Better understanding Frequency Drive Control Logic

[fastline12]

As more of an interest than anything, I have read that most modern VFDs may actually closely simulate true sinusoidal wave form by creating many voltage pulses along the wave form so instead of it actually being square wave output, it actually is pretty close to true sine wave output.

My confusion here is how in the heck does a transistor "ramp" up the voltage? It only knows on and off, how would it create anything but full DC Buss voltage when turned on? Are they possible using output side caps to help smooth the output as the cap charges?

 

[ozmosis]

I think you are mixing up current and voltage. It is the current waveform when imposing a PWM voltage output onto a motor's inductance, that is (near) sinusoidal.
There are some specialist type VFD's that can provide sinusoidal voltage output but in the main, the voltage comes out to the motor in short bursts and you vary the length of time they are on and off. Hence PWM (Pulse Width Modulation).
If you google PWM VFD explanation, you will see numerous sites from manufacturers and independents alike. Some reading material:

http://www.gambica.org.uk/web_image...tion_Voltage_Stresses_Report1_3rd Edition.pdf

http://www.danfoss.com/NR/rdonlyres/B07BD285-5895-48C9-AB04-2B0FBED854F6/0/vfdlesson3.pdf

 

[fastline12]

Pardon my ignorance here but I can certainly understand the current waveform being pretty much sinusoidal driving an induction motor just by the inherent power draw by the rotating motor but I would swear I read somewhere that VFDs operate at high frequencies because of the multiple pulses per cycle. It might also be that if a drive is running at say 240hz, the processor is required to handle the requirements of all three phases simultaneously. Otherwise I would think a simple 60hz operation would not take all that much switching speed.

what you are saying is that the voltage wave form would be rather square in nature from a VFD?

I will certainly read your links and read a bit more on the subject. Just something I was wondering I guess.

 

[itsmoked]

The voltage is switching fully ON and OFF just like you suppose above. What you may not realize is that it's PWM but not a constant PWM. Over the period of one synthesized sinewave the PWM changes the width of the ON pulses thru a complex pattern that creates the sinewave current.

And yes the CPU has to do that up to about 20kHz to run 60Hz motors. Once it's done the math for one phase the others probably get delayed copies.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[fastline12]

I think that is more or less the point of my questions. In simple theory and simplistic square wave PWM, a transistor array would only need to provide 60 pos pulses and 60 neg pulses per sec of the appropriate pulse width to approximate sinusoidal voltage. Technically a running motor's current demands would also cycle at slightly lagging from the power source but would still change.

I guess I am having trouble finding data and grabbing the reasoning for providing multiple switching events for each pulse and how that affects the current wave form or motor performance. I would imagine that if a VFD was actually running another VFD, this would not make much difference at all? The rectifiers would not really care and separate energy with no real discrepancy.

 

[Skogsgurra]

You need to get out of that "stepper motor thinking".

PWM is something quite different. Read this WEG-induction-motors-fed-by-pwm-frequency-converters-technical-guide-028-technical-article-english.pdf and pay special attention to the section about PWM voltage and motor current.

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.

 

[jraef]

Forget the square wave analogy, that has not been used for years now. Pulse Width Modulation (PWM) is what we are talking about, as osmosis said in your first relply. Read the referenced material before you get ahead of yourself and then come back with what you don't quite get after that. The basic PWM description is used in a lot of other things like power supplies, UPS systems, solar inverters, signal generators etc. It's good to have a clear understanding of the concept but it's too complex to go in to in a forum post. The literature is a better way to learn.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[fastline12]

I agree Jraef, if anyone else has some relative docs to absorb, feel free to post them.

jraef, on a sort of related subject, I wanted to check with you on an issue regarding a Fanuc alpha drive being run by an RPC. The RPC HP size is matched to the drive, not oversized. The drive and the rest of the system works without issue but in doing voltage testing, the L1-L3 voltage (L3=generated leg) dips to 170V at inrush and up to 290V under braking. All other voltages stay pretty stable, even L2-L3.

I am curious if an oversized RPC will be able to sink more power on L3 and stabilize inrush or if we need to look at other solutions?

 

[fastline12]

OK gents, where I am at is I now understand that square wave is Kmart today and nearly everything uses true PWM. By modulating the pulse width on the IGBT, you are able to control the voltage rise in the wave form. Short pulse - lower rise, wider pulse - higher rise. I am not yet sure what the optimal operating frequency or target pulses per cycle might be to operate around the 60hz range but still reading.

It sounds like one of the main issues surrounding inverters powering inductive loads is the reflected/inverted power pulses that get sent back to the IGBT and the device will not sink that power so it reflects back again and creates the harmonics common to drives that can damage systems and motor insulations. I am not yet sure what type of filtering is common in the 60hz range but it would seem either cable length tuning or filtering would be required for optimal safety of both the drive and driven device.

But how does a regen device work? Why must they be basically another inverter instead of just shorting the extra power from say the motor leads with digital switching, right back to the incoming power to the drive so it can be absorbed by the utility power? Do the regen units also have to run complex PWM to mimic the line it is putting the power on?

 

[Skogsgurra]

Sorry. You assume a lots of things instead of trying to understand.

PWM does not influence voltage rise at all. Every PWM pulse has the same risetime, independent of pulse width.

The reflections may be a problem, but have nothing with harmonics to do.

Wait with regen until you have understood the basic concepts.

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.

 

[fastline12]

OH, I guess I did not quite get there. Let do some more studying.

 

[dpc]

http://www.infineon.com/dgdl/AP1609...90004&fileId=db3a304412b407950112b40a1bf20453

Long link, but might be of some help.

 

[ScottyUK]

Have a look at

http://sea.siemens.com/step/flash/STEPACDrives/index.htm

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If we learn from our mistakes I'm getting a great education!