Total_newbie
Member
Members of this forum helped me out a great deal a couple of years ago with a question I had regarding acceleration during torque control of drives.
http://www.plctalk.net/qanda/showthread.php?t=126282&highlight=acs800
I am getting back to working on my software control of this drive (ABB ACS800), and I am seeing some behavior which I cannot understand. If drive is set to internal speed control (parameter 98.01) and I specify target speed with drive accel at max. rate - I see speed increase exactly at rate which I expect (based on specified motor torque and inertia which I have as load). E.g. it takes ~13 seconds to go from 0 to 740RPM, with constant speed ramp. (See attached pic - upper graph. Y -axis is 0.1X. Motor torque shown in brown is also 0.1X scale - so actually going to (-)100%.
However - if I switch to external (speed encoder) feedback (changing nothing else) - this same ramp takes twice as long. Also, accel rate is no longer constant - but increases with increasing speed. (bottom graph). Speed as sent from encoder is correct as measured with tach. (And encoder module was swapped for another one - no difference...). Motor torque still shows up as -100%.
Could anyone give me some idea as to why switching between internal and external (encoder) speed feedback should lead to such significant difference in accel rate? I cannot leave drive set to internal speed control, as this gives some error in speed at higher speeds and does not properly pick up zero speed.
(Note: I put this question up on another forum (Mr. PLC) - but didn't get any real ideas as to what might be the cause for this. Based on my experience in this forum - there are some experts who really know their drives...)
Thanks!
http://www.plctalk.net/qanda/showthread.php?t=126282&highlight=acs800
I am getting back to working on my software control of this drive (ABB ACS800), and I am seeing some behavior which I cannot understand. If drive is set to internal speed control (parameter 98.01) and I specify target speed with drive accel at max. rate - I see speed increase exactly at rate which I expect (based on specified motor torque and inertia which I have as load). E.g. it takes ~13 seconds to go from 0 to 740RPM, with constant speed ramp. (See attached pic - upper graph. Y -axis is 0.1X. Motor torque shown in brown is also 0.1X scale - so actually going to (-)100%.
However - if I switch to external (speed encoder) feedback (changing nothing else) - this same ramp takes twice as long. Also, accel rate is no longer constant - but increases with increasing speed. (bottom graph). Speed as sent from encoder is correct as measured with tach. (And encoder module was swapped for another one - no difference...). Motor torque still shows up as -100%.
Could anyone give me some idea as to why switching between internal and external (encoder) speed feedback should lead to such significant difference in accel rate? I cannot leave drive set to internal speed control, as this gives some error in speed at higher speeds and does not properly pick up zero speed.
(Note: I put this question up on another forum (Mr. PLC) - but didn't get any real ideas as to what might be the cause for this. Based on my experience in this forum - there are some experts who really know their drives...)
Thanks!