Forum Discussion
412 Replies
Salvo wrote:
Pretty much all modern controllers use high frequency. The higher the frequency, the smaller the magnetics become. Costs go down.BFL13 wrote:
There is high frequency PWM and "low frequency" PWM. The "PWM" 12v controller uses low frequency PWM to "control" when it is stopping voltage from going any higher after it starts controlling.
I suppose the "MPPT" controller uses high frequency PWM to keep at the MPP during Bulk, and "low frequency" PWM during Absorb and Float same as a "PWM" controller does---but I don't know.
BFL, are you perhaps saying both controllers use a highER frequency during bulk, and a lowER frequency during abs/float?- Sure, when in mppt mode, the controller will supply a little more current. But there's not much energy when the sun sets and goes through more atmosphere. The panel will be cooler and therefore have a little more power available.
pianotuna wrote:
Does the fact that MPPT goes "back on" in low light conditions (sunset etc) imply that it extracts a little more energy than a straight PWM?
Do all computer based MPPT units do this--or just the more advanced versions. - Pretty much all modern controllers use high frequency. The higher the frequency, the smaller the magnetics become. Costs go down.
BFL13 wrote:
There is high frequency PWM and "low frequency" PWM. The "PWM" 12v controller uses low frequency PWM to "control" when it is stopping voltage from going any higher after it starts controlling.
I suppose the "MPPT" controller uses high frequency PWM to keep at the MPP during Bulk, and "low frequency" PWM during Absorb and Float same as a "PWM" controller does---but I don't know. - I really don't know what you're arguing about. Yes, mppt and pwm controllers use different architecture to get to the same result. The end result is a constant voltage regulator when in absorb or float. Both use pwm to achieve their goal. Both controllers output equal amounts of power into the battery.
Of course the panel voltages will be different. The pwm controller uses 12V panels while the mppt controller uses a higher voltage panel. Panel voltage will be different. But who cares? Is this your sticking point?brulaz wrote:
Salvo wrote:
No, not correct. Both the pwm and mppt controllers use pulse width modulation to control output voltage during absorb and float.
The 3048 manual discusses changing pulse widths during the maximum power point discussion. Do you understand what changing pulse widths means? It's pwm!!! The mppt controller uses pwm to achieve the maximum power point and it also uses pwm to achieve constant voltage output during absorb and float.
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Again, forget output voltages. Look at panel voltages.
During the float or absorb stages the mppt controller maintains voltages on the panels that are different from the battery, whereas the pwm controller does not. They are obviously not doing the same thing.
The mppt controller is certainly using some form of pulse width modulation, but that doesn't make it's behaviour identical to a PWM controller during float or absorb states. - BFL said,
"There is high frequency PWM and "low frequency" PWM. The "PWM" 12v controller uses low frequency PWM to "control" when it is stopping voltage from going any higher after it starts controlling."
I think maybe that goes back to the debate over on/off shunt controllers vs. pwm. When pwm first came out, the 'shunt camp' argued on/off was in fact pwm, but low frequency pwm, not high frequency pwm. From what I can tell so far, it was never an issue of low vs. high frequency, but an issue of linear vs. switching technology. (but I could be mistaken about that). Switching, by its very nature IS high frequency. So there may be some confusion about what "high frequency" is even referring to? This is why I'm compelled to investigate the whole issue further. To this day, you can still buy shunt controllers, and some still swear by their superiority to "high frequency" pwm.
Morningstar's whole arguement for the superiority of pwm is based on findings by Sandia, that indicated two things, in particular;
1) "It appears the number of times a system cycles off and on, during a day in regulation, has a much stronger impact on battery soc, than the max voltage"
IOW, higher frequency pwm results in improved battery acceptance and thus higher soc, and
2) max voltage has little effect, while "reconnect voltage" has a strong impact on resulting soc.
This goes back to the other article, where the pro- shunt guy argued that pwm was being unfairly compared to on/off controllers with poorly calibrated max/reconnect setpoints. But either way, what we now consider as pwm is apparently high frequency pwm. Of course, I could be mistaken?
Is there generally a difference in the actual pwm frequency used by PWM and MPPT controllers? - Hi Salvo,
Does the fact that MPPT goes "back on" in low light conditions (sunset etc) imply that it extracts a little more energy than a straight PWM?
Do all computer based MPPT units do this--or just the more advanced versions. CA Traveler wrote:
Actual data can help with the understanding.
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Good idea.
Here's the Rogue graph I keep referring to:
brulaz wrote:
the pwm controller does not.
Voc is a different voltage than Vbatt. The pulses between Voc and Vbatt (abs or float) change width to limit current and maintain Vbatt.
Without a bucker the panel is at Voc or Vabs/float.
A scope is needed to measure Vpanel during PWM- Actual data can help with the understanding. This data is from a MS MPPT60 controller on 1/24/15 with clear skies and 3x250W panels. The batteries were fully charged and the large transitions were the result of adding/removing a 120A inverter load. The data shows the panel voltage and other changes as the controller adapts to the changing load.
brulaz wrote:
Salvo wrote:
No, not correct. Both the pwm and mppt controllers use pulse width modulation to control output voltage during absorb and float.
The 3048 manual discusses changing pulse widths during the maximum power point discussion. Do you understand what changing pulse widths means? It's pwm!!! The mppt controller uses pwm to achieve the maximum power point and it also uses pwm to achieve constant voltage output during absorb and float.
...
Again, forget output voltages. Look at panel voltages.
During the float or absorb stages the mppt controller maintains voltages on the panels that are different from the battery, whereas the pwm controller does not. They are obviously not doing the same thing.
The mppt controller is certainly using some form of pulse width modulation, but that doesn't make it's behaviour identical to a PWM controller during float or absorb states.
There is high frequency PWM and "low frequency" PWM. The "PWM" 12v controller uses low frequency PWM to "control" when it is stopping voltage from going any higher after it starts controlling.
I suppose the "MPPT" controller uses high frequency PWM to keep at the MPP during Bulk, and "low frequency" PWM during Absorb and Float same as a "PWM" controller does---but I don't know.
Wiki says there are a number of ways to do MPPT. One is to fiddle with panel voltage to find which voltage creates the most power.
