How Power Impulses Keep Electric Clocks Accurate (Jun, 1930)

How Power Impulses Keep Electric Clocks Accurate

WHY does an electric clock keep perfect time? Some of the so-called electric timekeepers are nothing but standard spring-driven clocks, equipped with an electric motor and a device to turn on the current at regular intervals and wind the spring. But others have no spring, no clock works, in the usual sense, and do not, as a matter of fact, either measure or keep time, yet they are always accurate.

Hidden away in the latter type is a little synchronous or cycle motor which is really acting as a meter, to measure something happening at the power station, and, he-cause that thing happens at regular intervals, the meter can be calibrated to record time.

The actuating impulse is the alternating of the current produced by the dynamos at the power house, which is why electric clocks can only be used on A. C. lines, and will not work in D. C. power districts. Most companies have standardized on A. C. of 60 cycles a second, and it is for this current that the standard clocks are built.

Dynamos vary but the variation is averaged up at the power house, so that if the dynamo gains two or three cycles in one hour it will close them the next. As it takes a change of 60 full cycles to throw the clock off one second, it can always be brought back to accuracy by this averaging process.

  1. Charlene says: February 2, 201110:42 am

    I had an electric clock that would have kept near-perfect time had it been on Mars. It ran 35 minutes late a day.

  2. Kosher Ham says: February 2, 201111:14 am

    Dare I ask if your electrical service is 50 Hz instead of 60 Hz?

    Mind you if you powered this clock from airliner’s 120 VAC outlet, you would probably get some interesting results with the 400 Hz power.

  3. MikeBurdoo says: February 2, 201112:26 pm

    If you see a standard 120VAC socket on an airliner, you can pretty much bet it is 60Hz as the aircraft have dedicated 60Hz converters with GFI’s. Likewise for 220V 50Hz outlets if specified by the customer.

  4. MikeBurdoo says: February 2, 201112:32 pm

    Now that I think about it, adding a few cycles to the line to keep the clocks in time sounds easy when there is only one power plant. It must get a bit more complicated when you have an electrical grid being fed by many power plants that may be separated by hundreds of miles.

  5. Kosher Ham says: February 2, 201112:57 pm

    The newer airliners perhaps (this would be an added accessory like an IFE system, etc.) I’m sure the outlets are clearly marked for what line frequency. On the KC-10, the outlets are not the standard house outlets and are marked 400 Hz.

    One bird that has really different AC system is the B-1B; it’s 220 V at 400 Hz; the only aircraft in the USAF to have this.

    PS, I have 20 years with Boeing.

  6. MikeBurdoo says: February 2, 20111:30 pm

    I agree 220, 400Hz would be a bit odd. 3 phase 400Hz with 208 line to line is pretty much the usual. I was also involved with aircraft avionics and power systems, 30 years with Learjet/Bombardier and 3 with I.A.I.

  7. JMyint says: February 2, 20112:58 pm

    MikeBurdoo, it was easier than you think. The 60Hz AC frequency was also used to sync the frame rate on the NTSc television system. So prior to Digital Television every TV set in the US, Canada, and Mexico synced to the same source.

    See the generators have to sync on the grid to prevent potential standing waves, spikes, and troughs.

  8. George says: February 2, 20114:34 pm

    The NTSC field rate was 60 Hz, but was changed to 59.94 in the color standard.

  9. John Savard says: February 2, 20119:50 pm

    It is interesting, but not surprising, that airplanes used 400 Hz in their internal power systems (to allow transformers, and the capacitors in DC power supplies, to be lighter in weight) and mainframe computers did the same for their internal AC power distribution systems.

  10. DrewE says: February 3, 20119:09 am

    JMyint — the frame rate of television (not just in the US) was chosen to be the same or nearly the same as the AC mains frequency so that any AC hum that got into the video signal would only cause stationary or nearly stationary artifacts, and thus be less noticeable than if it was rolling or flashing. I don’t think the actual signals were ever directly synchronized to the grid frequency, however; for sure, this wasn’t done at the receiver, as the relative phase of the RF signal and the AC signal is inconsistent from station to station (and from house to house or circuit to circuit in the house, because of the multiphase electrical grid).

    IIRC the change to 59.94 Hz was made to permit interlacing, it being the equivalent of eliminating one half a scan line per frame. The other approach would have been to alter the horizontal scan rate by a miniscule amount, but that was disallowed for nontechnical (bureaucratic) reasons.

  11. Charlene says: February 3, 201112:12 pm

    No, regular 60Hz service. My dad’s forays into amateur wiring were fraught with excitement (he once sawed through our house’s main electrical service wires while attempting to install a door, and somehow survived), but stepping down the frequency of one outlet was beyond even him.

  12. MikeBurdoo says: February 3, 201112:57 pm

    Charlene – It might have been an electric clock that actually used an electric motor that periodically wound the clock’s main spring. You would know if it kept running when he sawed through the electrical wiring. Then again, no one might have checked the time in the resultant excitement.

  13. Kosher Ham says: February 4, 20112:31 pm

    It is much easier to make converters these day with solid state devices; at the time that article was written, motor generators were often used to do the voltage/frequency conversion. Also at that time, some AC lines were only 25 Hz!

    Mike Burdoo: I’m curious about the 60 Hz on the bizjets– was that an engine driven generator or was it a converter?

  14. Hirudinea says: February 5, 20114:23 pm

    In Ontario before 1947 the AC was 25hz, what the hell were we supposed to do!?

  15. Orv says: February 9, 20115:34 pm

    This process is actually still done today, but now it has to be synchronized across the whole grid. Each grid (East, West, and Texas) has its own rules for how large the cumulative error can get before they initiate a correction; usually it’s a few seconds.

    An individual clock can still be inaccurate if it has too much mechanical drag, or (in the case of electric clocks) if it’s sensitive to noise on the AC waveform.

  16. Orv says: February 9, 20115:35 pm

    Meant to say “electronic clocks,” there. Motor-driven clocks ought to be pretty insensitive to noise.

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