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speed control problem of VFD

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Hi Guys

we have Allen Bradely Bulletin 1333 variable frequency dirve installed at fertilizer plant.

we r facing the problem of speed control of VFD. The display on the VFD shows the frequency. The frequency is varied in steps of 0.5 Hz (e.g. from 13 to 13.5 Hz and so on), and thats why spped varies in steps of 7 to 8 rpm. we want to change the speed in steps of 2 to 3 rpm.

Plz help me in this regard and tell me whether this is possible with any modification in the circuit (e.g. by inserting any resistor in series etc).

Quick response will be appreciated.

U can email me ur replies on:


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Hello faysalnaeem


Welcome to the forum.


I am not an expert with the Allen Bradley 1333 drive, (perhaps jraef can help) but I would be very surprised if it operates in 0.5Hz steps internally. There may be something in the control that causes this. How are you controlling the frequency input to the drive?


Best regards,

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The 1333 is adjustable in the same way as any VFD is, it can be (relativley) infinite. What you may have is a Multispeed setup where you click through a number of pre-set speeds, and someone has programmed those steps to be in increments of 5Hz as you say, however the 1333 was only capable of 4 preset speeds. If you are only using 4 speeds then it is merely a matter of reprograming the drive for the speeds you need.


The other possibility is that someone provided a series of resistors in specific steps feeding a voltage into the analog speed input. If that is the case, all you would need to do its replace those resistors with a potentiometer.

"He's not dead, he's just pinin' for the fjords!"
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