motor speed control help 1

[jcraft]

Just wanted to get some different opinions here as to the advantages and disadvatages to different methods. This goes back to a previous post I have about a vfd motor problem and the speed fluctuating. We have some plastic extrusion machines with newer vfd's. Both of them have pots that set the speed. My question is how accurate is analog vs. digital for this kind of motor speed control. Some questions are as follows
1) Advantage/diadvantage of analog pots for speed control vs. setting up a small hmi connected to the drive and using digital.

2) if analog is used does it provide less stability at varying loads or will the speed remain fairly constant in in either method. I have heard even with an analog signal the motor rpm's shouldn't vary more than +/- half an rpm. Not sure if this is true.

Thanks for the help as I am new to motor speed control and trying to get info on the different methods

 

[dV8r]

It's been my experience that an extruder must have an external speed reference to have the best speed control. This is because a lot of the variables that influence its speed are not necessarily electrical in nature but rather physical. Such as, friction, temperature, viscosity, etc.
Of course this is determined by how "accurate" you need to be.

 

[jde777]

If I remember correctly tach feedback gives speed control to .1% and encoder gives .01%.

Pots are generally used in this application because the process requires frequent adjustments by the operator. Usually a 10 turn pot. An HMI would not be ideal for this application in my opinion. Takes more time to enter speed in than just rotating a pot.

Both methods should provide speed control to the percentages above dependent of feedback device NOT reference.

 

[itsmoked]

I'd think this is a knob job too. Buttons end up having scrolling/translation issues.

Of course an analog signal is going to be more susceptible to noise or drift than a digital HMI but depending on the environment this may be entirely a non-issue.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

While I too think it's a "knob job" as Keith put it so eloquently, in direct answer to your question, digital speed references provide for mush greater accuracy than analog. The general rule is analog provides +- 1% accuracy, digital = +-.01%

 

[itsmoked]

<grin>

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[DickDV]

In a drive/motor system, you have three main sources of speed error. First is motor slip. Second is drive drift. Third is speed reference error.

Motor slip is all speed error and the level of drive technology determines how well the system eliminates this error. Generally, an open loop drive can eliminate 50% of slip error, mainstream sensorless vector drives can eliminate about 75%, ABB's DTC system can eliminate about 90%, and flux vector systems with encoders get it down to 1 pulse +/- one pulse non-cumulative. Analog tach feedback is a poor choice as these devices themselves are only about 1% accurate.

Drive drift used to be a big deal with changing temperature and varying input voltages but today is insignificant---parts per million!

Speed reference error takes two forms---noise on the reference signal which includes vibration on a speed pot making the signal jump all over the place and an often forgotten error, analog to digital conversion resolution at the drive analog input. Drives with 10 bit resolution divide the 10V reference into .01V steps which can be big error especially at slow speeds. Drives with 14 bit resolution divide the signal into .000625 steps which is clearly much better.

If you want to eliminate speed reference error and you only operate at a limited number of speeds, choose the fixed speed inputs and select the various speeds there. You will have no reference error at all if you do that. Some drives have as many as 15 fixed speeds available.