Setting functions
8.5 Motor control
Converter with control units CU250D-2
Operating Instructions, 04/2014, FW V4.7, A5E34261542B AA
137
Characteristics of U/f control
The inverter has several U/f characteristics. Based on the characteristic, as the inverter
increases in frequency the motor voltage rises.
The voltage boost of the characteristic improves motor behavior at low speeds. The voltage boost
is effective when frequencies < rated frequency
Figure 8-14 U/f characteristics of the inverter
The inverter increases its output voltage – also above the motor rated speed up to the
maximum output voltage. The higher the line voltage, the greater the maximum inverter
output voltage.
If the inverter has reached its maximum output voltage, then it can only increase its output
frequency. From this point onwards, the motor is operated in field weakening; this means
that the available torque linearly decreases with increasing speed.
The value of the output voltage at the rated motor frequency also depends on the following
variables:
● Ratio between the sizes of the inverter and motor
● Line voltage
● Line impedance
● Current motor torque
The maximum possible output voltage as a function of the input voltage is provided in the
technical data, also see Section Technical data (Page 325).
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