V/F Control; V/F Control With Linear And Square-Law Characteristic - Siemens SINAMICS G120 Operating Instructions Manual

7.6.1

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 required output frequency is calculated on the basis of the speed
setpoint and the number of pole pairs of the motor (f = n * number of pole pairs / 60, in
particular: f
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 is below the speed setpoint by the amount of the rated slip. If the load is driving the
motor (i.e. the motor is operating as a generator), the motor speed is above the speed
setpoint.
The characteristic is selected during commissioning, using p1300.
7.6.1.1

V/f control with linear and square-law characteristic

V/f control with linear characteristic (p1300 = 0):
Mainly used in applications in which the motor torque
must be independent of the motor speed. Examples of
such applications include horizontal conveyors or
compressors.
V/f control with parabolic characteristic (p1300 = 2)
Used in applications in which the motor torque
increases with the motor speed Examples of such
applications include pumps and fans.
V/f control with square-law characteristic reduces the
losses in the motor and inverter due to lower currents
than when a linear characteristic is used.
Note
V/f control with a square-law characteristic must not be used in applications in which a high
torque is required at low speeds.
Inverter with CU240B-2 and CU240E-2 Control Units
Operating Instructions, 01/2011, FW 4.4, A5E02299792B AB
= p1082 * number of pole pairs / 60). The inverter provides the two most
max
Functions
7.6 Motor control
169