Speed adjustment by External potentiometer only works through part of the range.

O

oughtsix

Plastic

Joined

Sep 1, 2009

Location

oregon USA

• Mar 28, 2011

• #1

I had a friend complain about the external speed control potentiometer only making speed adjustments through about a third of the range of the potentiometer.

After a little thinking I came up with this:
I just had a bit of inspiration re the speed adjustment issue with the potentiometer. There are two basic configurations for potentiometers: Audio taper and Linear taper. With linear taper pots the resistance is directly proportional to position of the pot. With an Audio taper pot the resistance is some function of a curve (very slow change at the beginning of the range and very fast change at the end of the range). From what you described it sounds exactly like you have an Audio taper potentiometer. I think if you swap your pot for a Linear taper pot you will get the speed control you are expecting.

FYI.

 

R

Robert Fournier

Aluminum

Joined

Feb 10, 2009

Location

Michigan, USA

• Mar 28, 2011

• #2

You never stated if you have a full range of speed control.

 

R

robvds

Cast Iron

Joined

Mar 21, 2011

Location

montreal qc

• Mar 28, 2011

• #3

you would have to clear the drive back to default to eliminate any special volt-amp-hz-range restrictions that might be programed into it. The pot must be capable of varying the 0-10volt dc or whatever this drive wants to control it.
Rob

 

The real Leigh

Diamond

Joined

Nov 23, 2005

Location

Maryland

• Mar 28, 2011

• #4

Make sure the pot is the correct resistance, usually 5,000 to 10,000 ohms.

If it is, then I would guess a taper problem. Or there may be limits set within the VFD preventing it from going full range.

- Leigh

 

M

MotorradMike

Aluminum

Joined

Jan 20, 2010

Location

Ontario Canada

• Mar 29, 2011

• #5

Taper isn't going to cause the VFD to ignore part of the rotation, it will simply cause the response to be non-linear.

I'd check the 'voltage bias' and 'voltage gain' settings.
I think what's happening is that you're applying 0-10V to an input which is set to respond to 0-5V.

 

The real Leigh

Diamond

Joined

Nov 23, 2005

Location

Maryland

• Mar 29, 2011

• #6

MotorradMike said:

Taper isn't going to cause the VFD to ignore part of the rotation ...

Click to expand...

It certainly can reduce the available control range to the point where the operator does not perceive a change.

There are a dozen different potentiometer tapers, a couple of which have 90% resistance at 50% rotation.

A couple of audio tapers have 0% change in the first or last 10% of rotation.

Any pot used as a VFD speed control should have a linear taper, no exceptions.

- Leigh

 

Last edited: Mar 30, 2011

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Miguels244

Diamond

Joined

Mar 27, 2011

Location

Denver, CO USA

• Mar 29, 2011

• #7

What is the brand and model?
There are a multitude of parameters, any one of which could exclude some speeds. The turn down may also be interfering with the pot input.

 

Jraef

Titanium

Joined

Aug 10, 2004

Location

San Francisco Bay Area

• Mar 30, 2011

• #8

Although I concur with The real Leigh on the taper issue, it's far more likely that it's just the resistance issue. You MUST read the manual to know what resistance the VFD input is looking for and match that. If for example it is looking for 10kohms and you use a 5kohm pot, at full resistance it is still 1/2 of the resistance the VFD is looking for.

 

The real Leigh

Diamond

Joined

Nov 23, 2005

Location

Maryland

• Mar 30, 2011

• #9

Jraef said:

... it's far more likely that it's just the resistance issue.

Click to expand...

As I suggested in post #4 above.

The VFDs I've encountered (by no means all that are available) will work with a range of pot values, typically from 1k (1,000) to 10k (10,000) ohms.

The pot is just a voltage divider with its wiper connected to a high-impedance input, so there are no loading issues.

- Leigh