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I have a well with a 1.5 HP pump. The pump is rated for 240V input. At the moment, the only nearby available power is 4-wire 208Y 3phase. I am considering connecting two of the three phases to the well pump, so it receives 208V where it is expecting 240V.

To what extent, and by what mechanism, will it hurt the pump if it is powered by 208V instead of 240V? Are there warning signs I can observe (like something electrical I can measure, or something I can observe about the flow of water) that would indicate whether this lower voltage is causing problems?

More generally, when an AC motor says it's rated for a certain voltage, without specifying a range, how tight is that calibration? Like, what is the "probably not much worse" voltage range? My intuition says "ohh eee ah around 15%" but I'm realizing I have no actual basis for that sentiment.

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  • $\begingroup$ Perhaps the Home Improvement Stack? $\endgroup$
    – Solar Mike
    Commented Mar 24, 2020 at 7:58
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    $\begingroup$ I would tag electrical-engineering $\endgroup$
    – SimpleJack
    Commented Mar 24, 2020 at 8:06
  • $\begingroup$ @SolarMike I considered home improvement, but I'm soliciting an understanding of the mechanisms of damage, and how they generalize to other situations, not just advice for my specific situation with the well. Also, 208V power is not typically available in a "home improvement" setting (My setting is a commercial farm). But maybe the electronics stackexchange would have been more appropriate. I'll wait for some answers to trickle in here first before posting over there. $\endgroup$
    – Diogenes Creosote
    Commented Mar 24, 2020 at 17:15
  • $\begingroup$ @SimpleJack electrical-engineering tag added, thanks $\endgroup$
    – Diogenes Creosote
    Commented Mar 24, 2020 at 17:15

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The 1.5 HP motor is built to draw a specific amperage at full load at 240V. Because power equals voltage times current, a lower than rated voltage will draw more amps through the motor. So if your supply is 208, your current will increase by a ratio of 240/208, or about 15%. Low voltage/high amps is a common failure mode for hand power tools being used with undersized extension cords.

If the motor is under a light load, you will likely never notice. But if it is running at full load, you might find it overheating, tripping overloads, or just generally failing early.

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  • $\begingroup$ Is there some way to determine whether a motor has too much current going through it, without ripping it open? Like, is temperature the problem? Something mechanical? $\endgroup$
    – Diogenes Creosote
    Commented Mar 24, 2020 at 17:27
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    $\begingroup$ If the motor doesn't have any overcurrent protection, then pulling too high a current can lead to high motor temperature, damage of the motor windings, etc. Depending on the motor setup, you may be able to measure the current with a proper clamp style ammeter. $\endgroup$
    – J. Ari
    Commented Mar 24, 2020 at 19:41
  • $\begingroup$ @AndrewCone, the typical way to check motor amps is putting a clamp-on meter over a wire. Not that well pumps will have to work harder when the water table drops (larger height to pump). $\endgroup$
    – Tiger Guy
    Commented Mar 25, 2020 at 3:37

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