Reactance in 3 phase motors running with pf correction capacitors connected and leading
Reactance in 3 phase motors running with pf correction capacitors connected and leading
(OP)
I have a problem explaining the problems of running three phase motors with capacitors on or near the connections and at a leading power factor. I have read it here but I lack the comprehension to explain it to someone else. Please can someone assist with the problems with this type of running phylosophy. It revolves around installing capacitors and not worrying when motors go low or no load and / or the installation goes low load. Thank you.





RE: Reactance in 3 phase motors running with pf correction capacitors connected and leading
Simple circuit for modeling the motor is magnetizing reactance Xm in parallel with load branch Rload.
Now connect Xc in parallel and focus on the Xc and Xm elements.
If it is overexcited, that means Xc < Xm at power frequency Fp
1 / (2*pi*Fp * C) < 2*PI*Fp*L
1 / (2*pi*Fp)^2 < L*C
1 / (2*pi*Fp) < sqrt(L*C)
(2*pi*Fp ) > 1/ sqrt(L*C)
We notice the right hand side is the LC resonant frequency. So overexcited system implies that at power frequency the LC circuit is being excited at a frequency above its resonanct frequency. When power is disconnected and motor coasts, actual frequency drifts down below Fp toward resonant frequency. As it passes through resonant frequency the magnitude will increase. Saturation limits the magnitude of the overvoltage but I gather it can still be on the order of twice normal voltage.
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(2B)+(2B)' ?
RE: Reactance in 3 phase motors running with pf correction capacitors connected and leading
That is a motor that will require 5KVAR to correct to unity at no load has 10 KVAR connected. I have never had a problem.
This was part of the 'Old School' art of power factor correction, when power factor controllers were too expensive for many customers and PF and PF penalties were calculated monthly based on the KWHr and KVARHr consumption.
Bill
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"Why not the best?"
Jimmy Carter
RE: Reactance in 3 phase motors running with pf correction capacitors connected and leading
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(2B)+(2B)' ?
RE: Reactance in 3 phase motors running with pf correction capacitors connected and leading
The capacitor is connected downstream of the contactor and the overload device. I have convinced them to set the overload correctly but am now trying to convince them that if the motor goes to light load or no load there is a problem. On Waross's reply is what I advised them to do, and that is, to correct only to the no load power factor to prevent a leading power factor at low load. I have heard it explained that there can be pole slipping and the chance of the resonant frequency causing massive current at the motor / capacitors that is not seen by the supply if it goes leading.
What Electricpete describes I've never seen before and will have to digest it and write it down on paper. I'm quite stupid. Hard work sees me through.
There is a debate on the issue of startup current when the contactor is closed. Does it draw more, same or less? I have said it draws more. I am building an irrigation Motor Assembly and will get a chance to test the installation as described on a 45kW, 4 pole, 400v motor while I have a power quality analyser connected. I hope to save the results and make the results known to anyone who wants them.
RE: Reactance in 3 phase motors running with pf correction capacitors connected and leading
The following document might be of interest: http://www05.abb.com/global/scot/scot209.nsf/verit...
RE: Reactance in 3 phase motors running with pf correction capacitors connected and leading
Pole slipping applies to sync motors, not to induction motors. I assume here we’re talking about induction motors unless stated otherwise.
Haven’t heard of that.
Fwiw, this link touches on a lot of the things we’ve been talking about
https://extranet.w3.siemens.com/us/internet-dms/bt...
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(2B)+(2B)' ?
RE: Reactance in 3 phase motors running with pf correction capacitors connected and leading
To my thinking, no problem unless and until the motor switches off. The only place that “no-load” comes into the picture is calculating a limit for correction (90% of no-load vars). It’s conservative because no-load vars are lower than full load vars. What problem do you envision at light load?
That’s not how I read Bill’s comment. He said he has overcorrected without problem. But I may be misunderstanding something.
Pole slipping applies to sync motors, not to induction motors. I assume here we’re talking about induction motors unless stated otherwise.
Haven’t heard of that. But it makes some sense that we do not preclude resonant condition if we don’t limit capacitance to keep the system resonant frequency above operating frequency
Fwiw, this link touches on a lot of the things we’ve been talking about
https://extranet.w3.siemens.com/us/internet-dms/bt...
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(2B)+(2B)' ?
RE: Reactance in 3 phase motors running with pf correction capacitors connected and leading
Induction motor. The corrected power factor at the motor will cause the current being supplied from the starter (Contactor and overload)to be lower. As the motor goes into overload condition it is not seen by the overload as the current has been corrected. Motor burns out.
The transients and settling is where Electropete is coming with the formulas. Need to study them. Resonance and Q factor are a serious concern for me as specifics with everything going on are too complex for my tiny brain. In Dunlop, Bulawayo, Zimbabwe we had a 3.3kV, 2.0MW machine that destroyed the contactors and made the motor vibrate so much it ripped bolts on the foundation. This was stated as being a Q factor problem. The cap bank was right next to the motor as was the switchgear.
I agree with Scotty about the inrush current. On starting the motor draws more. When you energize a capacitor, there is an inrush current. Putting the inductance with the capacitor it should not cancel out, but should increase the current. Happy with the AC6 specification.
RE: Reactance in 3 phase motors running with pf correction capacitors connected and leading
RE: Reactance in 3 phase motors running with pf correction capacitors connected and leading
In the preferred situation, the power factor correction capacitors are sized at or below 90 percent of the no-load kVAR
requirement of the motor. If the capacitors are too large, the motor can be subjected to self-excitation, which will result in
excessive voltages applied to the capacitors and motor. The capacitors are sized based on 90 percent of the no-load kVARrequirement because the manufacturing tolerance of the capacitors is –0 percent, +15 percent.
This is to prevent self excitation.
RE: Reactance in 3 phase motors running with pf correction capacitors connected and leading
The old school method of sizing capacitors with no sophisticated testing equipment was with a clamp type ammeter.
At no load, there is very little difference between the reactive current and the load current.
We calculated the KVARs based on the no load current and then used the closest sized capacitor.
If we were "Banking" VARHrs for power factor correction, we would select capacitors based on double the no load current.
A favorite spot for over correcting in saw mills was the dry kiln fan motors. They typically ran 24/7 most of the time.
The motors typically stopped and reversed direction every 4 to 6 hours.
As for starting current with capacitors, it has been proposed that large capacitor banks be connected to the motor terminals to supply the reactive current required during starting. The capacitors are cut out in several steps as the motor accelerates. I have read papers on this starting method but I have never seen it first hand.
The peak currents through the inductance and the capacitance may not be at the same point on the wave and may be subtractive rather than additive.
Bill
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"Why not the best?"
Jimmy Carter
RE: Reactance in 3 phase motors running with pf correction capacitors connected and leading
That's a good reference library you've linked to. I hadn't seen it before, but for anyone who wants to take a look here is the link to the index to the Siemens MV Tech Topics library.