Trying to understand the output

[Rjay]

Hi All,

Please help me understand.

Unidrive from Control Techniques rates the drive as a 200-240V input and 200-240V output.

Given it is AC, and using 240Vrms gives a peak value of 338VDC after full wave rectification (ignoring ripple, diode drops, etc.).

The PWM IGBTs switch to recreate 3 phase waveforms to the induction motor.

Here is my question....How can the output voltage be 240Vrms with a 338VDC bus to switch?

I must be missing something.

-Rjay

 

[leitmotif]

Granted each pulse will be + or - 330 but when you average them you will get roughly 240 VAC at 60 Hz. ratio of V/Hz is approx 4 for varying freq. If you put a shunt (resistor) in series with one leg of output and look at the voltage across resistor on a scope you should see a sine wave with a little ripple.

Most of the VFD manufacturers have pretty good stuff on the web on VFD operation - well worth reading.

Dan Bentler

 

[Rjay]

Hi Dan,

I understand that the PWM creates 3 phase sine waves with PWM ripple on the output but I don't see how to get approx. 676 V peak to peak sine wave (required to make 240VAC rms) by switching 338 VDC PWM into an inductive load (motor). This is the one part I am confused about.

Thanks again,

-Rjay

 

[Rjay]

Also,

The PWM will switch on & off so I suspect that the voltage on the scope will be a square wave switching between 0 and 338VDC. The current waveform would have the ripple sine wave. This compounds my confusion for rating the output at 240V.

-Rjay

 

[widelto]

Try this: http://www3.sea.siemens.com/step/pdfs/ac_drives.pdf

 

[Rjay]

Bill,

That helped a lot. My mistake was ground referencing the 338VDC (640VDC in the article...480V drive). I forgot that the current reverses in the IGBTs creating a positive and negative voltage swing to the motor. There is one very misleading graph in the article showing 640V spanning the PWM positive and negative pulses....ie. I don't think that is technically accurate. Either way, I get it now.

Thanks again,

-Rjay....This is a great forum!

Dan, I also get that you told me to look at one of these article. I had read these drive descriptions adnauseum but not seen the Siemens one. Whatever it was, it broke loose my misunderstanding...thanks.