- Location
- Illinois
- Occupation
- retired electrician
From a control design magazine article about soft starts and VFDs.
I don't think that is how they work.
VFDs and starting current
- Thread starter don_resqcapt19
- Start date
- Status
You're right - it isn't.
Whoever wrote that clearly doesn't have a clue.
article? https://www.controldesign.com/articles/2018/soft-starter-or-variable-frequency-drive/
although not the primary control mechanism the output will have a good amount of high order harmonics ~ switching freq which will result in a higher Z
although not the primary control mechanism the output will have a good amount of high order harmonics ~ switching freq which will result in a higher Z
Last edited:
The PWM switching frequency, commonly a few kHz, results in the current being a pretty good sinewave containing mostly fundamental power frequency.
I think this guy wrote it?
Mark Harshman, director, systems engineering, at Siemens: Starting a motor directly across the line can require an inrush current of 600% or more of the rated motor current. This current draw can result in significant voltage sag causing problems in a plant or the surrounding power grid.
Mark Harshman, director, systems engineering, at Siemens: Starting a motor directly across the line can require an inrush current of 600% or more of the rated motor current. This current draw can result in significant voltage sag causing problems in a plant or the surrounding power grid.
depends on load, etc
fft's show the story, not sure what 'mostly' is but v thd can be as high as 60% with i being 1/4 to 1/2 of that
I can't download the article in a legible form.
But if he wrote what the OP says, it is not correct.
And you and I both know that VFDs generally start from zero and increase in frequency and voltage are they run up. They are not (normally) designed for DOL starting.
Never measured anything close to that on the PWM drives we designed and built. The input harmonic current yes, but not the motor current.
And that's what we are looking at here.
take it up with him, I mean he is the director of systems engineering
if thd is only 10% at say 4 khz the weighted induct impedance due to each
fund ~ 0.9 x L
4k ~ 0.1 x 4000/60 x L , >7 times greater
I don't need to google it - it's a significant aspect of what I did for a living. Design, manufacture, test, install and verify the performance of variable speed drives mainly for industrial facilities.
I'd post you a link to our brochure but the mods might see that as advertising.
then you know this is a vsc not i sourced
so i is a result of v applied to the load
and z is a function of v and i: z = v/i
so even if i has lower thd it does not cancel v's impact
the motor acts as a low pass filter and since int(v/L dt) = i, averages or smooths
what he said is correct
higher order harmonics increase induct Z reducing i flow for a given v
That, in a nutshell, is why the current is close to sinusoidal.
You are wandering off the topic.
and why the Z is higher, reduciing i
it can be proven by generating a fourier series for v and i and reducing
high order v components will remain ~ Z
I would not have stated it the way he did, it is not 'wrong' nor the primary operating principle of a vfd
Smooths it.
it also lowers magnitude for a given v
i = v/Z, if Z increases and v is constant, i must decrease
- Location
- Illinois
- Occupation
- retired electrician
No it was a guy from DSL Electronic Systems.
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
180706-2037 EDT
I read the title of this thread to mean:
We have two black box systems to compare. Both produce comparable mechanical output after startup. One black box contains a standard induction motor, started by simply closing a switch (simple starter), or possibly it includes a relay contact type of soft start. The other box contains a VFD with control means to adjust startup, and an appropriate motor for use with the VFD.
Following this assumption, then it seems to me the question is --- how do the currents from the power company or other power source to the two different black boxes compare during startup?
Alternately:
In the quote of the first post "VFDs deliver much higher frequency source power ---- " we conclude the question is about motor input current, or in the case of the VFD it is also the VFD output current.
If motor starting is primarily controlled by ramping up of a low frequency apparent approximate sine wave current frequency to the motor by short modulated current pulses of a much higher frequency, then the motor wants to follow that low frequency. Per unit time there is not much energy required to slowly accelerate the rotor and load. Thus, not a lot of input current to either the motor or the VFD input is required compared to across the line starting.
When properly modulated the high frequency current pulses to the motor are averaged or smoothed by the circuit inductance to produced an approximate low frequency sine wave. A scope plot of motor current will show a relatively smooth low frequency current waveform, not short pulses, but with some of the high frequency as ripple.
I believe the quote of the first post is wrong because a high chopping frequency of current to a motor is present whether starting or not, just different modulation, and no mention is made of the synthesized low frequency sine wave current as the primary way that starting peak current is reduced.
What I have just said may be clear as mud. So if not clear question me.
.
I read the title of this thread to mean:
We have two black box systems to compare. Both produce comparable mechanical output after startup. One black box contains a standard induction motor, started by simply closing a switch (simple starter), or possibly it includes a relay contact type of soft start. The other box contains a VFD with control means to adjust startup, and an appropriate motor for use with the VFD.
Following this assumption, then it seems to me the question is --- how do the currents from the power company or other power source to the two different black boxes compare during startup?
Alternately:
In the quote of the first post "VFDs deliver much higher frequency source power ---- " we conclude the question is about motor input current, or in the case of the VFD it is also the VFD output current.
If motor starting is primarily controlled by ramping up of a low frequency apparent approximate sine wave current frequency to the motor by short modulated current pulses of a much higher frequency, then the motor wants to follow that low frequency. Per unit time there is not much energy required to slowly accelerate the rotor and load. Thus, not a lot of input current to either the motor or the VFD input is required compared to across the line starting.
When properly modulated the high frequency current pulses to the motor are averaged or smoothed by the circuit inductance to produced an approximate low frequency sine wave. A scope plot of motor current will show a relatively smooth low frequency current waveform, not short pulses, but with some of the high frequency as ripple.
I believe the quote of the first post is wrong because a high chopping frequency of current to a motor is present whether starting or not, just different modulation, and no mention is made of the synthesized low frequency sine wave current as the primary way that starting peak current is reduced.
What I have just said may be clear as mud. So if not clear question me.
.
Not the fundamental which is what matters for motor performance including starting.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
That entire article is basically responses from a variety of industry sources, all of them commercial in nature, but all of them correct, save one. Out of all of those responses, only that very first one is so egregiously wrong that it possibly will make intelligent people miss the points stated by the rest of the responders. It’s highly unfortunate that the publisher lead off with that bozo...
Not only was he wrong about how VFDs function, he was wrong about soft starters as well!
He had no business responding at all, let alone being the first response that readers see. That was a very poorly executed article.
Not only was he wrong about how VFDs function, he was wrong about soft starters as well!
What the heck is solid state series impedance? :dunce:
He had no business responding at all, let alone being the first response that readers see. That was a very poorly executed article.
- Status