CR4 - The Engineer's Place for News and Discussion ®


Previous in Forum: ACS850 VFD Reference Meet Problem   Next in Forum: Difference Between Ith & Iu
Close
Close
Close
4 comments
Anonymous Poster #1

VFD Output

04/08/2017 11:16 PM

Why the output V has more noise than the output current in a VFD? i.e. VTHD > ITHD? Is this because it is a VSI not CSI. Does a CSI have an output with ITHD < VTHD?

Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru

Join Date: Apr 2010
Posts: 5377
Good Answers: 521
#1

Re: VFD Output

04/09/2017 7:45 AM

Probably inductance in the circuit smooths the higher frequency current components.

Reply Score 1 for Good Answer
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 11562
Good Answers: 136
#2

Re: VFD Output

04/10/2017 9:41 AM

When did the CSI's show up? Did they find any evidence?

CSI output is usually contained in a clear plastic bag with evidence tape on it.

__________________
If it ain't broke, don't fix it. Just build a better one.
Reply Score 1 for Good Answer
Guru
United States - Member - New Member Engineering Fields - Power Engineering - New Member

Join Date: Sep 2006
Location: California, USA, where the Godless live next door to God.
Posts: 4366
Good Answers: 722
#3

Re: VFD Output

04/10/2017 12:28 PM

VSI drives are now almost always PWM output, meaning the bus votlage is fixed and the motor voltage is inherently a pseudo-sine wave, so the output VTHD is very high. It's the MOTOR that draws current through that as IF it were a sine wave, so because of the inductive time constant in the motor windings, think of the motor as a low pass filter for the output current in terms of I-THD. But that has less of an effect on what you see in terms of V-THD.

Nobody that I am aware of is still using a CSI for low voltage drives, those went out of production 20+ years ago, so I'm not sure why you care. In general though a CSI drive changes the voltage on the INPUT side, so the output is only modulating the current. So yes, there is less V-THD on the output side then. Some MV drives still use a CSI topology, but the modern versions now use PWM on the output as well, so the same thing applies in terms of I-THD, it's the motor that is acting like the filter.

__________________
** All I every really wanted to be, was... A LUMBERJACK!.**
Reply
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 11562
Good Answers: 136
#4
In reply to #3

Re: VFD Output

04/10/2017 1:44 PM

If the votlage is not lined up with the boltage, then sumbody gonna let de smoke out!

__________________
If it ain't broke, don't fix it. Just build a better one.
Reply
Reply to Forum Thread 4 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

James Stewart (2); JRaef (1); Rixter (1)

Previous in Forum: ACS850 VFD Reference Meet Problem   Next in Forum: Difference Between Ith & Iu

Advertisement