Over correction

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Bugman1400

Senior Member
Location
Charlotte, NC
Phil Corso said:
Bugman,

I strongly disagree! The magnetic-structure of an induction-motor is energized, hence excited, by the source its connected to.

Self-excited** is a condition in which an induction motor, operating as an induction-generator, in an isolated power system derives its excitation from shunt-capacitors or the natural capacitance of its connected power-line, if any!

** IEEE Std 100 "Standard Dictionary of Electrical and Electronic Terms"

Phil
Click to expand...

So, you indicate above that you are referring to the stator which is magnetized by the wdgs that are connected to the source. I agree with that. I still don't think extra PFC will affect that much. What I was referring to was the rotor that takes the rotating field established by the stator and uses that to develop the poles of the rotor so that torque can be applied to the rotor to drive the load. To me, that is a form of self-excitation that does not require slip-rings or another external source to establish the rotor poles.
 
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Phil Corso

Senior Member
Location
Boca Raton, Fl, USA
Bugman,

This thread started with a PFC-related topic. The PFC- doesn't "correct" the motor's PF! Instead, it compensates for the motor's PF, as "seen" by the source of supply! And it does that be off-setting the PFC's capacitive-reactance with the motor's inductive-reactance!

Phil
 
ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
Phil Corso said:
Tom, I've moved your measured values around a bit:

Before PFC:
286.5VLN, 34.9A, 33.2kVA, 26.2kW, 20.4kVAr, 079pf

After PFC:
286.5VLN, 36.2A, 27.2kVA, 26.3kW, 6.93kVAr, 0.97pf

1) If VLN is 286.5, then 3-ph, supply ph-ph voltage is ~496 Volts, Is this correct?

2) Assuming a NEMA-Premium motor, and using generic data, i.e., 460V, 4-Pole, Eff ~94%, then output is 24.6kW => 33Hp.
Thus if motor it operating above rating! I know Submersibles are unusual... and water, in this case, is the coolant, but...?

3) The PFC Cap factor, Q = [kVAr(b)-kVAr(a)]/kW = 0.511kVAr/kW, which is too high (using generic data)!
Can you get No-load current from Motor Mfg?

Phil
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1. According to the graph on my meter , 496 Is close enough.
2. Pump installers have a tendency to run motors well into the SFA. Suprised it isnt worse.
3. I do not know the mfg. No reply for my requested nameplate data. Suspect I am SOL for no load current.
 
kwired

kwired

Electron manager
Location
NE Nebraska
I don't necessarily know what long term local effects may be for overcorrection.

Most POCO around here have in recent years required correction for motors 50 hp or more. They also haven't been able to tell me how they are going to verify several years later that the correction installed today is still operational- but most have upgraded metering systems to ones that are capable of tracking this - they just are not using it for the most part yet - it will probably be used eventually and they will notify a user if power factor goes out of line from their requirements, and eventually penalize them if they don't fix it.

Most applications in this are are irrigation wells. Supply houses and their capacitor vendors seem to think you just use the horsepower of the motor and look at a chart and use the one they suggest. I have found that often yields a capacitor larger then necessary, usually only one or two standard stock sizes larger then necessary. It all depends on actual motor used, its actual power factor and efficiency - as well as consideration of actual load applied to it. If you size per nameplate data - you are sizing for a motor that is supplying full load at rated voltage - change any of those paramaters and you get different actual end results.

Most installers don't understand it, and just install whatever capacitor is on the chart the manufacturer supplies based on horsepower. They also don't have a clue how to adjust the overload setting if it is connected to load terminals of the starter, or they fail to provide proper overcurrent protection to the capacitor if not connected to the load side of starter. I have even run across some that are connected to line side of starter- and are online even if the motor is not running.

I'm sure the POCO has no problem with overcorrection, as it helps to make up for where they may need to apply their own correction, but outside of possible overvoltage conditions that may happen, your customer has nothing financially to gain from overcorrection.
 
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Phil Corso

Senior Member
Location
Boca Raton, Fl, USA
PtonSparky...

I believe you've misunderstood my request for voltage measurements. I just wanted to know the V L-N voltages for the two cases you presented in your 09-02-15, 07:33 post!

Phil
 
kwired

kwired

Electron manager
Location
NE Nebraska
Phil Corso said:
PtonSparky...

Presumably by no-load, you mean motor's not running! If data is not logged, then present it when when it's available!

Phili
Click to expand...

When he said irrigation season is over - he ultimately is saying those load conditions will not be present again until next spring or summer.
 
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