Functions
8.24 Open-loop and closed-loop control
Control Units CU240S
8-160 Operating Instructions, 11/2006, A5E00766042B AA
8.24 Open-loop and closed-loop control
Overview
There are several open-loop and closed-loop techniques for closed-loop speed and torque
control for inverters with induction and synchronous motors. These techniques can be
roughly classified as follows:
● V/f characteristic control (known as: V/f control)
● Field-orientated closed-loop control technique (known as: Vector control)
The field-orientated control technique – Vector control – can be further sub-divided into two
groups:
● Vector control without speed feedback (sensorless Vector control (SLVC))
● Vector control with speed feedback (Vector control (VC))
These techniques differ from one another both regarding the control ability and in the
complexity of the technique, which in turn are obtained as a result of the requirements
associated with a particular application. For basic applications (e.g. pumps and fans), to a
large extent, V/f control is used. Vector control is mainly used for sophisticated applications
(e.g. winders), where a good control and behavior in noisy conditions are required regarding
the speed and torque. If these requirements are also present in the range from 0 Hz to
approx. 1 Hz, then the speed/torque accuracy without encoder is not sufficient. In this case,
Vector control with speed feedback must be used.
8.24.1 V/f control
Data
P1300 Parameter range:
P1310 … P1350
Warnings: -
Faults: -
Function chart number: FP6100
Description
The V/f characteristic represents the simplest control technique. In this case the stator
voltage of the induction motor or synchronous motor is controlled proportionally to the stator
frequency. This technique has proven itself for a wide range of "basic" applications, such as
● Pumps, fans
● Belt motors
and similar processes.
The goal of V/f control is to keep the flux Φ constant in the motor. In this case, this is
proportional to the magnetizing current I
μ
and the ratio between voltage V and frequency f.
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