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

sunshine_eggo

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Oct 26, 2021
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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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bruceb58

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Of course it uses a ton more current. It's probably also blazing hot if you could touch it. It needs 3 phase power.
 

time2roll

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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.
 

FilterGuy

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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.
 
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