Sensorless vector control [t-head1]

The switching patterns for the inverter are calculated with the PWM (pulse width modulation) switching patterns. In voltage vector control mode, the amplitude and frequency of the voltage vector are controlled in relation to slippage and load current. This allows large speed ranges and highly accurate speeds to be achieved without speed feedback. This control method (U/f control) is the preferred method on a frequency inverter with the parallel operation of several motors.

In flow-regulated vector control, the active and reactive current components are calculated from the measured motor currents, compared with the values from the motor model and, if necessary, corrected. The amplitude, frequency and inclination of the voltage vector are controlled directly. This allows operation at the current limit and the achievement of large speed ranges and highly accurate speeds. Especially noteworthy is the drive’s dynamic output at low speeds, for example in lifting and winding applications.

The key advantage of sensorless vector technology is that the motor current can be regulated to match the motor’s rated current. This allows dynamic torque regulation to be implemented for three-phase asynchronous motors.

The following illustration shows a simplified equivalent circuit diagram for the asynchronous motor and associated current vectors:

In sensorless vector control, the flux-generating current i_µ and the torque-generating current i_w are calculated from the measured stator voltage u₁ and stator current i₁. The calculation is performed with a dynamic motor model (electrical equivalent circuit of the three-phase motor) with adaptive current regulators, taking into account the saturation of the main field and the iron loss. The two current components are set according to their value and phase in a rotating coordinate system (ω) to the stator reference system (α, β).

The physical motor data required for the model is formed from the entered and measured (self-tuning) parameters.