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3 phase Power filtering with unbalanced loads?

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max_torque:
The loads are unbalanced because they are controlled by the process and so dependant upon exact circumstance.

I already have snubbed Triacs, and a small upstream choke for each output (outputs are Line to Neutral)

My concern is the horribly noisy supply, because of the other (non certified and certainly non standard) equipment driven off the same supply, hence i want to robustly block as much of that noise as possible.  Most of the noise coming down the 3ph supply is common mode, and will have complex and changing harmonics depending on the operating state of various equipment within the facility.

Luckily most of the noise is also significantlyhigher frequency than the fundamental (50Hz) supply, so it ought to be possible to reject it reasonably easily?


MagicSmoker:

--- Quote from: max_torque on August 31, 2019, 01:12:02 pm ---The loads are unbalanced because they are controlled by the process and so dependant upon exact circumstance.
--- End quote ---

If you use a (relatively uncommon) 4-way common mode choke - so you can put inductance in series with each phase and neutral - then it won't matter that the phase loads are unbalanced because the neutral current will exactly cancel out the imbalance.


--- Quote from: max_torque on August 31, 2019, 01:12:02 pm ---I already have snubbed Triacs, and a small upstream choke for each output (outputs are Line to Neutral)

My concern is the horribly noisy supply...
--- End quote ---

Note that the primary purpose of triac/thyristor snubbing is to limit dV/dt during turn-off, not to reduce the emission of, or susceptibility to, noise.

That said, phase angle control of loads is itself a prodigious producer of noise, and presents a terrible power factor as well, so a filter is in order even if your concern about susceptibility is a bit misplaced.


EDIT - grammar

T3sl4co1l:
The complement to the parallel R+C snubber (for dV/dt) is the series L||R snubber (for dI/dt).  With both, you can save a lot of noise in the first place (i.e., from the TRIAC), and clean up what's left with whatever additional filtering.

Tim

NiHaoMike:

--- Quote from: MagicSmoker on August 31, 2019, 04:03:39 pm ---That said, phase angle control of loads is itself a prodigious producer of noise, and presents a terrible power factor as well, so a filter is in order even if your concern about susceptibility is a bit misplaced.

--- End quote ---
Hence why zero cross switching generally makes more sense than phase angle control for heating devices. It's very unusual for the thermal time constant to be small enough for that to make a difference in thermal regulation.

max_torque:
yup, we are not running phase angle control, just ZCS with a sort of integrated power kind of control scheme.

The device runs fine on the bench, and the traics and drivers all work and are snubbed correctly, my concern is as mentioned over what is going to come down the AC supply line in the actual test facility where the devices will be used. In that facility prototype high perfmormance power electronics and motor controls are developed and tested, often for motorsport use (including F1 / formulaE etc) and those DUTs are generally extremely noisy, because electrical noise is several orders of magnitude less important than other factors and the DUTs are not intended to be certified or type approvedin any way. As a result, all the switching devices run at high dv/dt and di/dt (to get the physical size and mass down, and to get the most robustness) and that causes a lot of noise to be passed back up the chain.  Various filters are in place, but the overall environment is still horendously noisy (we have to use fibreoptic CAN and ethernet to get datacoms around the test facility, but that of course also gives us a nice safe galvanic isolation for operators etc).

I've scoped the AC supply lines and it's not uncommon to see 50 to 100V spikes at hgih frequency on top of the normal AC waveform!

As a result i want to include a very robust supply filter to these units!   :scared: :bullshit:

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