The Volt-Hertz control is he most simple yet very effective way of controlling the speed of an induction machine. The reason for applying this circuit is the constant field in the motor.
In order to study the concept of the V/F control, as simple inverter component model including the V/F control is used. Here the VSI_V/F block models the voltage stiff inverter where the frequency and amplitude of the output voLtage can be controlled. The connection on the left side should be connected to the DC link and at the output connection on the right side an induction machine is connected. A simple mechanical load of J=100mKgm2 and friction of 2 [Nm/Rad/s] is used.
The ramping of the frequency and voltage is done using a signal block that provides the signal for the angular speed of the output voltages. The signal for controlling the amplitude voltages are made by multiplying the angular speed signal with gain K. If the start-up time is chosen long enough, the rotor speed will follow the angular speed signal.
A more detailed implementation with PWM and complete inverter modelled is given below. Here the V/F control takes as inputs the DC link voltage Vdc(left-top), the measured angular rotor speed ωr(bottom) and the proposed angular slip frequency ωs(left-bottom). The angular slip frequency is determined based on difference between the reference and the measured angular rotor speed. This difference is input to a PI controller, whose output is the angular slip frequency.
The output from the V/F control block are the duty-cycles for the modulator. The output from the modulator are the gating signals for the inverter. The DC link input current is measured in a parasitic inductance in the DC link. This series connection of inductance of 100μH and 10 mOhm models the losses in the bus-bar.
The simulation results are displayed where the measured currents and voltage to the induction machine are shown as space vectors. The current draws a circle with the radius equal to the amplitude and the voltage follows a hexagon, because of the 6 possible IGBT state combinations inside the inverter.
A conceptual model can be used to study the effects of the V/F control, where it can be determined how fast the drive will start and what current levels are reached. A more detailed model also reveals the harmonic content in the current
