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jhmumford
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Re: The grid affecting time
jhmumford   5/9/2013 1:55:29 PM
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This is odd.  I was told it was common practice to slow the frequency just a bit during heavy loads, then raise it to compensate during low demand (at night).  This was done to keep syncronous motor clocks from gaining or losing time, and that power companies were very careful about maintaining an average frequency of 60 Hz.  Maybe it was after this "experiment" that they started doing this?

bob from maine
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Re: The grid affecting time
bob from maine   5/9/2013 1:34:00 PM
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First of all, a generator running at 61HZ connected to a generator trying to run at 59HZ will turn the slower generator into a motor. You can't have a national grid with generators running at different speeds (frequencies). I believe I read the expirement in the 60's to vary grid frequency was a typical government attempt at trying to controll the laws of physics. It was ultimately unsuccessful and caused great upset in many industries. Today the national electric grid represents an extraordinally accurate time base. Crystal oscillators before the late 60's or early 70's required constant power to keep the temperature stable and had a significant warm-up time before they could be relied upon to be accurate. When telephone voice and data volume overhwhelmed the available bandwidth of telephone lines, MaBell developed a good-stable non-oven controlled series of crystal oscillators that allowed much better (and cheaper) equipment to permit multiplexing without the need to add infrastructure.

jljarvis
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Re: The grid affecting time
jljarvis   5/9/2013 12:59:32 PM
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Nope,  contemporary digital clocks are driven by a single IC which has a crystal timebase.   Power frequency insensitive.

Larry M
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Re: Motor terminology
Larry M   5/9/2013 12:44:14 PM
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Exactly.  As you (and I) said, rotor currents are induced by the stator magnetic field. That's why I confused induction and synchronous motors.

Incidentally, another use for sybchronous motors that only geezers like me will remember is driving phonograph turntables. Nothing worse than a record which plays too fast or slow.

Rob Spiegel
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Blogger
Re: The grid affecting time
Rob Spiegel   5/9/2013 12:11:49 PM
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That makes sense, Jim. But how does that affect digital clocks? Are they affected by the power source is they are not based on mechanics?

Stephen
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Re: Motor terminology
Stephen   5/9/2013 11:42:41 AM
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Yep, they're brushless. the rotor currents are induced by the stator magnetic field, no wired connection required.

Also if you turned over the field ass'y on an electric clock (some bigger wall clocks you could) you could make it run backward (just as accurate, as long as you renumbered the face!)

Stephen
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Re: Wakeup
Stephen   5/9/2013 11:39:39 AM
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Long term, the grid.


Of course before the 1980's the clocks were mechanical, syncronous motor driven not cheap quartz  crystal driven, though either's generally good enough for household use.

Stephen
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Re: The grid affecting time
Stephen   5/9/2013 11:24:17 AM
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those 1725/3450 motors aren't technically "syncronous" thought they follow the line frequency -- the slip varies sloghtly with load and the base slip varies slightly with design, particularly for motors w/ hard to start or impulsive loading.

The true syncronous motors (very low output power devices, as used in older electric clocks and timer gear motors) do sync up exactly to line frequency, and over long terms (days to weeks) the grid frequncy accuracy is VERY GOOD. in the short term it is more variable than say WWV, WWVH, CHU (Canada) or (better still) WWVB time signals, but for most applications this was not an issue.

The case cited here is an exceptional case, though 57-63 Hz shouldn't create issues w/ transfomers and induction motors as say 50 Hz could, but, when seconds are important, can cause the havoc the radio station had. Guess the grid was a little off on their high frequency corrections, but normally they wouldn't be so far off.

Now adays it's easy to come by *good* quartz controlled clock systems w/ WWWVB receivers allowing resync at night when VLF propagation is good across North America from the transmitter in Fort Collins, Colorodo.

Re: the grid business ISO NE has some interesting on line data and N.E. grid information at: http://www.iso-ne.com/sys_ops/index.html

 

 

Larry M
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Re: Motor terminology
Larry M   5/9/2013 10:57:32 AM
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I admit I'm not an expert in motors, but I've taken apart dozens of old clocks and never seen brushes in any of them.

DB_Wilson
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Re: Wakeup
DB_Wilson   5/9/2013 10:46:22 AM
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The power systems east and west of the Rocky Mountains have very few ties between them.  To avoid problems with massive power flows that would take a tie line out of service, the connections are made through dc connections.  At each end of the line is an ac to dc converter station.  The operaters agree on how much power is to be transmitted and set the equipment to accordingly.  The line can operate as a constant power source or sink depending upon the needs of the utilities.

Synchronous motors and induction motors are not the same.  Synchronous motors run at a constant speed that is determined by the power line frequency and the number of poles in the motor.  An induction motor's speed does change with loading.  Induction motors are less expensive to build than synchronous motors and a quite suitable for numerous tasks like fans, washing machines, and compressors.  For 1/4 horsepower to 2 horsepower motors, you may see a No-Load speed around 1795 rpm with the Rated-Load speed of 1750 to 1725 rpm.

The tiny synchronous motors in clocks were used for many, many years.  Only when production costs of the electronic clocks fell to near the costs of the motor driven clocks and the public demanded features not readily available in the motor driven clocks did the electronic clocks take over.  Even today, some electronic clocks use the power line as the primary time standard with a cheap crystal as a backup for power interruptions.

The power line is poor short term time standard but can be a very good long term standard.  Other than the experiment described in the article, the utility maintains a time accuracy of plus or minus 180 cycles (3 seconds).  (Utility standards assess time error as a number of cycles.)  Over short period, say an hour, a 3 second error is almost 0.1 percent.  Over a year, that 3 second error is slightly less than 0.0006 percent.

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