Using PWM to regulate power dissipation

[Jeroen Humasol]

Hi guys,

I'm trying to make an electronic load controller for a microhydro installation. However i'm having some troubles, with resonancy between the capacitors in my circuit and the internal inductance of my generator. To be honest, somebody told me that this was the problem, but i don't really understand how it's caused. And of course i even know less how that i could fix this

So here's the circuit:


Note: the inductance of the source is not drawn (because i don't know really know how much Henry it is). However the 'somebody' told me that i could calcultate it because the large currents where drawn at resonance frequency=1/sqrt(L*C).

The switch in the circuit is an IGBT in reality, we're switching it at 4kHz. We looked it up and it's certainly not this naughty boy that causes our signal to be this dirty:


Anyone that can explain me what is going on? (and if possible how to fix it)

Your admirer,

Jeroen Humasol

 

[Arouse1973]

Hello, you may need some capacitance after the diodes, I guess you want a d.c level?
Adam

 

[Jeroen Humasol]

Hi Adam, i'm sorry i just realised that the circuit i posted isn't completely correct, i tried to edit it but it seems that you can't edit your first post..

Heres the correct one:

 

[dorke]

Hi Jeroen,
It has everything to do with switching.
read this

 

[Jeroen Humasol]

Hi Jeroen,
It has everything to do with switching.
read this

Hello dorke, it's a bit late right now but i'll certainly read it tomorrow! thanks

 

[Jeroen Humasol]

Hello, you may need some capacitance after the diodes, I guess you want a d.c level?
Adam

I indeed need the diodes to get a DC current, but i don't really understand how a capacitance could solve the problem? Perhaps i wasn't completely clear: the measured current is the current drawn from the source. This is also the only current that really interests us, because it's that current that comes from our generator. We want this current to be as clean as possible to make life easier for our generator

 

[Arouse1973]

Hi Adam, i'm sorry i just realised that the circuit i posted isn't completely correct, i tried to edit it but it seems that you can't edit your first post..

Heres the correct one:
View attachment 26010

Add some capacitance to the output.
Adam

 

[Arouse1973]

Oh sorry I thought this was voltage noise on your output? What is the scope trace showing?
Adam

 

[dorke]

Oh sorry I thought this was voltage noise on your output? What is the scope trace showing?
Adam

I think it shows the generator's voltage at the input to the bridge.

 

[Arouse1973]

I think it shows the generator's voltage at the input to the bridge.

Right ok dorke, it wasnt that clear, thankyou!
Adam

 

[Jeroen Humasol]

I think it shows the generator's voltage at the input to the bridge.

I'm sorry guys, i was a bit tired and didn't mention it clearly: it's in fact the current that is drawn from our generator. So it's the current at the input of the bridge.

 

[Jeroen Humasol]

Hello Dorke, I read your article. But i don't really get it.

1) Their first suggestion is to add inductance. In our case it's the inductance that is causing the problem. Because if i understood my lab professor correctly then he told me that the source and the capacitor where exchanging currents. (reactive useless current at a frequency of omega=1/sqrt(L*C)). So if I add inductance, how do i know how much i should add, where is should add it and why should this work?

2)Their second suggestion is to use filters for specific frequencies. I think my professor said a similar thing. But how do those filters work? Are they just LC-low pass filters or am i missing something?

Thanks

 

[Jeroen Humasol]

I've some more questions: i'm trying to understant these filters. If you take this circuit:

This is a great filter if you only want low frequency voltage signals over your resistor Rload. But isn't this a horrible configuration for the current coming from the source? This because 'high frequency signals' are shorted to the ground because the impedance of C1 drops for higher frequencies. Because of this the voltage of the high frequency signals will only stand over the R1 resistor, which is a small resistor, so then follows that high frequency signals will creat high currents. Is this reasoning correct?

 

[Alec_t]

Why do you have the 100uF cap on the input to the bridge?

 

[dorke]

Hello Dorke, I read your article. But i don't really get it.

1) Their first suggestion is to add inductance. In our case it's the inductance that is causing the problem. Because if i understood my lab professor correctly then he told me that the source and the capacitor where exchanging currents. (reactive useless current at a frequency of omega=1/sqrt(L*C)). So if I add inductance, how do i know how much i should add, where is should add it and why should this work?

2)Their second suggestion is to use filters for specific frequencies. I think my professor said a similar thing. But how do those filters work? Are they just LC-low pass filters or am i missing something?

Thanks

Well,
I don't think your professor is correct on this one.

The circuit isn't resonant(it theoretically could be ,but isn't).
The inductor isn't the cause of this it is the switching of the load that causes the Harmonic Distortion you notice(THD).

You thus have the case of the fundamental desired signal and it's harmonics which should be eliminated.

A series inductor is the simplest way to create a low pass, filter that is the reason increasing it is recommended.
Series Resistive filters are very wasteful since they will waste at the fundamental frequency as well.

The way to view this(just a view) is like so:



The way to approach and solve this problem is in the frequency domain.
We recently had this thread with similar problems.
You can adopt the method suggested in it to measure in the frequency domain.

 

[Jeroen Humasol]

Hey dorke,

I think my professor is correct. This because as a test we only connected the voltage source to the capacitor. And this was the result: (the fucked up sinus is the current, the smooth one is the voltage)

for all clarity at that point the circuit was:

 

[dorke]

No he isn't.
Here is why:
The circuit as you describe it(#16) is a linear one.
In that kind of a circuit all currents and voltages should be pure sinusoidal and at the fundamental frequency only .
That should be the same thing in the case of a resonant circuit as well.
The distorted current is clearly not that,it probably contains the 5th and 7th harmonics.
Have you tried to watch it in the frequency domain?

You should further investigate the behavior of your generator.
It must be introducing some non-linear elements.
You can use a pure ohmic load and measure.

Is this generator something you have more data about?

 

[Jeroen Humasol]

No he isn't.
Here is why:
The circuit as you describe it is a linear one.
In that kind of a circuit all currents and voltages should be pure sinusoidal and at the fundamental frequency only .
That should be the same thing in the case of a resonant circuit as well.
The distorted current is clearly not that,it probably contains the 5th and 7th harmonics.
Have you tried to watch it in the frequency domain?

You should further investigate the behavior of your generator.
It must be introducing some non-linear elements.
You can use a pure ohmic load and measure.

Is this generator something you have more data about?

To be honest, i don't really know how to watch it in the frequency domain. Can i do this with an ossiloscope? Or should i use the program that you suggested in the other post? About the generator, i can't acces it right now so i can't give anymore information about it. Next week i'll return to ESAT (the lab where i perform the tests and then i can investigate it. Thanks anyway for the help so far! I'll keep you up to date next week

 

[Jeroen Humasol]

One more question, this generator will be generating power for an entire village. So the load of the village will be parallel to the dump load. It will be mainly used for lightning. Do you think that the load of the village can cause such great harmonics that the generator gets damaged? (it's a one phase generator)

 

[Arouse1973]

Every time you convert AC to DC using diodes aren't you are adding a non linear load into the mix?
Adam