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Sherlock Ohms

What's the Time?

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Larry M
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Platinum
Re: The grid affecting time
Larry M   5/9/2013 9:00:24 AM
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Synchronous motors are very common.  You see them on, e.g., bench grinders and drill presses and furnace blowers. The label plate always (here in the US at 60 Hz) shows the RPM as 1725 or 3450 rpm. That's a slight lag from the expected 3600 rpm--that design lag is what causes these motors to rotate; they are trying to "catch up."

Besides the label plate, you can recognize these motors because they are brushless: no sparking commutator on the armature, as you would find in a vacuum cleaner or power drill. Brush-type motors are called universal motors because they can operate on AC or DC. Synchronous motors (sometimes called induction motors because the field current induces a current in the rotor causing rotation) are, of course, AC only.

You may recall an article by Margery Connor a year or so ago, wondering how she could vary the speed of her drill press with a light dimmer. Because it was driven by a synchronous motor, she couldn't. It was 1725 rpm or nothing. Had it been a universal motor, she could have done so, since their speed and torque are related to the rms voltage applied.

 

 

Larry M
User Rank
Platinum
Re: The grid affecting time
Larry M   5/9/2013 9:01:34 AM
No, no, Debera. Not the current. Synchronous motor speed is related to the frequency of the applied AC voltage, not the magnitude of the voltage or current.

Eric Tucker
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Iron
Synchronous clocks
Eric Tucker   5/9/2013 9:06:38 AM
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  One of the early manufacturers of synchronous clocks would send salesmen to the operating engineers of power plants and give them one of their clocks, with the advice "If the clock is slow, just go in and speed up the generator 'til it catches up." Very clever marketing.

   The same company went on to become the manufacturer of the first commercially successful electric organ which used a syncronous motor to drive its tone generator. Examples of these nearly 80 years old (I own a 1947 model myself) are still running great -- and in tune!

Battar
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Platinum
Wakeup
Battar   5/9/2013 9:07:40 AM
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El-Cheapo clock radios take the 50/6o Hz from the input transformer secondary and use is as a clock frequency instead of a crystal. The Clock/display driver IC usually has a 50/60Hz select pin which is jumpered on he PCB according to the market. These clock radios are notoriously inaccurate, because 50Hz is never quite what it says. It's worse if they go into backup/power out mode using a backup battery because then they rely on an R-C circuit. I understand all this happened 50 years ago, but even then, wouldn't a broadcasting station rely on at least on independent accurate clock source?

jljarvis
User Rank
Gold
Re: The grid affecting time
jljarvis   5/9/2013 9:15:15 AM
Perhaps a subtlety that y'all didn't pick up on:  THE ENTIRE NATIONAL POWER GRID EAST OF THE ROCKIES WAS INVOLVED IN THIS EXPERIMENT.   There being no HV transmission lines over the rockies, the left coast was... left out. 

The idea was to see how much fuel could be saved if you ran just a few Hz slower in peak times (rather than having brownouts)....  and catching up in null times like 3 am.   They may have cared about the clocks, or they wouldn't have tried to catch up.   At least, I figued they tried, but missed it by 5-10 seconds. 


No big deal for your kitchen clock, unless you wanted the coffeemaker to come on precisely when network news did.  :) 

The interesting thing to me is that I never saw mention of a final report on the experiment.

 

Larry M
User Rank
Platinum
Re: Wakeup
Larry M   5/9/2013 9:29:39 AM
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What would you have had, 50 years ago, for an alternate clock source? Nothing was available. The pennies-cheap 32,768 kHz crystals and IC counters and LCDs that enable $1.00 watches and giveaway promotional stick-on clocks had not yet been cheaply mass produced. Timepieces were notoriously expensive and inaccurate. Think of the Rolexes and Omegas of the time.

I take issue with your statements regarding the "El-cheapo" clock radios. I have a couple of these (all under $US 10.00), from the usual Asian sources. They rely on the 60 Hx here in North Carolina, which is dead accurate. And if we do have a power outage, the 9-volt battery drives one of those 32,768 kHz crystals, not an RC circuit. I reset these exactly twice a year, at the beginning and end of the Daylight Savings Time period.

warren@fourward.com
User Rank
Platinum
Re: The grid affecting time
warren@fourward.com   5/9/2013 9:31:07 AM
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Au contraire, mon ami.  The power company cares very much about the time.  Just pay your bill late and see.

krruss
User Rank
Iron
Motor terminology
krruss   5/9/2013 9:41:29 AM
Synchronus motors and induction motors are not the same.  An induction motor runs behind the synchronus speed, a syncrhonus motor is locked to it.  An induction motor is the most common motor in the industrial world.  

An induction motor (squirrel cage motor) does not have brushes.  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.  The generator at the power station is obviously syncrhronus.  As noted in this story, some clocks are as well.  In 28 years, I think generators and an old motor in the power lab at the University of Wyoming are the only large syncrhonus motors I have ever seen.  

The poster is correct that the small throw away motors in vacuum cleaners, mixers, etc are universal motors.  

At every power station I have ever been in, there is a meter generally on the back of the board where they track the frequency over a 24 hour period.  Generally, the deviation is 10ths or 100ths of a hertz.  Since the normal deviation is so small, I think the author showed pretty good trouble shooting skills finding this problem

 

Battar
User Rank
Platinum
Re: Wakeup
Battar   5/9/2013 9:54:23 AM
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Larry,

       I would expect a commercial broadcast station to invest in the required equipment to maintain accurate time. After all, they already have an accurate clock source to drive the transmitter, don't they? This is a commercial enterprise we're talking about, not a HAM operator.

Hard to believe your clock radio normally uses the mains frequency as a time base in preference for its' on-board watch crystal. Which do you think is more accurate?

Larry M
User Rank
Platinum
Re: Wakeup
Larry M   5/9/2013 10:02:20 AM
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The mains are more accurate in the US. Those crystals are very accurate when they are on your wrist, held to constant temperature.  Less so in an outdoor transmitter hut or a room where air conditioning might not be present or cycles on and off.

Want to test it?  Get two of those cheap crystal-controlled watches. Set them to the same time. Leave them in the same environment for a few days to assure they track. Then put one in the refrigerator or freezer and the other in a warm spot, e.g., in proximity to an incandescent light bulb. Check them after two days and notice how far apart they have become.

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