Smoothing Capacitor

[Geno]

I finally have almost all the pieces in place to provide up to 40 amps (surge) at 120 volts of computer grade power to the house from the Listeroid/ST5 combo. It works and now I just have to make it safe and reliable. The last big piece was an Outback Flexmax 60 and last week I saw a new one on ebay for $425.00 so I snapped it up.

Power will run 75' to the basement where it will enter a transformer and get reduced to 70 vac. It then goes to a high power bridge rectifier which puts out 96 vdc. This goes into the Flexmax which charges a 48 volt battery bank for the 48 volt, 5.3 kva ups.


I tested the Flexmax on the bench without filter caps and it powered up and ran. I know I need smoothing (electrolytic?) caps to reduce the ripple but I'm a bit confused on how to size them. The best I could come up with based on info I found on the web was around 10000 mfd with a voltage rating somewhere above my high limit. I won't be charging at more than 2000 watts and usually less. Can anybody shed some light on the cap question?

Thanks, Geno

[Crumpite]

Geno,

Just hooking a big cap up won't reduce your ripple very much at the amperage level you're running.
The method I'd use is to hook a big series inductor in between your bridge and your inverter.
Then you hook your cap up just before the inductor.
The inductor will when work with the cap where the inductor will reduce the ripple during the high current part of the waveform, while the cap works during the low current portion.
Inductors work better as the amperage in increased unlike caps which are very much the opposite.

The amount of inductance and capacitance can be calculated but you'll need to know what percent ripple you need.
Unless there is a specification in your inverter's manual you'll likely need to experiment.

Large transformers can be used as inductors by using different connections of the windings.

But if the inverter is working as it is, why do you think you need filtering ?
The KISS principle is your best friend !
Hope this helps a bit,
Daryl

[mobile_bob]

5.3kva ups?  my bet is it is a Best Power ups or one of the units from whoever bought out Best.
if this is the case you have a really tough and well built unit.

the batteries should do a lot to smooth the power sufficiently for the inverter section.

bob g

[BruceM]

As Bob suggests, your battery charger is a PWM type unit, and is unlikely to complain about the "lumpy" DC source, since it's "banging" your batteries with short high voltage spikes anyway. Your battery will see only this, not likely the ripple on your AC supply to the PWM charger.  The batteries act as a huge capacitor, you don't need to add any for their sake, in this configuration

If the charger unit is not happy (hot or ) with what you have now,  If you add capacitance to your linear supply, your power factor will suffer, as the caps take a big hard gulp only when voltage swings above the cap voltage. A cap filter will also boost the average DC voltage roughly 2/3-3/4 of the peak rectified AC, depending on the loading.

Adding a inductive filter is, as Daryl says, the prefered solution if cleaning up the rectified AC is needed, and your transformer output is high enough that you don't need the capacitor boost.  It will give you DC volts the same as your AC volts, with a lovely PF.   I think Daryl made a typo on the location, though, the inductor as a filter on a linear supply goes after the bridge, before capacitors, if any, not before the bridge, in any textbook or design I've ever seen or built.

[Crumpite]

As Bob suggests, your battery charger is a PWM type unit, and is unlikely to complain about the "lumpy" DC source, since it's "banging" your batteries with short high voltage spikes anyway. Your battery will see only this, not likely the ripple on your AC supply to the PWM charger.  The batteries act as a huge capacitor, you don't need to add any for their sake, in this configuration

If the charger unit is not happy (hot or ) with what you have now,  If you add capacitance to your linear supply, your power factor will suffer, as the caps take a big hard gulp only when voltage swings above the cap voltage. A cap filter will also boost the average DC voltage roughly 2/3-3/4 of the peak rectified AC, depending on the loading.

Adding a inductive filter is, as Daryl says, the prefered solution if cleaning up the rectified AC is needed, and your transformer output is high enough that you don't need the capacitor boost.  It will give you DC volts the same as your AC volts, with a lovely PF.   I think Daryl made a typo on the location, though, the inductor as a filter on a linear supply goes after the bridge, before capacitors, if any, not before the bridge, in any textbook or design I've ever seen or built.

I guess I need my descriptions to be more clear !
BruceM is perfectly correct...

The lineup is AC to bridge rectifier then inductor and then perhaps a cap.
As BruceM mentions, if the cap is first then you can get a higher voltage out due to the cap charging up to the voltage peak, rather than the smaller RMS value.
(I think you can avoid a bad power factor if the cap/inductor ratio is just right, though I confess I've never seen it done, so I might be wrong)

Daryl

[Geno]

Thanks for the replies.

The UPS is a Best Ferrups 5.3kva that one of my clients has had since the mid 90's. It ran like a champ till I conned them out of it a year or 3 ago. It survived multiple lightning strikes to the building over the years. It's a good unit but is set up for 208vac input which is one reason I'll be powering it with 48vdc

Last night I put 600 watts through the Flexmax and into the batteries for 30 or so minutes. The UPS wasn't connected. It went through the bulk, absorb and into float stage just the way I thought it should. It was 55°F in the basement. The Flexmax fan never turned on and the rectifier heat sink only got warm to the touch, just sitting on the bench, with under spec wires.

My lack of knowledge here and especially on what kind of "smooth" power the Flexmax will accept and run with properly is my main concern. The Outback forum says the unit expects flat, dc power but there's nothing I can find which shows smoked units from less than flat power sources. Numerous posts say caps have been added in applications like mine which is why I started this thread.

Thanks, Geno

[BruceM]

Geno, what is the specified DC voltage input range of the Flexman charger?  If you want to be "kind" to it, then inductive filter, then capacitors too if you like.  A surplus transformer can be used as the inductor.  I'll help you pick one if need be, bigger is better and you'd like one where the winding you use has at least double the DC current rating of what you're using, though if a toroid, I'm getting away with current equal to ratings.  You can use a secondary winding, but you won't get as many Henrys...so the transformer gets even bigger. You can measure the inductance in Henry with an LCR meter, a handy tool.  If you undersize, you'll get too much voltage drop, it will get hot, and you won't get the filtering you should.  

I think you should try using it briefly at a higher charge rate and see if there is a problem, or find me the specs on the input DC requirements. If you can get the service manual, and it had a schematic, I can see if there is a design problem with "lumpy" DC.

Any capacitance before the inductor, in a linear power supply, will negatively affect PF.  AFTER the primary inductor filter, capacitance can be added without screwing the PF pooch, as long as the primary inductor is sized properly. In my 120V battery charger, I do cheat on PF a bit and switch modest capacitance values of motor run caps in before the first stage inductive filter, as a way of dynamically regulating charge voltage and current. That and switching from multiple transformer windings (which I also do), is the only way to dynamically regulate the output of a linear supply, short of pissing energy away via heat  (big Bipolar transistors in parallel as a series regulator).

With two stages of inductive filtering, and only 10,000 uF between them, I get under 3mv of ripple (typically 1.5mv) on my battery voltage while charging.  (12amps max, at 156V)

[Geno]

Max input is 150vdc. It will charge 12-60 volt battery banks. I cannot find anything in the literature or the forum on ripple specs but they do state it is designed for PV only with the exception of 3, Outback approved, hydro vendors. One guy on the forum said Outback does test them on dc power supplies but he didn't elaborate further. I've only made a quick web search but haven't found any schematics yet.
I don't want to add anything to my system I don't need. I'll try the higher current charging later today and I should have the UPS hooked up as well.

http://www.outbackpower.com/pdf/specs/flexmax.pdf
http://www.outbackpower.com/pdf/manuals/flexmax.pdf

Thanks, Geno

[BruceM]

The issue is what is the minimum DC specified, as you said you were using a 70VAC transformer.  You would be safer being near the high end of the voltage range with "lumpy" DC.

With a 150VDC max voltage input, you could eliminate the transformer if you are running 120VAC on your generator head, just bridge rectify, and filter using that 70VAC transformer's secondary winding. This would give you a pretty clean, 120VDC output.

But you may be fine as you are.

[Geno]

Minimum recommended volts to the Flexmax is 24v if you're charging a 12v battery bank. The charging circuit will turn on at  ~14 input volts with a 12v battery bank. The recommended voltage goes up from there depending on battery bank voltage.

I must be missing something. If I rectify the 120vac I get 166vdc output. If I then put the + dc line through a winding on the transformer the voltage stays the same. It may be smoother but the voltage is still too high.

Thanks, Geno

[BruceM]

See if this helps explain:

When you rectify 120VAC, you get 120VDC RMS, with a peak DC voltage of 169 volts.  If you fillter that (MUST HAVE LOAD or VOLTS TOO HIGH) with an inductor, the RMS DC voltage will be 120VDC, with some ripple, the peak will be some volts higher depending on your inductor Henrys.

It seems your charger should be able to handle the 70VDC for your 48V battery, based on the specs for 24V.  Test it and see if you have to add an inductor as filter. I think you might be fine as is.

[mbryner]

I'm trying to do the same thing, but with a MX80 Flexmax80 charge controller.   I'd like to take power from a ST7.5 head and rectify it and run it into the MX80.

It appears we have 2 threads going that are similar:
http://www.microcogen.info/index.php?topic=350.15

it is designed for PV only with the exception of 3, Outback approved, hydro vendors.

and one of them is the guy who installed my system, Alternative Power and Machine here in Southern Oregon.   I discussed feeding ST head power into the Flexmax80 after rectification and he thinks it should work fine.   We discussed filtering it first but didn't discuss whether or not it should be with a capacitor or inductor.    Looks like after reading this thread, I need to go back to me college electronics books and review inductor filters.   Thanks guys!

Marcus

[Geno]

12/5/09
Please excuse my overabundance of caution but when the meter says 169 volts and the manual states 150 volts (over and over) I don't proceed. In any event.....

Earlier today I got the ups hooked up. I then discharged the battery bank. I've been charging the batteries from the 70v tap on the x former for an hour or 3 now. I've gone up to 1200 watts and the Flexmax works fine. It's just warm to the touch. I need more cooling on the rectifier if I want to push the charge current. Your earlier assumptions are correct at this time. The Flexmax isn't having any problems.

12/6/09
I reduced the charge to 500 watts before I went to bed and sometime later the charge cycle completed and it went to float. This morning the Flexmax is barely warm and pulling about 60 watts. Now I have to get everything mounted and safe.
Marcus: It's reassuring to hear what your installer says about this.

Even though the UPS will provide clean power it's quite a pig and uses 300 watts with the inverter on and no loads attached. It also hums and buzzes quite a bit. But it was free.

Thanks, Geno

[BruceM]

That's great news, Geno.  Nice work on the wise, careful testing.

Pity about the UPS 300 watt (!) standby draw, makes you want to round up the villagers and go after the idiot designer with torches and pitchforks. 

[Geno]

I didn't expect the UPS to be terribly efficient. It's old and I doubt it was designed to be run for long periods on the inverter. I'll be running it at less than 1/2 capacity average with short bursts to 3/4 load. It will be nice to be able run my larger loads without having to run around turning off the other loads. Time will tell but I might have to put some heat sinks on the transformer in the UPS. It got pretty hot with an 1800 watt load in about 90 minutes. Even if this UPS doesn't work out I'll have everything in place to put in a different inverter.

Thanks, Geno