Capacitor in DC link of the VFD? 3

[jammy0786]

can any1 tell me what is the use of Capacitor in DC link of the VFD? How come a capacitor works on DC? also how does a VFD controlled motor gets its required reactive power as it is being supplied directly from an inverter which isn't a source of reactive power?

 

[itsmoked]

The DC link capacitor is vital to the VFD function. The input power is rectified and stored in said capacitor. The inverter's output is supplied from the capacitor. That's why the inverter can take in a fixed frequency of power and synthesize any other frequency for output - it's using that DC source for the raw material.

All capacitors will work with DC. They work like a fast charging/discharging battery.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

also how does a VFD controlled motor gets its required reactive power as it is being supplied directly from an inverter which isn't a source of reactive power?

IF it is a PWM inverter, then the output fundamental component should similar to an ideal voltage source. A voltage source delivers whatever power (real or reactive) is demanded by the load, right? Or did you have a different type inverter / vfd output stage in mind.


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(2B)+(2B)' ?

 

[jammy0786]

thanks @itssmoked but why do we need to store the DC power when our input is continuous online, we can keep rectifying and then keep inverting?

Thnaks @electricpete yes I'm using a PWM inverter. doesn't the DC link capacitor supply the required reactive power in VFD?

 

[John2025]

The caps there to smooth the rectified voltage. Without it you'd have a waveform that was just a bunch of half waves varying from 0 to peak voltage and back 120 times / second. Google DC power supplies for a better explanation. Reactive power only applies to the output to the motor, not the input caps.

 

[waross]

Reactive power is imaginary. That said, the imaginary model simplifies calculating the effects of a phase shift between the voltage and the current.
Consider first principles:
When a voltage is applied to an inductor such as a motor winding the current does not rise instantly. The current rise due to the application of a square wave front follows a curve based on something like (one minus the reciprocal of the base of natural logarithms).
Apply a sine wave and the current curve becomes somewhat more difficult to plot. When the voltage is dropping, the induction tends to keep the current flowing.
Now you can try to calculate the effect of reducing the voltage applied to an inductor on a point to point basis and plot the curve, both when there is a path for the current supplied by the induction coil (the DC link capacitors) and when there is not a path for the current supplied by the inductor (inductive kick).
Or take advantage of a lot of difficult mathematics done in the past by some unsung genius and use the imaginary concept of reactive power (and the DC link capacitors to provide a path for the inductor supplied current).
BTW Does anyone know who developed the concept of reactive power?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Skogsgurra]

Don't know. But C. P. Steinmetz is one of the suspects.

I like the obvious and no-nonsense explanation that Pete gave us. A voltage source delivers what the load asks for. Be it active or reactive power - or a mix. No need to try and visualize how power flows in switches and capacitors.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Hello Gunnar. I was thinking Stienmetz or someone of similar ability.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jammy0786]

thax every1 for the help that clears a lot of doubts.

 

[cswilson]

On a related note, several years ago there was a lot of buzz about "matrix converters", which supposedly could perform this functionality without a DC link and associated capacitors by permitting each of the three line phases to be connected to any of the three motor phases through a "matrix" of power transistors.

I haven't heard anything about these lately. It is an idea that has fizzled?

 

[jraef]

Ended up being too expensive for the purported benefits. Yaskawa still makes and sells them (the AC7 Series), but they still make and sell VSI technology as well, and continue to develop it, whereas they developed the one Matrix drive and it stagnated. As I have heard it told, it works out OK on 4 quadrant applications, an inherent quality of that technology, but for the vast majority of applications where that is unnecessary, it's just too expensive by the time you overcome the other inherent challenges, such as the output voltage being 87% of the line voltage, which means boosting the line voltage going in.

This is a good compendium of why I think.

http://www.eurekamagazine.co.uk/des...ology/the-truth-about-matrix-converters/3206/

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington