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.
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!
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.
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.
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