Ripple current ?

#345

On a simple FWB rectifier with a high? C filter cap....... is the result? ?RMS? I(C1).....? the same as the AC ripple current going into the cap ????

On another note ( using the same above config)? I noticed that the RMS xfmr current I (T1)? x RMS V(T1)? ?results in a LOT more rms AC power.....? ? vs just the V(L1) x? I (L1).....? like a lot more, like 53.7% more.
Put another way? DC load power / AC power? = .65? ?Am I assuming the Power factor is just .65??

On another note ( using the same above? FWB + filter cap config).....when comparing V (T1)? ?vs I (T1)? ...under full load..... that the peak of the current pulse sits dead center in the middle of the (now flattened to a straight line)? peak of the voltage.?

I believe the flattened? Voltage portion is what is causing the harmonics going back into the mains V.?

It also appears that there is no leading / lagging going on.? The top of the current pulse is dead center in the flattened portion of the top of the voltage peak.?

Since there appears to be no leading / lagging..... where is the lousy...'apparent power factor'? coming from ???

The config is as follows.?
Plate xfmr? with 240 vac input? ( 60 hz)....and a 5225 vac secondary ( no load).? Hypersil C core xfmr.?

.1 ohm dc pri resistance.? And a 6.0 ohm dc sec resistance.? ?Source resistance = 53.39 ohms.? Sec winding rated at 3 amps CCS.? ?Xfmr weight = 253 lbs.?

FWB consists of 24 x 6A10 diodes ) 1 kv piv / 6 amps CCS / 400 amps surge rated)? in series? for EACH of the 4 x legs of the FWB.? ? Then two identical FWB assy's are wired in parallel. 192 x diodes in total used.

Filter cap is? 800 uf? ( 25 x 10,000uf? @ 450 vdc lytics in series, each with a 100k @ 3 watt, 1%? MOF across each cap? ( aka...'eq' resistors).? 2 x banks of caps are wired in parallel ( 400+400 = 800 uf).
Each cap is just .0159 ohm for ESR? @ 120 hz.? ?Total for the 50 x lytics =? .19875 ohms.? Each lytic is 3.0" diam x 8.625" tall.. .and weigh 3.3 lbs each. Ripple current rated at 21.6 amps CCS.

I have been using PSUD-2 in various high voltage configurations for eons now.? FWCT, FWB, FWD.? ?Plane C filter....and also a C-L-C? configuration .? ?For the C1-L-C2 config, I noticed that the C2 cap has to be a bare minimum of at least the same value as the C1 cap....and preferably a lot more.? ?If the C2 cap is lower than the C1 cap value, the output DC? voltage oscillates wildly, till it finally settles down.? ?I also have the option of shunting out the L ( via a vacuum relay) to turn it into one big C filter.? ?I also installed an adjustable spark gap across the 4H choke terminals.? ( choke is also a hypersil C core type, 127 lbs, and rated for 3 amps CCS? and 10 kvdc).? ?For safety purposes, the entire choke is well insulated from the bottom of the rack cabinet.?

I have built plenty of HV? B+ supplies over the years.? I'm just not clear on the 'apparent PF' issue, V flattening / harmonics . AC ripple measurements.

Any insight would be greatly appreciated.?

#346

On Fri, Dec 6, 2024 at 01:26 PM, Jim VE7RF wrote:

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Hi Jim,

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On a simple FWB rectifier with a high? C filter cap....... is the result? ?RMS? I(C1).....? the same as the AC ripple current going into the cap ????

Yes, that's exactly right. Ripple current limit is specified by the manufacturer to limit the effects of heating so RMS is the way to measure this.

?

On another note ( using the same above config)? I noticed that the RMS xfmr current I (T1)? x RMS V(T1)? ?results in a LOT more rms AC power.....? ? vs just the V(L1) x? I (L1).....? like a lot more, like 53.7% more.
Put another way? DC load power / AC power? = .65? ?Am I assuming the Power factor is just .65??

?

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This isn't power factor (that's a whole different ball game involving an overall phase difference between V and I), it's the ratio between RMS and average power. For the load, RMS and average will be pretty much the same. For the transformer, the conduction angle can be quite short and this inflates the RMS power (short and peaky signals have a higher ratio of RMS to average, that's just the way the mathematics works).

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Since there appears to be no leading / lagging..... where is the lousy...'apparent power factor'? coming from ???

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Again, there's no power factor situation here - that's why you're not seeing any lead/lag. The maximum current will peak during maximum voltage; this is where the voltage difference between the transformer and V(C1) is at its highest. Thus causing the current to flow.

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For the C1-L-C2 config, I noticed that the C2 cap has to be a bare minimum of at least the same value as the C1 cap....and preferably a lot more.? ?If the C2 cap is lower than the C1 cap value, the output DC? voltage oscillates wildly, till it finally settles down

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That's correct, making C2 bigger than C1 will damp down the oscillations and lower the resonant frequency. C1-L-C2 is effectively a tuned tank circuit, so will always be resonant to a greater or lesser degree.

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Hope the above is helpful. As a general suggestion, try and make your conduction angle larger - check I(T1) as short high current spikes are unhelpful. Try making C1 smaller still, allow plenty of ripple on C1 as the combo of L/C2 will cut the mains hum down to get the conduction angle up. The larger conduction angle will (tend to) make the power supply more efficient .

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Regards,

Duncan

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#347

On 12/6/2024 11:12 AM, Jim VE7RF via groups.io wrote:
the Power factor is just The classic definition of Power Factor I learned as a boy applies to Linear components (L R C) and continuous currents (no rectifiers). Same for "lead/lag": does not cover the case of rectifiers.

The usual reason to think Power Factor is because a classic Electric Utility Power Meter can measure both Real and Imaginary power. When a customer sucks huge gulps of Imaginary energy (induction motors at light load) the line losses are real and significant, and the company bills for that energy. Traditional Residential customers had only resistance loads (lamps, heaters, stoves) or mild inductance (sub-HP motors), and didn't understand the concept, so were only charged for Real power (but at a higher rate to cover the multitude of little losses in residential service). (I hear some new 'smart' meters do count imaginary energy in residential service, progress!?)

Your 16KW max load would cost $4/hour here. OK, if it is 24 hours a day (I was in a discussion about the closing of a broadcast radio system) that's $100/day, $3,100/month, or the salary of an asst manager. OTOH if it is a RadarRange which runs 15 minutes a day to cook large meals that's a dollar, and probably insignificant. Any way they meter it. (But taking 15KW peaks on a 150W average billing will get into a Demand Charge.)

Yes, as you move from the raw-rectified AC along to the smooth DC your cap values should increase. I don't have a Law for that, but it seems to work that way.

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Ripple current ?