3 phase motors for dummies (I'm the dummy, need help)


Happy Breffast!
Oct 26, 2021
Friend has a deep well pump. He has no technical knowledge of the system.

Based on the specs, the pump motor is 3 phase 208-230VAC.

He has been feeding it with 230VAC from a split phase 120/240VAC generator via L1 & L2. This is not in doubt. I've seen it running; however, I have NOT seen the well head area, so I don't know what the wires from the generator are connected to.

I know that there are methods that can allow single phase to power a 3 phase motor like a VFD, though my understanding is tenuously tenuous.

The most confusing thing is that this pump uses way MORE current than the motor specs indicate. Motor is supposed to pull 11-12A, but it pulls 18-19A. Have confirmed those numbers on the output side of the generator with clamp ammeter. He also recently installed a dedicated LF 6kW inverter to drive it, and the 6kW inverter reported it's at 76% load, so ~4500W, which is consistent with the generator load.

Anybody have a guess as to why? Is this what happens when you try to drive a 3 phase motor with single phase 230?

The pump is about 10 years old, so I would expect some deterioration, but I would not expect nearly 2X the draw, so that's what's driving me to wonder if it has something to do with driving a 3 phase motor.
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Solar Enthusiast
Mar 17, 2021
Of course it uses a ton more current. It's probably also blazing hot if you could touch it. It needs 3 phase power.


Photon Sorcerer
Mar 20, 2021
18 x 2 = 36
12 x 3 = 36

Looks about the same power. I read some can be rewired to run on split phase. Maybe this was done during installation.


What, me worry?
Nov 26, 2019
Los Gatos CA
Keep in mind the current on a 3 phase motor is always going to be a lot less than an equivalent single phase motor.

This calculator says a 3phase 12A motor at 90% efficiency is around 5HP. A 5HP motor is around 3728W and with 90% efficiency it will draw ~ 3728/.9=4142W.

Coming at it from a different angle the 18Ax230V=4140W which is in line with the previous calculation of 4140.

So.... the currents sound about right and there is almost certainly a VFD type device between the generator and the pump.