PF 4M Par# 446 PWM Frequency

MJC

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M
Join Date
Mar 2011
Location
ILL
Posts
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Hello again, I have a question about PWM frequency setting of a Powerflex 4m drive (22F-B012N103) Par#A446. I recently installed a VFD for a customer that wanted to be able to control the speed of a exhaust fan. Everything runs smooth except for the noise from the motor (high pitched squeal/whine) that in my understating is caused by the PWM frequency. The work area is relatively quiet and customer complains of the noise that is emitted from the motor. I was able to minimize the noise by adjusting the PWM Par 446 to a value of 7kHz default is 4kHz, however I have minimal understating of the PWM frequency and its results on the motor, there for I'm turning my questions for the gurus of this great site.


Can anyone please explain the Par 446 and its adjustment results on the motor.

Also would anyone know of a way to minimize the noise emitted from the motor due to what I think could be the cause of the PWM frequency.
Thanks in advance.
 
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Can anyone please explain the Par 446 and its adjustment results on the motor.
Click to expand...

This sets your PWM Carrier Frequency. Note that if you raise the PWM Carrier Frequency, there is De-rating required. Here's a little light reading on the subject. Do a Google Search for even more.

http://www.reliance.com/pdf/drives/data_sheets/D7171D.pdf

Also would anyone know of a way to minimize the noise emitted from the motor due to what I think could be the cause of the PWM frequency.
Thanks in advance.
Click to expand...

See the AB/RA Tech Note 472890. Access is everyone. There are a few more Tech Notes that would probably apply to your situation.

How far is it from the VFD to the Fan Motor?

Stu....
 
Thanks for the info will read up on this.
How far is it from the VFD to the Fan Motor?
Click to expand...
About 25 ft.
I not sure if this will also affect the noise but the wires for the supply and load are ran in the same conduit and they are of THHN type.
 
MJC said:
About the same distance of 25 ft
Click to expand...

I believe he PWM frequency can also be set as low as 2kHz which may produce a lower pitch (less annoying) sound and should not reduce drive efficiency as much as the higher frequency. Adding an output reactor may also help with the noise issue but not as cheaply as changing a parameter.

Having the VFD output and input in the same pipe means that more induced voltage "noise" is going to get onto the distribution system. This may or may not be of any consequence, but is something to keep in mind if other problems develop.

Changing the output wiring to a shielded cable would reduce the risk of this trashy signal interfering with other stuff in the plant. But, chances are, it wouldn't fit in the same conduit and might be more trouble than it is worth.

Most all of our drive to motor connections are THWN. You can expect the wiring to fail prematurely. That still might be years, so it may be difficult to sell the bean counters on the high dollar wire which is called for.

If I had a nickel for every time I had to troubleshoot a drive and found perforated insulation...

In our plant there are at least 100 drives wired this way, so even if I can convince them to let me start using good shielded cable, I will retire before we could get all the runs replaced.
 
I believe he PWM frequency can also be set as low as 2kHz which may produce a lower pitch (less annoying) sound and should not reduce drive efficiency as much as the higher frequency. Adding an output reactor may also help with the noise issue but not as cheaply as changing a parameter.
Click to expand...
Thanks Okie, however setting the PWM frequency to 2kHz makes even more noise at lower frequency, you are correct the pitch isn’t as high but it is definitely more annoying and louder then a setting of 7kHz, I played with the frequency settings 2-10 kHz and found that a setting of 7kHz works out best for low and high speed of 30 to 60Hz. It is definitely higher but emits much less noise.
 
Okie, with the distance of 25 feet I will definitely look into changing the load side cable to a shielded type. I do have a load reactor here someplace that I could put on just to see if it will reduce the noise emitted by the motor. But I will wait for anyone else to chime in on the issue.
 
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