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linear shunt failure

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Steve Conner

Thu Aug 20 2009, 03:38PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, I say that's no different to just driving the IGBT gate with whatever signal you would apply to the DTC pin, suitably processed so that it can only be on or off, and not assume any voltages in between. (and suitably amplified, for instance by a gate driver IC)

You'd have to do the same processing for the signal to drive the DTC pin anyway, and if you do the processing right you have what we've called hysteretic or bang-bang control.

I'm not going to write any more about this for at least a week, maybe Dr. Slack or someone else would like to have a go.

Dr. Slack

Thu Aug 20 2009, 06:24PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Maybe let's try seeing a real circuit, and analysing what it will do.

Build this, and it will work. At least the topology will, I've left all the component values at their default settings, part of your job is to get the values right so it does what you want. Not a 555 in sight, they're far too complicated for me, unnecessary and insufficient.

X1 forms a triangle wave oscillator. This will run at 30kHz (when the values are correctly chosen). The feedback to the +ve input makes it a schmidt, and defines the upper and lower voltages. For R1, R3 and R4 equal, it's about 33% to 66% of rail. Use R1 bigger to reduce the voltage swing if you want. Choose C1 to get the frequency right.

By inspection now you can see that if the bus voltage is low, the +ve input will be low, and the comparator output will never go high. If the bus voltage is high, then it will be on permanently. The clever bit is to choose resistor values for R5, R6, R7 such that a bus voltage of 260v gives 33% rail, that is just about to start turning the IGBT on, and that a bus voltage of 280 or 300 (or whatever you choose) gives 66% rail. At this bus voltage the IGBT will be on 100% of the time. At intermediate voltages, the duty cycle will vary to control the mean current.

It's possible that with the triangle generator running 33 to 66% of rail, it's not possible to choose R5/6/7 correctly, in which case you choose different values for R1/3/4 to shift the voltage of the triangle wave up or down, larger or smaller amplitude. Maybe a zener in series with the DC bus sense would burn off some voltage. However, all that's the easy bit.

I've shown default 324s in this diagram, though comparators would be a better choice than op-amps, I'm not certain a 324 will go cleanly rail to rail at 30kHz. You may want to use something with a higher output current to actually drive the IBGT gate.

Like Steve I think I'll give this thread a break for a week or so. Good luck. Post again when you've analysed what this circuit does, and calculated a plausible set of component values.

IamSmooth

Sat Sept 12 2009, 05:29PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I just wanted to update those that have helped me:

I finished the board and tested the circuit at 250v with a 30A load. The circuit performed well. It kicked on transfering to my load and kicking on the cooling fan. It went through a hysteresis point and after about a 1/2 second delay it turned off. I could see my lights dimming from the sudden load.

If anyone is interested in a final schematic I can post it; otherwise, thank you all for your help and hopefully this will work.

M_Gargantua

Sat Sept 12 2009, 10:28PM

Registered Member #276 Joined: Thu Mar 02 2006, 04:40PM
Location: NY
Posts: 5

Is your turbine self excited or can you isolate the excitation field coils? If you can then you can then its a simple task of varying the field excitation to maintain 260V.

Steve Conner

Sun Sept 13 2009, 07:17PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I think he used permanent magnets, good luck varying the excitation on those! Neo magnets are probably better for this application than electromagnets.

IamSmooth

Sun Sept 13 2009, 08:35PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

The design is an axial flux generator using N42 neodymium magnets set up as a three phase Star system.
The project is here and at

if you are interested.

IamSmooth

Mon Sept 14 2009, 11:20PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I recently cannibalized my old linear shunt controller. When it failed the unit stayed on and shunted at all voltages. I suspected the mosfets were all blown. I just tested them and seven of the eight worked; I can't find the eighth. I don't understand. I also tested the diodes that provided the gate voltage and they worked, too.

Anyway, the IGBT shunt controller is working so far.

Steve Conner

Wed Sept 16 2009, 02:22PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, Ockham's Razor says that the missing eighth FET was failed short. With paralleled devices, the first one to fail short will protect the others.

IamSmooth

Sat Oct 24 2009, 10:40PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

While working on something else I found the eighth MOSFET and tested it. Sure enough, it failed as a short.

Sulaiman

Sun Oct 25 2009, 01:00PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Just a few thoughts;

If you do not furl the blades then the tower has to be very strong.
If you build a strong tower wouldn't larger blades with furling be more cost-effective?

1 kW isn't much .. about the same as an electric heater !!!
Making/mounting a 1 kW resistor is more expensive than buying a 1 kW heater,
I'd prefer the old-fashioned red-glowing type to the newer bright-glowing types for durability.
You can use electric heaters in series or parallel for different voltages or powers.
You can load the dc bus or the generator directly.
You can use triacs (more rugged than mosfets) if on the ac side.
e.g. Resistive divider / capacitor / diac / triac / heater ... 6 components per phase.
A zero-crossing ssr and heater per phase with some triggering circuitry etc. etc.

I have a tree-element electric heater which could be rewired as three separate loads for three phases, it's the bright type but only cost GBP11,.. much cheaper in springtime.

For experimentation I would want the pitch of the blades to be continuously variable
to get the most from any wind speed.

A brake/clamp on the rotor is another option.

Amazing how projects expand to fully utilize available time and money

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