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Faunsdale, AL | You cannot compare them accurately by amps, only by watts.
The idler motor pulling 18 amps per leg was probably pulling watts equal to about 1/3 of the volt-amps it pulls. That is the "real" power the motor actually uses and you pay for vs the "apparent" power it appears to be drawing when you multiply volts x amps. The difference is reactive power that magnetizes the motor and is conserved. In other words, you have to supply it for the motor to run, but it takes almost no power to create it at the power plant. Reactive current circulates between power plant and motor, so the wires etc have to be sized to handle it, but again it doesn't use up fuel at power plant, so the cost of those reactive amps to the utility is minimal.
Some utilities bill you for the apparent power you use (volts x amps) as a way to cover the cost of the lines and transformers needed to supply the real power (watts) AND the reactive power that circulates along with it. Others use a "demand" charge along with the traditional kWhr charge (real power) to do the same thing.
Additionally, an unloaded motor had a very low "power factor" in other words it's doing very little work just spinning the shaft.....Some windage from its fan and a little bearing drag is all the real work it's doing, so the magnetizing or reactive current it's drawing is the biggest part of the total amps you will measure on the wires WITH AN AMP METER. You need a power meter not an amp meter.
Most motors have a power factor value listed on the tag. This will be something like .92 meaning AT FULL RATED LOAD AND VOLTAGE, 92% of the amps it pulls will be doing work. The rest is reactive current circulating through the motor to magnetize it. At less than full load, the motor pulls less amps and the reactive current is a greater percentage of the total (power factor is lower). They will also have an efficiency rating listed. This is where the single phase motor may fall behind a little. The 3 phase motor can be manufactured to be more efficient at using electrical power to do work than a single phase motor.
So without a power meter, it's pretty useless trying to compare single phase vs 3 phase and unloaded vs partially loaded vs fully loaded motors.
Edited by ccjersey 2/14/2019 10:13
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phase converter
