What is auto-tuning to a VFD?
Auto tuning a VFD is a process by which a drive measures the impedance of a motor for the purpose adjusting the motor control algorithm. The measured value may be matched to known impedance for a given motor size and used in determining voltage and current relationships at different speeds. Ultimately, this allows for more effective driving of a motor load as well as better speed regulation specifically when running without feedback ( open loop).
When not to auto-tune?
- Auto-tunes are generally to be performed when the motor is cold. Auto-tuning with a hot motor may result in a variance in impedance which will subsequently cause the execution of a motor control algorithm which does not accurately match the true motor impedance.
- When multiple motors are connected, an auto-tune will result in the reading of multiple motor impedances connected in parallel. Some auto-tune functions match impedance readings to known typical motor impedance values ( for instance a typical NEMA B motor). As such, the reading of multiple motor impedances in parallel can not be matched to a known motor impedance value or may match a different type of motor. This results in an unsuccessful auto-tune which may be signified by a higher than usual noise levels.
The biggest mistake often made with a sensorless vector drive is to allow it to run on the default motor model instead of taking the time to tune the drive to the specific motor that is being used. Many VFDs now come with auto-tune features, but surprisingly few people take advantage of them. The result is performance that is barely better than a scalar drive at a higher cost.