Buck converter for small wind turbine project

[Burnit0017]

Hi, adding fourth stator. This will allow a 8 coil per phase configuration. I am hoping this will lower the voltage and increase the current when used with the buck converters. But I am not sure. I still have to attach the end cap. 

I am planning to add a micro to the buck converter. Before I can do that the circuit needs a voltage limiter at the input. The max rating for the mosfet is 500 volts. I am searching for a circuit that will trigger at 400 volts. Any help will be greatly appreciated. 

[rossw]

I am searching for a circuit that will trigger at 400 volts. Any help will be greatly appreciated.

Find any circuit that will trigger precicely at a known voltage you can work with that's less than 400V, and make a simple voltage divider. Eg, if you can find something that triggers sharply and reliably at 240V, then a simple divider of 180K at the "top", 270K at the "bottom" will give you exactly 240V at the tap with 400V at the top, but you would need half-watt resistors

[tomw]

I am searching for a circuit that will trigger at 400 volts. Any help will be greatly appreciated.

Find any circuit that will trigger precicely at a known voltage you can work with that's less than 400V, and make a simple voltage divider. Eg, if you can find something that triggers sharply and reliably at 240V, then a simple divider of 180K at the "top", 270K at the "bottom" will give you exactly 240V at the tap with 400V at the top, but you would need half-watt resistors

Ross;

That is just the ticket and  deceptively simple. My first thought was a zener type circuit like on the analog input of the PLC boards we are messing with.

Tom

[rossw]

Ross;
That is just the ticket and  deceptively simple. My first thought was a zener type circuit like on the analog input of the PLC boards we are messing with.

Crossed my mind too, Tom - but then I was concerned that the original poster may have been wanting it in an AC application. The Resistor divider works fine for AC or DC. Capacitors are possibly better for AC because they won't dissipate any power, but they're not much good at DC

[Burnit0017]

Hi, I found a crow bar circuit that may work for the voltage limiting at the input of the buck converter. ZD1 will be replace and a load can added between the Q1 and ground. When Q1 is triggered the load may act as brake for the turbine and reset the circuit. Comments welcome

http://www.wiringdiagrams21.com/2009/11/28/a-typical-crowbar-circuit-diagram

[Burnit0017]

http://www.wiringdiagrams21.com/2009/11/28/a-typical-crowbar-circuit-diagram



Hi, I plan to alter the circuit to include two 200 volt zener diodes in series to replace ZD1. I am not sure if the zeners will brake down at 400 volts.

I also found a link where a high voltage laser circuit used a opto-coupler to trigger the SCR when the zener diode brake down occurred. 

The strategy at this time is to use a load between the output of Q1, that will act as brake for the turbine when the SCR is trigged and include a opto-coupler circuit to provide the trigger signal. Then the circuit will reset when the turbine will slow down.

There maybe better solutions. I have orders some parts for experimentation.

[rossw]

http://www.wiringdiagrams21.com/2009/11/28/a-typical-crowbar-circuit-diagram

Hi, I plan to alter the circuit to include two 200 volt zener diodes in series to replace ZD1. I am not sure if the zeners will brake down at 400 volts.

This circuit is indeed simple and generally fairly reliable. However a couple of things I'd just like to point out.
Zener diodes don't have a particularly sharp knee, so as long as you're not too concerned about the exact voltage it triggers at, you're fine.

The other thing I'd be slightly concerned about is that if your diodes break down (and putting 400V DC into a circuit like that is a fairly taxing environment), then you have the potential for a LOT of magic smoke.

A few things, in no particular order. The SCR doesn't need much gate current to trigger, so I'd put a fairly high value resistor in series with the zeners, on the "hot" side as a safety device. The 1K resistor will (obviously) need some careful calculation. Remember to calculate things like: how much current will flow if you get (say) 450V, if the zeners will survive it and how much power that resistor will need to dissipate.

If you want are more precise control of your voltage, rather than the crude-but-effective zener triggered crowbar, consider using an opamp or comparator. With a suitable divider (eg, 470K top, 6K8 on the bottom) 400V DC in would give you 5.7V on the tap. Thats easily in a safe and comfortable range for a simple comparator (eg, LM311). With its gain, you will have quite accurate and repeatable trigger points, and a nice sharp turn-on to drive your optocoupler.

[Burnit0017]

Hi, I found a solution that will work on the AC side of the bridge rectifiers and reset after it triggers.

http://www.vawts.net/t45566226/us-vawt-limiter/

I not familiar with Triac’s or Diac’s. I will have to do some reading, now that I know what I am looking for. Comments welcome

[Burnit0017]

Hi, I order the parts before I understood Steve’s circuit. I do not know if the Zeners will breakdown at 400 volts. The trigger signal is from the DC side at the buck converter input. L1 is from the AC side; single phase; from the PMA. If the SCR fires it will reset when L1 returns zero. Load could be a heating element so the PMA stator would stay cool. May be a possible solution for the input over voltage protection circuit. Comments welcome

[rossw]

The trigger signal is from the DC side at the buck converter input. L1 is from the AC side; single phase; from the PMA.

The diagram, as you have it, seems certain to cause you grief.
IF you get the SCR to fire, it will do so only on every half cycle. That'll only pull down the volts on that half cycle, the other half will continue to deliver full volts.

The trigger circuit, as you have it, is unikely to work if your PMA is as I understand it. If L1 is either end of the same single phase coil, your SCR current will also have to travel through the lower diode in the rectifier, etc.

From what I've seen of your project, I'd be advising the use of triac rather than scr, and a simple opto-coupled trigger from the (now isolated) DC circuit.

[Burnit0017]

  Hi, thank you for the very sound advice. I did find a PDF that shows the method. I will have to order the parts and I post the results when available.  Thank again.

[Burnit0017]

concept drawing for test OVP circuit

[rossw]

concept drawing for test OVP circuit

Aka, "Magic smoke" circuit

Recognise that it's a "concept", but:
* you can't trigger two SCRs back-to-back like that with the circuit as shown.
* Your opto is going to be FAR too soft to trigger cleanly with the 2x200V zener arrangement
* I still don't "get" what "L1" is where the load connects. How do you plan balancing the load across 3 phases?
* your comment "heat will be dissipated in the load, not the stator" is wrong. If you are dissipating any power, and your stator has any resistance, then you will be dissipating power in the stator. OK, you may dissipate less this way than say, a dead short...

[herbnz]

concept drawing for test OVP circuit

Hi
My thoughts on this OVP .
As the transfer of AC involves losses due to the high frequency in the multipole generators we tend to use. I always have the rectifiers at the generator and bring DC down.
Therefore I would be looking for a clamping circuit on the DC. SCr's can still be used but as they latch on a second SCR is required reduce the current below holding value. Its been a long time since I have worked with SCR's but I remember the second leg charged a capacitor and when the turn off SCR is pulsed it attempts reverse bias the main SCR. I will look in old texts etc. Or a mosfet  or bipolar transistor could be used.
Many thanks for the info you are producing on these buck converters. I have been playing with MPPT & HV for a few years now but cheated by using a 150 v commercial solar unit Tristar .

Herb

[herbnz]

concept drawing for test OVP circuit

Hi Found cct At http://www.cavehill.uwi.edu/fpas/cmp/online/el10c/gibbs/Thyristors_files/image013.jpg

In normal operation the SCR is subjected to forward voltages which are below its breakover voltage and the SCR is made to turn on by the application of a suitable gate current. This gate current is usually made high enough to insure that the SCR is switched to the on state at the proper time. Furthermore, the gate current is usually applied for just an instant in the form of a current pulse. A constant gate current is not required to trigger the SCR and would only cause more power to be dissipated within the device. Once the SCR turns on, it can be turned off only by reducing its forward current below its respective holding current value.

 

The SCR is primarily used to control the application of dc or ac power to various types of loads. It can be used as a switch to open or close a circuit or it can be used to vary the amount of power applied to a load. A very low current gate signal can control a very large load current. The SCR in Figure 3 is basically used as a switch to apply dc power to the load resistor (R2) but in this basic circuit there is no effective means of turning off the SCR and removing power from the load. However this problem could be easily solved by simply connecting a switch across the SCR. This switch could be momentarily closed to short out the SCR and reduce its anode-to-cathode voltage to zero. This would reduce the SCR's

 forward current below the holding value and cause it to turn off.

 

A more practical SCR circuit is shown in Figure 6. With this circuit, mechanical switches have been completely eliminated. In this circuit SCR1 is used to control the dc power applied to load resistor RL and SCR2 along with a capacitor (C) and a resistor (R1 are used to turn off the circuit. When a momentary gate current flows through SCR1, this SCR turns on and allows a dc voltage to be applied to RL .


http://www.cavehill.uwi.edu/fpas/cmp/online/el10c/gibbs/Thyristors_files/image013.jpg

Figure 6

This effectively grounds the left hand side of capacitor C and allows it to charge through resistor R1. This, in turn, causes the right hand plate of C to become positive with respect to the left hand plate. When a momentary gate current pulse is applied to SCR2, this SCR turns on and the right hand plate of the capacitor C is grounded thus placing this capacitor across SCR1. The voltage across capacitor C now causes SCR1 to be reverse- biased. This reverse voltage causes the forward current through SCR1 to drop below its holding value thus causing this SCR to turn off and remove power from RL. Therefore a momentary gate current through SCR1 will turn on the circuit and a momentary gate current through SCR2 will turn off the circuit.