Locking out frequencies on a Powerflex 753

zacslade

Member
Join Date
Oct 2014
Location
Phoenix, AZ
Posts
84
Hi all,

I have been asked to prevent a Powerflex 753 from running in a specific frequency range (between 31-50Hz). I've never heard of that kind of setting on a VFD before -- especially since it seems to me that the VFD has to be in that range for a period of time if it is accelerating to 60Hz. But I'm thinking maybe I can get away with preventing the VFD from remaining in that range for any period of time.

Have any of you ever done something similar? How did you do it?

Thanks,
Zac
 
I have never seen this setting in a VFD.

This can be programmed in a PLC easily, if there is one controlling the VFD output.

If not, the only way I can think of is to add an interposing PLC.

Maybe someone else here knows of a brand that does this.
 
Good point. Also, additional information: There isn't a PLC in this application. The VFD speed is controlled by a pot.

But that gives me an idea: I could maybe change the pot so that it can't go above 30Hz, and then add a "full speed" button to get up to 60Hz.
 
I have seen that setting in 1336's I would imagine it exists in the 753. I've never seen it used for so wide of a range though.

It's used to prevent running at an rpm known to produce vibration. Obviously it has to pass through that range on the way to full speed, but why do they want skip that much of the usable range?
 
I checked what Steve said above in a PowerFlex700 manual:

Some machinery may have a resonant operating frequency that must be avoided
to minimize the risk of equipment damage. To assure that the motor cannot
continuously operate at one or more of the points, skip frequencies are used.
Parameters 084-086, ([Skip Frequency 1-3]) are available to set the frequencies to
be avoided.
The value programmed into the skip frequency parameters sets the center point
for an entire “skip band” of frequencies. The width of the band (range of
frequency around the center point) is determined by parameter 87, [Skip Freq
Band]. The range is split, half above and half below the skip frequency parameter.
If the commanded frequency of the drive is greater than or equal to the skip
(center) frequency and less than or equal to the high value of the band (skip plus
1/2 band), the drive will set the output frequency to the high value of the band.
See (A) in Figure 31.
If the commanded frequency is less than the skip (center) frequency and greater
than or equal to the low value of the band (skip minus 1/2 band), the drive will
set the output frequency to the low value of the band. See (C) in Figure 31.
 
Aabeck has pretty well covered how it works, but just one other consideration -

If your drive speed is controlled by a pot, and you're using this approach to skip such a wide range of frequency (30-50Hz), you'll introduce a large "dead spot" in the pot. Let's say your pot increases the frequency by 10Hz for every full turn. For the first three turns, it'll increase the speed uniformly. Then for the next full turn it will do nothing. Then you turn it just a little more, and it'll shoot straight up to 50Hz. This may not be desirable for your application. Same thing when turning down - if your operator is trying to turn it down quickly, they may panic or think something is wrong when they turn the pot a whole two turns from max, and the drive still doesn't drop below 50Hz.

As others have mentioned, if you can give a little more detail about why you're trying to do this, some other creative solutions may be forthcoming!
 
It's funny you mention this.

I saw this parameter in an ABB ACS310 the other day that I was programming.

I never knew a parameter like this even existed!
 
Interesting timing, we had a customer ask us last week if we have any "critical speeds locked out?" I said no since I had never heard of it. To CMB I asked the senior engineer and that's how he described it, speeds that are known to cause vibration can be skipped.
 

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