Closed-Loop Control; V/F Control; Typical Applications For V/F Control - Siemens Sinamics G120 CU240E Operating Instructions Manual

Functions

5.7 Closed-loop control

5.7 Closed-loop control
Overview
There are two different open-loop and closed-loop control techniques for frequency inverters
with synchronous and induction motors.
● Closed-loop control with V/f-characteristic (called V/f control)
● Field-oriented control technology (called vector control)
5.7.1

V/f control

5.7.1.1

Typical applications for V/f control

V/f control is perfectly suitable for almost any application in which the speed of induction
motors is to be changed. Examples of typical applications for V/f control include:
● Pumps
● Fans
● Compressors
● Horizontal conveyors
Basic properties of V/f control
V/f control sets the voltage at the motor terminals on the basis of the specified speed
setpoint. The relationship between the speed setpoint and stator voltage is calculated using
characteristic curves. The frequency inverter provides the two most important characteristics
(linear and square-law). User-defined characteristic curves are also supported.
V/f control is not a high-precision method of controling the speed of the motor. The speed
setpoint and the speed of the motor shaft are always slightly different. The deviation
depends on the motor load. If the connected motor is loaded with the rated torque, the motor
speed about the rated motor slip is below the speed setpoint. If the load is driving the motor
(i.e. the motor is operating as a generator), the motor speed is above the speed setpoint.
108
Frequency inverter
Operating Instructions, 08.10. 2008, t.b.d.