Can shunt capacitors share voltage between phases?

Discussion in 'General Electronics Chat' started by Robert Emmet, Oct 9, 2014.

1. Robert Emmet Thread Starter New Member

Oct 9, 2014
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Kind Sirs:

I have symmetrically connected three 35 ufd motor run caps between three 480 volt phase lines. While monitoring the phase-neutral voltages in an ON/OFF test, data shows that, with the caps in the circuit, the three WYE voltage magnitudes are closer to the mean value. 274, 277, 280 become 276, 277, 278 with caps.

How do the caps charge and discharge between the AC phases to achieve this result. Please suggest a mechanism for this process (quantitative, if possible) and provide references for further study.

Thanks, RTE

2. alfacliff Well-Known Member

Dec 13, 2013
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ac flows through capacitors instead of "charging" them. the same effect of having three same value resistors instead of caps.

3. Robert Emmet Thread Starter New Member

Oct 9, 2014
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Thanks Alfacliff, I am familiar with the model that AC charges and discharges caps 60 times each second. Does the AC current flow through my caps because they are not perfect caps and have some resistance ? Is the leakage current that is flowing through the caps and balancing the phase voltages? RTE

4. alfacliff Well-Known Member

Dec 13, 2013
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ac current flows through the caps because thats what caps do, they pass ac current in the amount allowed by their capacitave reactance. the three phases are not in phase, they have a phase difference, therefor a voltage difference between phases.

Sep 22, 2013
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6. #12 Expert

Nov 30, 2010
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No. It's way bigger than that. Motor run capacitors commonly pass several amps of current. A 35 uf cap has 75.8 ohms of reactance at 60 Hz. Put 275 volts across that and 3.6 amps will flow.

7. profbuxton Member

Feb 21, 2014
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I would disagree with the statement that capacitors "pass" current. Capacitors have an insulating medium between their "plates" called a dielectric. It does NOT allow current to flow. A capacitor connected to an AC source can only charge up and discharge its "plates" at the source frequency which will appear, to an AC current meter as if a AC current flows. A current AC or DC will only actually "flow" in this circuit if the dielectric is compromised(leaky).

8. Robert Emmet Thread Starter New Member

Oct 9, 2014
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Profbuxton,

These shunt capacitors have two phase voltage sources driving the charge/discharge process. How is one phase voltage locally influenced by the other through the capacitor as the data suggests?

RTE

9. profbuxton Member

Feb 21, 2014
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That could be difficult to answer without more info on the source of the supply and maybe its impedance and any load on the supply. The voltage across the caps would need to be analysed as a vector diagram. When you say ON/OFF test do you mean all three caps are switched into/out of circuit or only one at a time?
Capacitor quality (leakage) may have an influence but that can be easily checked.

10. #12 Expert

Nov 30, 2010
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Two of my meters disagree with you.

If you think current is only the passage of electrons, then no electrons are flowing through the capacitor. For practical purposes, a meter, a motor, or a person in series with this path will believe current is flowing.

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Last edited: Oct 11, 2014
11. profbuxton Member

Feb 21, 2014
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#12, I do agree that "for practical purposes", a current appears to "flow through" the capacitor but I contend that what you measure is the cyclic charge and discharge of the capacitor at ac frequency. As a exercise try connecting this to a reversing dc supply of some kind and you will get the same basic effect as you increase the frequency. Maybe connect it to a variable frequency supply and see the result.

12. #12 Expert

Nov 30, 2010
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Much like the cyclic magnetism in a power transformer, no electrons pass from the primary winding to the secondary winding. If you want to believe that no current flows, that's up to you, however, after 40+ years of doing this, I don't think I'm going to teach myself the difference between alternating charges and electron flow by doing an experiment.

13. t_n_k AAC Fanatic!

Mar 6, 2009
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Imagine a series circuit in which there is an AC voltage source, a light globe and a two terminal black box. You are told the black box encloses a single passive element which could be a resistor, an inductor or a capacitor.
The lamp glows indicating Joule heating due to the AC current flowing. What is the most likely component in the black box?
As far as I understand the issue, one makes reference to displacement current within the dielectric which is indistinguishable from the current flowing in the external circuit.

Last edited: Oct 11, 2014
14. t_n_k AAC Fanatic!

Mar 6, 2009
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Unfortunately this has wandered away from the OP's original question. As indicated earlier there is insufficient information available to establish what could correctly explain the OP's measurements for the two circuit conditions.

15. MrChips Moderator

Oct 2, 2009
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The electrons do not have to cross an insulating barrier in order to transfer energy. As long as the electrons move back and forth, energy is transfered.

Same thing happens in a power transformer.

16. Lestraveled Well-Known Member

May 19, 2014
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Robert Emmit
Do you have a resistive load on the three phases while you conduct this experiment? If there is no load, then I suspect that the balancing effect that you see when you add the capacitors is due to reactance in the cabling.

17. Robert Emmet Thread Starter New Member

Oct 9, 2014
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Dear AAC,

The meters show AC current passing in both directions through the cap leads in the 60 cycle charge-discharge process. There is zero current flowing through the cap because the

18. Robert Emmet Thread Starter New Member

Oct 9, 2014
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Dear Fellow Travelers,

I really appreciate your interest and suggestions concerning the cap-voltage data that I shared with you last week. Tony Kuphaldt was a true genius to have created this forum because by sharing electrical ideas freely and anonymously, many experiences will move the general understanding forward. The problem I have now with sharing more of the circuit, is that my BOSS found out that I had originated a thread on allaboutcircuits.com, by sharing some data, and he hit the ROOF. Evidently he has grandiose about the commercial value of this project, and ORDERED me to be silent until he establishes that HE is the first to describe the complete circuit which he considers his invention. He has formed a group of other lifetime IEEE Members for this purpose and to protect my paycheck I must remain silent until further notice. After he has protected himself, maybe I can convince him that continuing this excellent thread will develop useful additional understanding and application prospective for the HIS circuit invention,

So long for another week. I will try to calm down the BOSS,

RTE, OP