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    Howcome 3 phase 6 pulse rectifier fed loads dont need PFC

    Hi
    Attached is the current in a single source of a three phase star connected supply feeding into a 3 phase rectifier which runs into a resistive load……as you can see.. the current is nowhere near sinusoidal…….howcome three phase 6 pulse rectifier fed loads don’t need power factor correction?

    Schem and ltspice sim also attached

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    Re: Howcome 3 phase 6 pulse rectifier fed loads dont need PFC

    howcome three phase 6 pulse rectifier fed loads don’t need power factor correction?
    I don't unterstand the question. Which application are you talking about? How do you conclude that it doesn't need PFC? According to which power quality standard?


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    Re: Howcome 3 phase 6 pulse rectifier fed loads dont need PFC

    The power factor is in fact close to 0.9 - that is considered good enough for most industrial apps, above 16A per phase harmonics regs don't apply.


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    Re: Howcome 3 phase 6 pulse rectifier fed loads dont need PFC

    After rectifying AC supplies to DC...
    A capacitive load (or capacitor filter) causes supply Ampere waveforms to get out of alignment with supply voltage waveforms (characteristic of power factor error).

    Current flow becomes spikes occurring at peak voltages of the 3 waveforms. Simulation portraying this type of power factor error:

    Click image for larger version. 

Name:	3-phase star 6 diodes 230 VAC 1000uF cap 50 ohm load.png 
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ID:	157527

    The above is different from an inductive AC load creating power factor error from an AC supply.



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    Re: Howcome 3 phase 6 pulse rectifier fed loads dont need PFC

    above 16A per phase harmonics regs don't apply.
    Thankyou, i believed this to be the case.
    Doesnt it seem odd though?...i mean harmonics are supposed to cause heating in supply system transfromers...so you would have thought above 16A per phase needed it more?
    Sometimes one wonders what is the basis behind these (very expensive) regulations.



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    Re: Howcome 3 phase 6 pulse rectifier fed loads dont need PFC

    Hi,

    Thankyou, i believed this to be the case.
    Doesnt it seem odd though?...i mean harmonics are supposed to cause heating in supply system transfromers...so you would have thought above 16A per phase needed it more?
    Sometimes one wonders what is the basis behind these (very expensive) regulations.
    I wonder, too.

    I've designed high power control units. They don't care much about "mains quality", because they are exclusively used in a chemical plant with separate grids. They have an extra 10kV / 400V grid for "dirty" power applications ... besides the usual grid for PCs and so on.
    With an extra "dirty" grid it makes sense not to care about quality ....

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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    Re: Howcome 3 phase 6 pulse rectifier fed loads dont need PFC

    As per schematic, it's not < 16A per phase. According to IEC/EN 61000-3-2 class A equipment, absolute harmonic current limits apply.

    Click image for larger version. 

Name:	class A.PNG 
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ID:	157529

    The limits are undershot by far. However, if you increase load current by factor 10 or so, and it's not professional equipment, class A limit values would be exceeded.

    You can use .four 50 20 I(V4) in LTspice to see the harmonic currents. Listed peak values have to be multiplied with 0.71 to compare with RMS limit values.

    I'm not sure which real application is modelled by the circuit. Most technical devices (DC/DC, VFD) have input capacitors and thus much higher harmonic currents.

    Applications not governed by harmonic current regulations (professional > 1 kW, > 16A phase) however don't have a free ride for unlimited mains "pollution", they need a permission by the utility company.



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    Re: Howcome 3 phase 6 pulse rectifier fed loads dont need PFC

    the caps on 3 ph rectified mains are often not bigger than 10uF as they don't need to be bigger - there is no gap in the mains supply as there is for single phase - the nominal is 565VDC and the ripple ( no cap ) is 15% (rms ) or so ...



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    Re: Howcome 3 phase 6 pulse rectifier fed loads dont need PFC

    Yes...and Delta connected 3 phase sources give better harmonics...but still nowhere near sinusoidal.
    I agree, they are actually limited, but its still surprising...i mean, industry and commercial is some 65% of global electrical energy useage...you'd think the harmonics would need to be improved, to stop those supply transformers from overheating.

    The most stringent harmonic regs get imposed on stuff that is residential..and this is only 12% (China) to 37% (USA) of electrical energy useage.
    I suspect residential supply transformers are a little bit less robust than ones for industrial areas.

    https://www.sciencedirect.com/topics...ty-consumption
    Last edited by treez; 29th January 2020 at 19:36.



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