Static phase converter

[ccjersey]

I have had a 3hp 3 phase motor running seasonally, intermittent (half hour runs separated by hours) for several years now driving a centrifugal pump. It is powered by a Phase o Matic or some such brand static "phase converter" which is essentially a start relay used to apply voltage (phase shifted by a capacitor) to the 3rd phase to the motor.

Used as directed, the converter manufacturer does not recommend loading the motor to more than 2/3 of it's rating. I cannot determine actual loading of the pump with the information available, but to go by the performance of the motor and the fact that it has survived in this service for more than a few years, it must be fairly lightly loaded.

I have had a run capacitor (25uF) between one wire of the single phase supply and the 3rd (synthesized)phase to the motor. This arrangement produces ~2 amps on that phase and ~9 amps and ~7 amps on the other two which are powered directly off the single phase supply. I can increase the run capacitance (50 uF)and get in the range of 6 amps on the synthesized phase, but the high amperage phase will rise to ~12 amps if I do that. All amperages measured by a clamp on digital meter.

Wye connected TEFC NEMA 145T motor, dual voltage running on 208-230 (2 parallel wye connection) supplied with ~245 volts single phase. Rated amps at 230V is ~7.6 amps and at 208V is 8.0 amps. Service factor 1.15

Am I better off with some amperage on the 3rd phase from the "run" capacitors being in the circuit, but causing an imbalance/high amperage in the other two phases, or would the motor be better off with no voltage on the 3rd phase and balanced/lower amps on the two single phase powered phases.

I know a VFD with single phase input would be just the ticket, but would like to optimize this arrangement.

Thanks!

 

[jraef]

Unbalanced current creates what are called Negative Sequence currents in the lower amperage phases. These then in turn create negative pulses of torque, making the motor essentially "fight itself" when operating, which raises the motor winding and rotor temperature. If the motor is lightly loaded, i.e. the 2/3 figure they told you about, then the added heat caused by the negative sequence current is able to be absorbed by the motor's mass and dissipated with little damage. The closer ANY of the phases get to full loading, the less ability you have for the motor to absorb the added heat. So from that standpoint, the method that results in lower currents in all 3 phases gives you a better chance for long term survival.

That said, if your duty cycle is really as low as you say, you may not live long enough to see the difference it would make... That's a very low duty cycle, probably well within a motor's ability to absorb and dissipate the heating effects even if fully loaded on the "real" phases.


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[OperaHouse]

Let me explain that a different way. The capacitor is just to get the motor started. There are people who wrap a rope around a three phase motor shaft to get it spinning and then apply power. Think of the motor as a rotary transformer. The two phases have to supply the third and the normal winding current should not be exceeded.

If more power is needed add an additional three phase motor with no load and start that first. That will increase the available current by 2/3 of that motor.

 

[vinmann]

If the load is fairly constant, take a volt reading from the 3rd leg to each of the other legs. Adjust the run capacitor to achieve balance or prevent over-voltage on the 3rd leg.