48V charging via ST generator and solar charge controller (no ST mod)

[BruceM]

I'll think about it some more, Marcus.  Earlier in this monstrous thread or another, we found a non-MPPT charge controller that was $200.  It's a Tristar60 also, but not MPPT, which doesn't matter for this application. 

Here's the $200 Tristar 60 unit:  http://www.wholesalesolar.com/products.folder/controller-folder/tristar-60.html

Let me sleep on your current limit issue and see if I can think of another solution.

[mike90045]

Non MPPT chargers (PWM - Pulse Width Modulation) work by directly connecting the input (PV or generator) to the battery terminal.  The lower the battery voltage, the more time the terminals are connected [ by a FET ]. Max source current will flow till the FET or FUSE burns out.  The fet also works as the nighttime blocking diode. 

[Halfcrazy]

Are the transformers really rated in max watts or do they have a max current? my logic is here you will be doing half the amps at the toroid's that you will be doing going into a 48 volt bank. Also for an interesting project like this we at MidNite solar may put you on a short list of First run pre production units for a reduced cost. Shoot me a quick pm with a few highlights of the project and I will see it gets to the rite people. This program was designed for industry professionals but we intend to open it for a few interesting cases to help us make a better mouse trap.

I should also state there will be 3 voltage limits a 150vdc a 200vdc and a 250vdc here is a little primer on the Classic's progress.  http://www.midnitesolar.com/pdfs/officialMidNiteSolarClassicwatch.pdf

[mbryner]

From the ebay listing:  "It has a dual primary for both 100 -120 VAC or 200 - 240 VAC 50/60 Hz operation.  The secondary is 40 VAC center tapped at 30 ampere current rating."   Doesn't say any more on the label on the transformer itself either.   I'll PM you halfcrazy.

[BruceM]

Mike's post made me check the Tristar 45 and 60 to see if they had charge current limiting on the bulk charge phase.  It looks like they do not, unless that is one of the PC RS-232 software programming options, which seems unlikely. They only do PWM after bulk charging is complete.  So the Tristar PWM controllers would not be useful for this application.  Thanks Mike.

PWM IS used for current limiting, Mike, it's just that these units have no bulk charge max current limiting, they are only doing the fixed voltage charging phases by PWM. 

I'll think some more about other means of current limiting.

[mike90045]

Mike's post made me check the Tristar 45 and 60 to see if they had charge current limiting on the bulk charge phase.  It looks like they do not, unless that is one of the PC RS-232 software programming options, which seems unlikely. They only do PWM after bulk charging is complete.  So the Tristar PWM controllers would not be useful for this application.  Thanks Mike.
WM IS used for current limiting, Mike, it's just that these units have no bulk charge max current limiting, they are only doing the fixed voltage charging phases by PWM. 
I'll think some more about other means of current limiting.

PWM does not limit current. It limits the TIME the current is applied. 
The FET is turned Full On or Full Off, it can't be set to 82% on. It can give the effect of being throttled to 82%, but it's full power for only 82% of the time. 

Looking at the MS 45 & 60 MPPT controller, they output full array power in bulk.

If you use the 45MPPT - it will self limit in Bulk, to 45A   Which may meet your transformer power spec.

[BruceM]

Mike, PWM is used for current limiting in virtually every SMPS out there, including the DC supplies in the PC you're using.  An inductor is used to limit the instantaneous current (current averaging); it can be on either input or output.  So current is really being regulated- the load or source, depending on the design, does NOT see high current pulses.  I hope this make sense to you now.

In our case, we will have a huge inductor on the input to Marcus's MX80. The AC supply won't see current pulses at all.

For high frequency switchers, the inductors get to be very small, thus the ever increasing speeds of switch mode converters and supplies.

[mike90045]

I'll have to dig into my Trace C-12 again, I don't think there is an inductor in it.  Just the FETs, at a high fq switch rate at absorb and float.

What is SMPS - switch mode pwr sup?  Yes, those are full of chokes and transformers, and that's how the MPPT chargers work, in buck mode.  The upcoming Midnight Classic will have buck & boost modes, for even more charging options.

[BruceM]

Yes, SMPS is switch mode power supply.  Without an inductor to do current averaging, a PWM PV charge controller would blow out radio reception in the whole neighborhood-  square wave power switching on a long unshielded wire (like a PV lead in)  is a no-no.  

[BruceM]

Marcus, One approach to current limiting the AC charger input is to add an adjustable dropping resistor in series to your harmonic setup, so that you can have the extra resistor shorted by a switch or NC relay for AC operation, and open when charging batteries, so that your load is kept to the max wattage of your transformers.  This would not work with the boost type MPPT regulators, but would for the buck types like your MX80; as voltage drops to nearer battery voltage, current will be limited.

Another approach would be to have a mechanical switch or lever which would adjust the governor speed down to accomplish the same thing (only if harmonic regulator).

With an AVR, the 120V voltage sense circuit would have to be modified to allow regulation at a lower voltage.  A resistor switched in to the adjustment trim pot circuit would suffice. 

The other alternate would be a current limiting switching regulator on either the 240VAC side or DC side.  Not an option for your project, I think.

[mbryner]

Thanks Bruce.   Those are good suggestions.      The resistor approach would involve a pretty big, hot resistor.    Why is the switching regulator not an option?  Cost?

One thing I hadn't mentioned because I don't have enough info on it yet:  I ran across a electrical engineer friend from the hospital in Lowe's on Sunday.   (We don't see each other enough, and like to speak Swiss German together.)   We got to talking and I mentioned this part of my project.   He says he has a "large" variac sitting at his house, but doesn't remember exactly how many amps or volts its specs are.   He'll be bringing it out to my land next Sunday and we are going to look at it.   It might be just the ticket?   If it has high enough amp rating and I can dial down the voltage, I may not need those toroids or a current regulator because I could just use the max current input the MX80 is set at (i.e. the 3400 watts of my PV array), correct?

[BruceM]

A dropping resistor in the harmonic circuit is not that big and if sized properly, not that hot.   You only need a small voltage drop, and excitation current isn't that high.  Many of us already have a dropping resistor for harmonic operation.  About 10 ohms, 100 watt adjustable wire wound unit is my WAG.

A variac is an autotransformer- so there is no secondary winding and no transformer isolation.  This would be problematic for a 240V source setup as you must have one leg tied to your battery ground.  If your setup was 120V, the variac was 120V and neutral was already tied to the battery bank ground, then you could use a variac. 

The problem with a switching current regulator is that it would have to be custom built, I don't think it'sl likely find an off the shelf one.  You don't want to pay for custom engineering/fabrication work, it would make a second charge controller or a 240VAC-48V charger look like peanuts. 

[mbryner]

Got it.

Re: a dropping resistor in the harmonic circuit: that's why I was commenting in another thread earlier:
http://www.microcogen.info/index.php?topic=614.msg7191#msg7191
Except I didn't know what size resistor to use, so I suggested using a dimmer and you replied recommending a variac.    If you think a 10 ohm 100 watt resistor is appropriate, I'll try it.

Re: a variac in the output:  IIRC, the battery bank negative is tied to bldg ground, which is also tied to PV ground and neutral in the breaker panel.    If it's a high voltage variac, that opens up another option.

Whew, this thread is ponderous, but lots of good info here, at least for me...

[BruceM]

Remind me, please, what's the faceplate volts and amps on your ST-5.  I'd like to do a calculation before you order a dropping resistor.

[mbryner]

I oversized and went with a ST 7.5, just to make it more complicated!   I'll look at it next time I'm out there, but stuck at work in hosp for next few days.