Last week I discussed a very common measurement of drive regulator performance. That is the step response or Bandwidth of the regulator. This applies to the torque regulator, speed regulator as well as the tension regulator. We are looking for a nice exponential response without overshoot and with a known time constant (seconds) or Bandwidth (radians/second).
In preparing this blog entry, I reviewed speed regulator tuning instructions from 6 major VFD vendors representing Europe, North America and Asia. It is clear from the documentation that the drive engineers are all aware of tuning requirements and standard measurements including step response. It is evident that they do not wish to publish more information than absolutely necessary. The published instructions do not provide guidance in selecting the desired regulator bandwidth or provide assurance that default tuning performance has been achieved. We expect that drive techs working in web handling have received additional training.
Brand A1 measures the motor inertia in seconds by measuring the acceleration torque. The drive tech enters the desired (specified by engineering) Bandwidth (BW) in radians/second and the speed at which to perform tuning. Procedures are listed to set gains and time constants. If the drive runs smoothly, we are done. The response is not verified automatically.
Brand A2 measures the motor inertia in seconds by measuring the acceleration torque. Detailed instructions are not provided, but typical speed regulator responses to a step are shown. The manual indicates trying for the fastest regulator (lowest time constant) possible with only a small amount of overshoot. The manual refers to the time constants (seconds). The response is not verified automatically.
Brand P provides good detail on most of their auto tune function. The detail is weak when it comes to the speed regulator. A short description including units is provided for all parameters and measured values. The drive tech enters the speed at which to perform tuning. The manual refers to the time constant (seconds). The response is not verified automatically.
Brand S describes a simple “pass/fail” auto tune. A manual tune is also described with figures showing step responses. The manual indicates trying for the fastest regulator (lowest time constant) possible. The manual refers to the time constants (seconds). The response is not verified automatically but in manual tuning, the trend is available.
Brand T1 performs an auto tune but spares us all the details. Proportional and integral gains are calculated, but not even units are scaled into %.
Brand T2 measures the motor inertia in seconds by measuring the acceleration torque. The drive tech enters the desired (specified by engineering) Bandwidth (BW) in radians/second and the % torque at which to perform tuning. Gains and time constants are calculated. The response is not verified automatically. The manual recommends trending the actual step response.
Brand T3 performs an auto tune but spares us all the details. Proportional and integral gains are calculated, but not even units are not provided.