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Recently I was thinking about a generator project and realized that is beyond my current understanding of AC power. I have been doing some research since but I am really having a hard time understanding the frequency part of AC power sources. From what I understand the frequency of an AC signal is the number of full +/- voltage cycles in a signal.

To make discussion simpler lets consider an A/C generator that is driven by a constant 1500 RPM. If that generator has has 4 poles this would result in 50 Hz, since there are 6000 magnet switches every minute, which is 3000 full voltage changes a minute which is 50 changes a second.

Now lets assume I have the same sort of generator (size, etc) but it has 12 poles like an usual car alternator, it will produce 150Hz AC power. At this point I am really wondering why any appliance would care. If you consider it within a full second nothing really changes, there is the same amount of time in positive voltage and negative voltage as if it would be running with 50Hz. It just switches three times more often.

Why exactly do A/C appliances care how often the voltage inverts with A/C power sources?

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  • \$\begingroup\$ Have you heard of impedance/admittance of capacitive/inductive loads? These are propotional/inverse propotional to the frequency. You need to learn not only about generation, but about AC physics/electronics in general. \$\endgroup\$
    – Eugene Sh.
    Aug 28, 2020 at 19:03
  • \$\begingroup\$ My college education did not go much past average DC education, and I think it shows... \$\endgroup\$ Aug 28, 2020 at 19:05
  • \$\begingroup\$ ... which means I heard of the terms you mentioned but have no idea what they mean. However I do not expect you to explain them to me as they are apparently already very well explained. \$\endgroup\$ Aug 28, 2020 at 19:08
  • \$\begingroup\$ Fortunately these times it is really easy to fix as all of the information is readily available on any level and in any format you might prefer :) \$\endgroup\$
    – Eugene Sh.
    Aug 28, 2020 at 19:08
  • \$\begingroup\$ For example, let's reverse the generator example. If you have a 3-phase or synchronous motor that runs at certain RPM when fed with 50Hz, it won't run at the same RPM when fed with another frequency than 50 Hz. So some devices care. \$\endgroup\$
    – Justme
    Aug 28, 2020 at 19:08

2 Answers 2

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Some appliances care, some don’t.

AC induction motors run in sync with the line frequency (less a few percent of ‘slip’, depending on load.) This is set by the number of poles they have.

So an induction motor designed to run on 60Hz will run slower on 50Hz. This isn’t usually a problem as this essentially de-rates the motor. On the other hand, running a 50Hz motor on 60Hz might be a problem as the motor can deliver more power than it’s designed for.

Universal motors won’t care.

Most lighting won’t care. Arc lights will flicker slightly more on 50Hz.

Appliances that internally convert AC to DC (including home computers and other electronics) won’t care, mostly. Power supplies that support 50Hz need more input capacitance than 60Hz ones.

IT gear power supplies have largely migrated to designs that support all worldwide line voltages and frequencies, so they’re designed specifically to not care. Specifically, they have bigger capacitance rated at higher voltage to accommodate both 50Hz and 230/240V as they are rectified to DC.

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    \$\begingroup\$ When you say "don't care" it might be true for small deviations from the rated frequency (like 50Hz vs 60Hz). But when it comes to 50Hz vs 150Hz (or 1000Hz or whatever), the picture would be different. I doubt 50Hz rated AC/DC converter will work at all on 150Hz supply. \$\endgroup\$
    – Eugene Sh.
    Aug 28, 2020 at 19:36
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    \$\begingroup\$ Well, from what I understand 50Hz is not 50Hz anyway and it will fluctuate quite a bit, but it will be in a reasonable range, right? \$\endgroup\$ Aug 28, 2020 at 20:17
  • \$\begingroup\$ Depends on the design. There are AC-DC supplies that work on both shore and aircraft / shipboard power that can accommodate 50-400Hz. As long as the input caps have low enough ESR to deal with the ripple it's not such an issue. \$\endgroup\$ Aug 28, 2020 at 21:41
  • \$\begingroup\$ At some frequency (starting likely around 1000Hz) the input rectifier diodes of the usual AC/DC convertor may refuse to work this fast. Or they may work for a while and then overheat. \$\endgroup\$
    – fraxinus
    Sep 29, 2021 at 8:09
  • \$\begingroup\$ @YanickSalzmann: Normal AC power distribution will be extremely close to the "advertised" frequency - 60 Hz in North America or 50 Hz in most other areas. Many electric clocks use the power line frequency as their timing reference - if the power frequency varied significantly, the clocks would show the wrong time. A clock from a 50 Hz area would run fast in a 60 Hz area. \$\endgroup\$ Jan 29 at 4:00
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Magnetic iron cores are rarely efficient at spanning over a decade or maybe 2 of frequency unless ferrite. So automotive alternators are not as efficient as fixed f types. Eddy current losses tend to increase exponentially with frequency and Z also increases with f until self winding parallel resonance (LC).

200 to 400 Hz magnetics in iron cores have thinner laminates and ferrites also change composition with RF frequency with lower mu and more conductive particles. So the big torroids once used in audio power amps or SMPS are high mu until they start to exceed a certain f.

The main problem with magnetics and high current is if you lower the frequency too much, they can saturate and turn into resistors, so V/F ratios are used in variable speed motor AC controllers.

But if your are just thinking heaters, they work on DC.

Read the appliance label near cordset for safety approvals.

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