A "simple" 32VDC 30A dimmer or PWM automatic power cutoff strategy?

[thos37]

I'm an amateur in electronics and am trying to teach my self by building some circuits for a human-powered music project that I've been involved in recently. I'm looking for some guidance and advice from those of you who have more experience and skills on a good approach to a few issues.

I have a pedal power system composed of 4 bicycles with 36V 3 phase AC rear hub electric bike motors used as generators, running in parallel through 3-phase full wave diode rectifiers into a 32V 58F super capacitor bank (2 parallel sets of 2x16V 58F Tecate super caps in series), and from there into a 24VDC to 240VAC inverter (with 22v-32V input range). The inverter powers a 1200W (peak) PA sound system.

In the current system, a simple microcontroller circuit (ATTINY85 ADC input from a voltage divider) monitors the voltage on the caps and switches off relays to disconnect the bikes from the caps if the voltage reaches 32V. This keeps the music going when the pedalers provide too much power.

One problem with this is that the relay cut-off creates a very sharp and unpleasant "edge" that makes pedaling uncomfortable during the switching. Another problem is that the power factor is not very good, and you can even feel the pulses as each generator pole reaches its peak output and a surge of current enters the capacitor.

I'm looking for some hints or clues towards an appropriate solution for some kind of automatic "dimmer" circuit that can handle 20-30A, 22-32V DC, reduce the current when the voltage on the caps approaches 32V, and, optionally, improve the power factor.

I've looked at SEPIC and Buck-Boost circuits in particular, since the inverter can handle anywhere from 22V to 32V, and the generators put out power from less than that up through 36VDC. Would an MPPT also help to get the most out of the generators at their sweet spot? Is there anyway to do the dimming other than with a buck-boost or sepic type of circuit?

Thomas

 

[kabiru]

What did you really want,is it boost coverter or the dimmer circute that can with stand 30Amps?

 

[thos37]

Hey Kabiru,

Thanks for the reply!

I want to build a dimmer, but only IF it's significantly easier to make than a boost/buck switching converter. If it's not, then I'd rather build the boost/buck converter since it can also do power factor correction and potentially MPPT.

T

 

[kabiru]

I suggest you use the boost/buck converter because control both your current and your voltage at the same time.

 

[DXNewcastle]

I agree with kabiru, but there is another approach that you might want to consider before you make a decision:

Your description of the sudden loss of load on the cyclists effort reminded me of the method used in some railway power systems. They use a resistive load which is progressively switched into the traction motors' circuits when breaking (the motors are powered directly from the on-board generators during acceleration). You could consider applying a resistive load to the cyclists at the point when your 'batteries' approach full-charge. This could be implemented progressively in your project, either through (large, 1kWatt) variable resistances or through electronic-controlled high speed switching (such as PWM control).

I wonder if a progressive switch between the 'batteries' and a resistive load might help in proving a smoother transition between full load and no load? Though in terms of maximum long-term efficiency, I am inclined to favour adding more battery capacity, with the one aim of taking advantage of as much human effort as possible while it is available, and therefore the designer must provide as much storeage for that potential power as possible.
Perhaps you can combine the 'dimmer' concept to move energy to resistive loads when battery storeage is fully charged and the improvement of efficiency by capturing more energy when its being generated.