SimPowerSystems

A Field-Oriented Variable-Speed Induction Motor Drive

This case study illustrates a variable-speed induction motor drive using field-oriented control. In this control scheme, a dq coordinates reference frame locked to the rotor flux space vector is used to achieve decoupling between the motor flux and torque. They can thus be controlled separately by stator direct-axis current and quadrature-axis current respectively, as in a DC motor. This figure shows a block diagram of a field-oriented induction motor drive:

Figure 2-21: Field-Oriented Variable-Frequency Induction Motor Drive

The induction motor is fed by a current-controlled PWM inverter, which operates as a three-phase sinusoidal current source. The motor speed is compared to the reference * and the error is processed by the speed controller to produce a torque command Te*.

As shown below, the rotor flux and torque can be separately controlled by the stator direct-axis current i_ds and quadrature-axis current i_qs, respectively.

Figure 2-22: Field-Oriented Control Principle

The stator quadrature-axis current reference i_qs* is calculated from torque reference T_e* as

where L_r is the rotor inductance, L_m is the mutual inductance, and |_r|_est is the estimated rotor flux linkage given by

where _r = L_r / R_r is the rotor time constant.

The stator direct-axis current reference i_ds* is obtained from rotor flux reference input |_r|*:

The rotor flux position _e required for coordinates transformation is generated from the rotor speed _m and slip frequency _sl:

The slip frequency is calculated from the stator reference current i_qs* and the motor parameters.

The i_qs* and i_ds* current references are converted into phase current references i_a*, i_b*, i_c* for the current regulators. The regulators process the measured and reference currents to produce the inverter gating signals.

The role of the speed controller is to keep the motor speed equal to the speed reference input in steady state and to provide a good dynamic during transients. It can be of proportional-integral type.

Variable-Frequency Induction Motor Drive

Modeling the Induction Motor Drive