Control Loops; Current Loop; Speed Loop; Flux Loop - Siemens 6SR41 series Product User Manual

Application Specific Features

7.2 Control Loops

7.2 Control Loops
The NXG Control includes three main control loops that are defined in the following sections.
7.2.1

Current Loop

The current loops form the innermost loop of the NXG control system. It is essential that
these loops are stable. When the current loop gains are very low, then the drive output
currents do not have a sinusoidal waveshape, i.e., dead-bands can be seen around the zero-
crossings, and the peaks are not smooth but appear flat. On the other hand, when current
loop gains are too high, then a high frequency ringing appears on the sinusoidal current
waveform. IOC trips can also occur if this is the case.
Default values of the current loop gains are sufficient for most applications. Tuning may be
required for high performance applications and when output filters are used.
7.2.2

Speed Loop

Control of motor speed is accomplished with the speed regulator. The output of the speed
loop forms the torque current command. The default speed loop gains work well when the
inertia of the motor and the load are reasonable, i.e., motor and load have similar inertia.
Examples of applications where this is not the case are given below. Speed loop gains
require tuning when its output shows significant oscillations during small changes in speed
command.
ESP applications have motors with very low inertia. In such applications, the speed loop
gains can be safely reduced by a factor of 5 or more from their default settings.
Fan applications have motors with very high inertia. In such applications, the speed loop
gains are typically reduced (by a factor of 2 to 5 from their defaults). These applications, in
general, do not require fast/sudden response, and a reduction in speed loop gains prevents
large/sudden changes in the torque current command.
7.2.3

Flux Loop

Regulation of motor flux is accomplished with the flux control loop. The output of the flux loop
forms the magnetizing current command. The default flux loop gains work well for most
induction motor applications. With synchronous motors, lower gains should be used. Flux
loop gains will require tuning when the regulator output shows significant oscillations during
steady state operation.
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Operating Instructions, Version AE 12/2009, A5E01454341C
Product User Manual