82 Vector Control Of Ac Motors

construction of the induction machines are an advantage over the DC and PM machines.

The key variable for control in speed- and position-controlled applications is the torque. Although torque is never measured directly, torque estimators from machine models are frequently used to generate the current commands. A current controller in the innermost control loop regulates the motor current by comparing the command currents with feedback current measurements coming from the sensors. Speed control, if necessary, is achieved in the outer loop by comparing command speed signal with the feedback speed signal. With the two loops arranged in a cascade, the speed controller output of the outer loop is the current command for the inner loop. In certain high-performance position-controlled applications, such as in the actuator drives for accessories in EVs and conventional vehicles, the position is controlled in the outermost loop, putting the speed controller in an intermediate loop. The ability to produce a step change in torque with a step change in command generated from the outer loops represents the degree of control over the motor drive system for high-performance applications. Vector control in induction motors enables the machine to produce step changes in torque with instantaneous transition from one steady state to another steady state, which dramatically improves the dynamic performance of the drive system.

The objective of vector control or field orientation is to make the induction motor emulate the separately excited DC motor or the PM brushless DC motor. To understand vector control, let us revisit the torque production mechanism in DC machines. A simplified diagram of a DC motor with the field produced by separate excitation is shown in Figure 8.14. The field flux linkage space vector ~k> is stationary and is along the ¿/-axis of the motor. The armature current space vector > is always along the g-axis due to action of the commutator and brushes, even though the rotor is revolving. The orthogonality between the field and armature current ensures the optimal condition for torque production, providing the highest torque-per-ampere ratio. The electromagnetic torque of the DC machine is given by

where kT is a machine constant depending on construction and size of the machine. Vector notations are dropped, because these variables are constants in DC machines. The armature and field circuits in a separately excited DC machine are completely independent or decoupled, allowing independent control over torque and magnetic field. Independent flux control is especially desirable for EV-type applications, where flux weakening is used at higher speeds above rated torque conditions in the constant power region of torque-speed characteristics. The constant power range helps minimize the transmi s si on gear requirements for propul si on drive s.

In the case ofPM brushless DC motors (i.e., PM trapezoidal machines), the rotor position sensor and power electronic converter replace the mechanical commutators and brushes of DC motors and work in synchroni sm to maintain the orthogonality between stator current space vector and the rotor flux vector on an average basis. The back-emfs in these machines are trapezoidal and not il 'n

□ Field mmf,}./

FIGURE 8.14 Torque in a separately excited DC machine.

sinusoidal; hence, vector control is not possible. Torque ripple is a dominating problem in PM brushless DC motors, because square-wave currents are used for torque production in order to synchronize with the trapezoidal back-emfs. In contrast, the orthogonality between armature and field mmfs is continuously maintained in DC commutator machines that help deliver smoother torque. However, in the case of PM sinusoidal machines, the back-emfs are sinusoidal, and using vector control, smooth torque control can be achieved like in an induction machine.

In light of the discussion presented above, the primary requirements of instantaneous torque control are controllability of the armature current, a controlled or constant field flux, and an orthogonal spatial angle between stator mmf axis and rotor mmf axis. In the case of DC and PM brushless DC motors, the last two requirements are automatically met with the help of commutators and brushes, and position sensors and inverter switching, respectively. However, bear in mind that orthogonality is maintained on an average basis only for PM brushless DC motors, the effect of which shows up in performance. In the case of induction machines and PM sinusoidal machines, these requirements are met with the help of dq-models and reference frame transformations. Instantaneous torque control is achieved when the three requirements are met at every instant of time. Note that the armature of a machine is the component that carries the bulk of the current delivered by the source. In the case of DC machines, the armature is in the rotor, while in the case of AC machines, the armature is in the stator. The control of armature currents is achieved with the help of current regulators, such as the hysteresis current regulator or a PI current regulator. Armature current control is necessary to overcome the effects of armature winding resistance, leakage inductance, and induced voltage in the stator windings. For field-weakening operation, the rotor flux needs to be reduced, which is achieved through field current control. The task is simple in separately excited DC machines. In induction and PM sinusoidal machines, dq modeling that decouples the torque and flux, producing components of currents and subsequent control of these components, help achieve the obj ective.

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