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Drive autotune

By Clarence Klassen 
I occasionally run encounter Adjustable Speed Drives (ASDs) running with only the basic drive setup. That includes motor nameplate data and scaling for correct RPM at top line speed. This is not adequate for web handling. We need to have our drives tuned.Drive vendors try to make this easy for the commissioning personnel, but the procedure is different for each vendor.

Every drive has a number of regulators which must be tuned in sequence. AC ASD drives typically have a torque loop, a speed or velocity regulator and may have a tension or position regulator.

The drive vendor usually specifies the requirements for the torque loop. Motor nameplate data is required. In most cases, a torque loop tuning procedure makes accurate measurements of the motor characteristics affecting torque and uses these in tuning the torque regulator. This may be called “motor identification”, “motor tests”, “vector tuning”, “motor data optimization”, etc. This test applies power to the motor but generally does not rotate it. The torque loop is tuned for fast response or high bandwidth usually measured in radians per second (rad/sec). Many VSD’s provide 300 rad/sec or higher response for the torque loop.

The speed or velocity loop is tuned next. The desired response or bandwidth may be selected for the application. We want a fast response. This may be 100 rad/sec for servo applications, 5 rad/sec for most web handling equipment, and 1 rad/sec for large paper mill equipment. Autotune for the speed loop generally accelerates the motor to measure the load inertia. Before running this test, enter the maximum safe speed, accel rate and decel rate, and torque limits. Keep the area safe with caution tape and watchers since the normal stop buttons may not be functional while tuning.

The tension regulator is tuned last. Generally, there is no autotune for the tension regulator – it must be tuned manually. The line has to be threaded to tune the tension regulator. The response of the tension loop must be slower than the response of the speed regulator unless special control engineering calculations have been made.

Tuned drives will provide much better tension control for web handling.

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