Constant current source Inverter

[ESMAT 1995]

i was reading about CSI and the first paragaph said "A constant current source is assumed here, which may be realized by using an inductance of suitable value, which must be high, in series with the current limited dc voltage source. " and it says nothing else about it , so clearly it's a basic concept, but i have no idea how that works, so can anyone explain to me please?
the link to the PDF
http://www.nptel.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Power%20Electronics/PDF/L-39%28NKD%29%28PE%29%20%28%28EE%29NPTEL%29%20.pdf

 

[berkeman]

Welcome to the PF.

clearly it's a basic concept, but i have no idea how that works,

Me neither. What's a Current Source Inverter? Can you post a scan of the page you are reading, along with any schematic diagrams?

 

[ESMAT 1995]

Welcome to the PF.

Me neither. What's a Current Source Inverter? Can you post a scan of the page you are reading, along with any schematic diagrams?

sure, i will edit the post adding the link to the PDF

 

[CWatters]

Are you familiar with the use of a capacitor to stabilise a voltage? Well an inductor does the same thing but for currents rather than voltages.

 

[Windadct]

CSI is the name of a general thyristor (SCR) inverter, DC to AC converter. This inverter need a constant current source to "force" the thyristors to stay in conduction mode, until the voltage in the load forces the commutation.

The typical design is just a traditional rectifier, with a large inductance in the DC link.

 

[Windadct]

OH -- for your referencing pleasure, I found this, apparently legal, PDF of THE GE SCR Manual...

 

[ESMAT 1995]

CSI is the name of a general thyristor (SCR) inverter, DC to AC converter. This inverter need a constant current source to "force" the thyristors to stay in conduction mode, until the voltage in the load forces the commutation.

The typical design is just a traditional rectifier, with a large inductance in the DC link.

So how is this "constant current" provided?
thanks by the way for this great reference

 

[ESMAT 1995]

Are you familiar with the use of a capacitor to stabilise a voltage? Well an inductor does the same thing but for currents rather than voltages.

i am familiar to the use of inductors as a filter for ripples, is that what you mean?

 

[CWatters]

Yes. Inductors don't "like" changing currents. They try and maintain an established current if the source is removed (at least they do for as long as they have sufficient energy stored in them).

Compare with a soothing capacitor, they try and maintain a voltage when the source is removed.

 

[CWatters]

PS an inductor can't behave like an ideal current source any more than a capacitor can behave like an ideal voltage source. The inductor only has to provide current for awhile as Windadct describes.

 

[Windadct]

Keep in mind - Constant Current in this case only need to be considered in the a relatively small time frame, clearly this is not a "Constant Current" over time period over much more than one cycle, or anywhere near a second. But in the time frame of a switching event needed to make the inverter work, it is not anything close to an IDEAL Current Source. In the link, the energy source for the system,is a traditional voltage source, with the addition of the inductor, the source for the Inverter behaves as constant current.

 

[ESMAT 1995]

Keep in mind - Constant Current in this case only need to be considered in the a relatively small time frame, clearly this is not a "Constant Current" over time period over much more than one cycle, or anywhere near a second. But in the time frame of a switching event needed to make the inverter work, it is not anything close to an IDEAL Current Source. In the link, the energy source for the system,is a traditional voltage source, with the addition of the inductor, the source for the Inverter behaves as constant current.

so i started reading a little in that reference but it could take me months before i can truly understand what's going on, but i found this link too on future electronics describing power inductors as " A power inductor maintains a steady current in an electrical circuit with a varying current and/or voltage."
Did he mean that it will maintain for a very short period of time or is that another case?
http://www.futureelectronics.com/en/inductors/power-inductors.aspx

 

[CWatters]

Did he mean that it will maintain for a very short period of time..

Yes and no.

Suppose you have a current source of 5A with some noise on it (+/- 0.1A). Putting an inductor in series will reduce the noise so the output is a constant 5A. It can do that indefinitely.

On the other hand if you have an intermittent source that delivers 5A for awhile and then switches off, the inductor will try to maintain 5A after it switches off but it can only do that until it runs out of stored energy.