Hz it might run 61Hz, It will not be running at 55 Hz. Motors will burn out.

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Platinum
WHAT TIME IS IT
5/9/2013 5:56:16 PM
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Great post Jim.  Believe it or not, I had a similar experience with an ERC II (electronic range controller) when I worked for GE.  We absolutely could not understand why our on-board clock was losing time.  Not much time but it would lose approximately 10 to 15 seconds per day.  We contacted Robertshaw and they were baffled also.  One of our engineers, a EE type, had come to us from Louisville Power.  He made several phone calls and discovered the problem was just as you mentioned an input of 50 to 55 Hz instead of 60 Hz.  That solved our test problem.  Again, great post

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Platinum
Re: Motor terminology
5/9/2013 5:11:22 PM
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Ah yes, the good old days.  We had one of those that would play a stack of four 20" glass 78 RPM records.

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Iron
Re: Motor terminology
5/9/2013 4:42:34 PM
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Larry M

Ok, I stand corrected - However, my point was to correct the idea that a synchronus motor and a squirrel cage motor are the same.

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Platinum
Re: Motor terminology
5/9/2013 4:31:30 PM
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krruss wrote: "A synchronus motor does have brushes, and requires a dc voltage on the rotor coil to lock it to the system frequency.  Since it does have brushes, and requires a separate power supply for the rotor, they are actually not very common."

Wikipedia suggests that there are several varieties of synchronous motor. One type, DC-excited, is produced in sizes greater than 735 W. Three other types of non-excited motors, reluctance motors, hysteresis motors, and permanent magnet motors. do not use DC excitation and do not have brushes. They rely on either induced rotor current or permanent magnets in the rotor. Each has different self-starting characteristics.

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

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Platinum
Re: Synchronous Clock
5/9/2013 4:24:37 PM
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Ahh yes. I'm also a geezer and I had one of those. Wikipedia suggests that small synchronous motors can be self-starting if load and the moment of intertia of the rotor are sufficiently low.

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Gold
Re: The grid affecting time
5/9/2013 4:14:17 PM
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I'm not sure it was ever common practice, but I don't really know.  What was mystifying to me was how they were able to move the whole grid in a controlled fashion.

Someone mentioned the tendency for slower generators to be 'pushed' by faster ones.  So perhaps there was a self-synchronizing effect across the grid.   The freq. shift must have been quite slow.  I've got a contact at Public Service Energy Group who would be in a position to know what happened.

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Platinum
Re: The grid affecting time
5/9/2013 3:25:07 PM
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I've designed dozens of clock-based appliances, and they all used line cross for a time base. Most of it is about money; if it plugs into the wall then line cross is free and a crystal costs money, a lot of money. Then there are some practical aspects of using line cross; the software can determine 50Hz or 60Hz using the microcontroller's internal oscillator, multiplexing the display can be synced to the line to prevent flicker with fluorescent lighting, if you know where line cross is you can phase fire triacs, you can "walk" relays by firing them at different parts of the linecycle to minimize relay wear and you know when the power has been interrupted long before VCC fails because line cross has ceased before the power supply bleeds off (handy for saving stuff in EEPROM and shutting the system down nicely).

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Platinum
Synchronous Clock
5/9/2013 3:08:13 PM
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My parents had a synchronous clock on the mantel when I was growing up in the 40s & 50's.  It kept perfect time, but whenever we had a power outage the clock had to be reset and a small knob on its back had to be rotated to get it going.  It was a common prank to hold the knob to stop the clock at noon, and then spin the knob in the opposite direction to make it run in reverse.  We could tell what time it was by mentally transposing the numbers on its face, but it used to drive my old man crazy.

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Platinum
Re: Wakeup
5/9/2013 2:25:29 PM
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Your proposed "experiment" could be flawed (depending).

Putting two oscillators of the same frequency (mechanical or electronic) near each other will demonstrate a strange affect. They will sync with each other. This gives an additional edge to the two time pieces tracking together in the same environment.

The experiment would be valid if the two time pieces are verified to track closely (but not exactly) with same temperatures - but physically separated enough to eliminate this effect.

The syncing effect is more pronounced with certain types of electronic oscillators. But can be demonstrated at some level in nearly all types.

I suspect using watches with metal cases and a modest distance would provide most of the required isolation to make the experiment valid.

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