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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.

Larry M
User Rank
Platinum
Re: The grid affecting time
Larry M   5/9/2013 9:00:24 AM
NO RATINGS
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.

 

 

RPLaJeunesse
User Rank
Iron
Re: The grid affecting time
RPLaJeunesse   5/9/2013 8:36:32 AM
NO RATINGS
Come on, this is supposed to be an engineering site, think basic electricity. If your generator is loaded down, it slows down. If your grid-tie lags the rest of the grid, you draw current from it. Your electric company buys power just this way. If you have excess power available, speed up the genny and pump the juice to the grid. Your electric company makes money just this way. This buy/sell scheme has almost trumped the goal of keeping a constant number of power cycles (at 3600 a minute) per day. All those millions of clocks? They don't really care if you are on time or not!

Charles Murray
User Rank
Blogger
Re: The grid affecting time
Charles Murray   5/8/2013 7:43:24 PM
I'm with you, Rob. I didn't know that the power to the clock would affect motor speed, either. I learned something new here.

Debera Harward
User Rank
Silver
Re: The grid affecting time
Debera Harward   5/8/2013 5:16:10 PM
Rob these electric clocks works on electric current that powers them and changes time any variation in the rate of current may cause the clock either to move faster or slower .

tekochip
User Rank
Platinum
Re: The grid affecting time
tekochip   5/8/2013 4:33:37 PM
NO RATINGS
They keep they number of cycles accurate over the course of a day, but I can't imagine these days they would run at a lower frequency during the day.  Tell me they don't still do that.

warren@fourward.com
User Rank
Platinum
Re: The grid affecting time
warren@fourward.com   5/8/2013 2:29:37 PM
I think the power company knows very well how we depend upon their 60 HZ being 60 Hz.  I would think, however, that lowering the freqency would raise current on brown-out days.  It sure decreases the efficiency of the transformers in the circuit. 

It is like the politicians who got around the Constitutional requirement to have Congress declare war.  Just don't call them wars!  Actually, it isn't like that at all.  Sorry.

Hummm...

jljarvis
User Rank
Gold
Re: The grid affecting time
jljarvis   5/8/2013 2:18:18 PM
Rotation of Synchronous motor shafts are sync'd to the frequency of the power source.  Rob the motor of 2 cycles per second, times 3600 seconds in an hour, and pretty soon you have a fair number of revolutions!   Let that go on for 6 or 8 hours, and you've got an error on the order of 7- 10 seconds.

It evidently depends on the number of poles in the armature, and the clock gearing as well, although I never had time to model the whole system.

 

jljarvis
User Rank
Gold
Re: The grid affecting time
jljarvis   5/8/2013 2:10:04 PM
NO RATINGS
We're talking 5-7 seconds accumulated error in a few hours.  It was real obvious!

Rob Spiegel
User Rank
Blogger
Re: The grid affecting time
Rob Spiegel   5/8/2013 2:05:56 PM
I didn't realize the power to the clock wlould have a direct affect on the speed of its motor. I thought the clock would be either off or on, and that it it was on, it would move at a specific speed.

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