Now we connect a cable, enter a few nameplate parameters, and start the autotune. The motor grunts a bit, then accelerates and stops. A message indicates “Auto Tune was successful”. Does anyone know what just happened?
First of all, the auto tune for each drive was designed by expert control engineers to do some very specific measurements and calculations to tune drives to the motors and loads to which they are connected. Auto tune does a good job in most circumstances.
Some of the jobs of auto tune are to measure the magnetizing current required to provide the correct flux at various RPM’s. It must measure stator time constants required in modeling the torque for the motor. It must measure the inertia of the roller connected to the motor in order to provide adequate torque for changing speed without producing an overshoot.
In most cases, the auto tune makes a measurement, then performs a calculation to set time constants and regulator gains. Usually the calculations are not checked. This usually works well. When it doesn’t, the drive trips and presents an alarm. Auto tune may fail if the drive is connected to a very old motor. It may fail if there is excessive coupling or gearbox backlash. It may fail if there is a torsional resonance between the motor and its load. Sensorless operation fails more often than when an encoder is used.
Another way the auto tune may fail to meet the needs of web handling is if the response or bandwidth of the speed regulator does not match the target response or it has a large overshoot. I like to check the result of auto tune by charting the response to a step in speed reference. This takes less than a minute if trending and drive master software is setup up.
The chart below shows a speed regulator response with a target of 5 rad/sec. Only 1.6 rad/sec was realized.
Sunday, November 29, 2015 8:40 AMSir i am using a DCS 800 drive for 140 kw dc motor this drive was not tuning the armature tune fail Field auto tune ok . motor was running at 500rpm directly and drive was tuning with other motor. ple suggest what is the problem in motor or drive