Allen Bradley 1336

Try using a burrito instead....

--Ed. C.

"Try using a burrito instead...."

LOL, took me a LONG time to figure out that joke.

_{For others who didn't, look at the OP and look for a misspelled device name.}

Is the controller set to Proportional-only, or has it got some Integral action as well?

For SVC mode, it is absolutely critical that you tune the drive to the motor. Did you do that?

And are you sure that in programming, you have selected SVC mode? Just because the drive is capable of it does not mean it is enabled. From the description, it seems to me to be acting as if it is in Scalar control mode (V/Hz). SVC should be plenty capable of holding an accurate speed at that level. You really only need the encoder feedback if you want the speed or torque to be accurate at 1Hz and below.

And lastly, if it is in SVC mode, are you sure it is in speed control mode or torque control mode? If in torque mode, speed becomes secondary.

PS: the 1336 is now so old it is totally obsolete and AB has ceased supporting it. Might be time to think about an upgrade.

JR is right on! To answer your question completely does depend on the answers to his questions. However, in general the error to which you refer has to do with the slip in the motor required to produce the torque necessary to move your conveyor. The actual speed is going to be dependent on the load. Even if you go with a newer VFD, this will still be the case. Yes an encoder will allow more accurate speed control is you set up the 1336 correctly. However, if you don't tune the system properly then you will likely end up with a variation in speed that constantly changes (oscillation in the control circuit). So, adding feedback may not be your best solution since tuning can be a major pain... If you must have better control, you might consider replacing your induction motor with a synchronous motor. You should also spend sone time with the 1336 manual which you can download at Rockwell.automation.com. Not the installation manual but the larger file.