Siemens SINAMICS G130 Engineering Manual page 28

Fundamental Principles and System Description
Engineering Information
Diagram of the basic structure of the V/f control method
The voltage is adjusted as a function of frequency according to the V/f characteristic with the aim of maintaining the
motor flux as constantly as possible at the rated flux value, irrespective of speed or frequency.
At low frequencies, the ohmic stator resistance of the motor in relation to inductance is not negligible, which means in
this case that the voltage of the V/f characteristic must be boosted relative to the linear curve in order to compensate
for the voltage drop across the stator resistance.
At high frequencies, the maximum possible output voltage V
knee point in the V/f characteristic. The knee point generally corresponds to the rated operating point of the
connected motor. If the frequency is further increased beyond the knee point, the ratio between voltage and
frequency decreases due to the constant voltage. The motor flux is then also reduced, causing the motor to operate
in the field-weakening range.
V/f control modes are available in the standard firmware of the SINAMICS converters described in this engineering manual
(SINAMICS G130, G150, S120, S150) in the vector drive object. The following V/f control modes can be selected:
·
V/f control with linear characteristic
·
V/f control with parabolic characteristic
·
V/f control with freely parameterizable characteristic
·
V/f control for high-precision frequency-controlled drives in the textiles sector
·
V/f control with independent voltage setpoint
In order to optimize the performance of drives operating in V/f control mode, the following functions have been
provided in the SINAMICS firmware:
·
Slip compensation: For the purpose of increasing accuracy of speed, the frequency is adapted as a function
of load current in order to compensate the slip of the connected asynchronous motor.
·
Flux current control (FCC): For the purpose of increasing accuracy of speed, voltage and flux are adapted
as a function of load.
·
Resonance damping: The resonance damping function dampens electromechanical oscillations in the
frequency range up to a few tens of hertz.
·
Current limiting control: The current limiting control prevents the connected asynchronous motor from
stalling and thus functions as stall protection.
The advantages of the V/f control method lie in its simplicity and its ability to withstand parameter fluctuations, such
as changes in resistance caused by temperature rise or changeover of the motor operating on the converter.
Furthermore, this control method fully supports converter-fed operation of multi-motor drives. Its disadvantages lie in
its lack of precision and dynamic response, particularly at low speeds and in the field weakening range.
SINAMICS Engineering Manual – November 2015
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Ó Siemens AG
of the converter is reached, resulting in a horizontal
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