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I have a submersible pump for which I'm trying to determine the variability in flow rate, to high precision. Since this is just an impeller driven by an AC motor, what is the % error in frequency of a US wall outlet? Is there a commonly accepted 'ballpark' value, such as 60 hz +/-0.5 hz?

My suspicion is that for most electrical grids, this largely depends on the citywide usage throughout the day.

I apologize because I have an inkling that my question is poorly presented -- that being said, if anyone knows of where I can find real-time data regarding the West Los Angeles electrical grid, I'm all ears. Thanks!

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    \$\begingroup\$ Related: mains frequency accuracy worldwide. Answers cover Australia and North America. \$\endgroup\$
    – The Photon
    Commented Sep 21, 2018 at 1:33
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    \$\begingroup\$ Also, there is no "West Los Angeles grid". Pretty much the entire western US and Canada from the Rockies west are a single synchronized grid. \$\endgroup\$
    – The Photon
    Commented Sep 21, 2018 at 1:37
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    \$\begingroup\$ You'd be better off measuring the flow rate directly, rather than trying to predict it from things like mains frequency and motor characteristics. \$\endgroup\$
    – Dave Tweed
    Commented Sep 21, 2018 at 1:51
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    \$\begingroup\$ "an impeller driven by an AC load" - what do you mean by that? \$\endgroup\$
    – Ale..chenski
    Commented Sep 21, 2018 at 3:50
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    \$\begingroup\$ Related: in Europe, there recently was a situation where the frequency of the grid deviated by 0.004Hz for a few months, resulting in mains-referenced clocks running up to 6 minutes late. Eventually, the frequency of the grid was raised by 0.01Hz for a period of time, allowing clocks to catch back up. \$\endgroup\$
    – Tom van der Zanden
    Commented Sep 21, 2018 at 7:50

4 Answers 4

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Typical accuracy of the AC Mains in North America is extremely accurate. Over a long period of time (hours), total error is such that an electric clock with synchronous motor has exactly zero error.

The instantaneous frequency may vary by some tiny fraction of one Hertz but the long term error is actively corrected.

I'm assuming that your impeller pump uses an induction motor. The slip in that motor will introduce far more error in your measurement than frequency changes of the AC Mains.

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  • \$\begingroup\$ Is the error held to zero for timekeeping (as wikipedia implies), or is there any other reason? And would you assume every country has zero long term error, or is it a very hard task which only more advanced countries can accomplish? (en.wikipedia.org/wiki/Utility_frequency) \$\endgroup\$
    – piojo
    Commented Sep 21, 2018 at 3:57
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    \$\begingroup\$ @piojo check out the info in the other comments, but you may be better, if you are certain it has not been asked before, to ask your own new question. \$\endgroup\$
    – Solar Mike
    Commented Sep 21, 2018 at 4:58
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    \$\begingroup\$ Your answer would be better if you explain WHY it is so stable - that it is actually the best way to make sure that all grids have enough power. If more power is consumed than produced, the frequency will dip untill consumption meets production. And opposite, if less power is consumed than produced, frequency will increase. Power consuption will increase b/c frequency, and again we are at equality. (But the utilities and the power companies will work to reverse any changes to frequency) \$\endgroup\$
    – Stian
    Commented Sep 21, 2018 at 7:13
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    \$\begingroup\$ @piojo You are right, the same could be achieved with synchronization only. Well, timekeeping is reason enough for me \$\endgroup\$
    – Maple
    Commented Sep 21, 2018 at 8:00
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    \$\begingroup\$ To add some support to the answer, I had an alarm clock made in the early 1980's that kept time almost as well as my cell phone. I don't remember ever really adjusting the time. The clock was basic and ran off the freq of the mains. I used it until it recently (mere months ago) had a component fail, causing it to gain hours a day. On the other side of things, my 3 year old microwave has all kinds of features and gains about a minute a month. \$\endgroup\$
    – computercarguy
    Commented Sep 21, 2018 at 14:28
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Frequency of AC power grid in North America is monitored by a network of Universities. You can see the live gradient map at FNET here in real time. It is maintained to about 0.1% across the continent.

enter image description here

However, this doesn't look like having any relevance to accuracy of impeller-based water pump, since the flow will depend on pressure differences and viscosity. Maybe a peristaltic-based pump might give you some accuracy proportional to AC mains frequency, but it will give you a constant flow rate, so it is totally unclear what you are trying to measure.

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If your submersible pump uses an induction motor - more than likely - then its rotational speed will be mains frequency minus slip frequency. The slip frequency will vary with load, including motor/pump friction, pump head and viscosity of water which varies with temperature.

Mains frequency drift won't be your problem.

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The answer is that a impeller pump is almost certainly not a positive displacement pump. Thus, the flow will vary upon back pressure, viscosity and inlet pressure.

In addition, the motor is not a synchronous motor, so the speed of the motor will vary upon load (=flow rate).

In short the frequency deviation of the mains network is the least of your problems when you try to calculate flow rate from rated motor speed.

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