Better understanding Frequency Drive Control Logic
Better understanding Frequency Drive Control Logic
(OP)
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?
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?





RE: Better understanding Frequency Drive Control Logic
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://w
http://ww
RE: Better understanding Frequency Drive Control Logic
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.
RE: Better understanding Frequency Drive Control Logic
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
RE: Better understanding Frequency Drive Control Logic
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.
RE: Better understanding Frequency Drive Control Logic
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Better understanding Frequency Drive Control Logic
"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: Eng-Tips.com Forum Policies
RE: Better understanding Frequency Drive Control Logic
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?
RE: Better understanding Frequency Drive Control Logic
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?
RE: Better understanding Frequency Drive Control Logic
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
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Better understanding Frequency Drive Control Logic
RE: Better understanding Frequency Drive Control Logic
Long link, but might be of some help.
RE: Better understanding Frequency Drive Control Logic
----------------------------------
If we learn from our mistakes I'm getting a great education!