INDUCTION MOTOR CONTROL BY SELECTION OF INVERTER STATES

8.1
As shown in topic 7, induction motors in field-orientation ASDs are current controlled, that is, the control system produces reference values of currents in individual phases of the stator. Various current control techniques can be employed in the inverter supplying the motor, all of them based on the feedback from current sensors. Operation of the current
control scheme results in an appropriate sequence of inverter states, so that the actual currents follow the reference waveforms.
Two ingenious alternative approaches to control of induction motors in high-performance ASDs make use of specific properties of these motors for direct selection of consecutive states of the inverter. These two methods of direct torque and flux control, known as the Direct Torque Control (DTC) and Direct Self-Control (DSC), are presented in the subsequent sections.
As already mentioned in topic 6, the torque developed in an induction motor can be expressed in many ways. One such expression is
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where sr denotes the angle between space vectors, Xs and Xp of stator and rotor flux, subsequently called a torque angle. Thus, the torque can be controlled by adjusting this angle. On the other hand, the magnitude, \s, of stator flux, a measure of intensity of magnetic field in the motor, is directly dependent on the stator voltage according to Eq. (6.15). To explain how the same voltage can also be employed to control ©sp a simple qualitative analysis of the equivalent circuit of induction motor, shown in Figure 6.3, can be used.


From the equivalent circuit,

we see that the derivative of stator flux reacts instantly to changes in the stator voltage, the respective two space vectors, vs and p\s, being separated in the circuit by the stator resistance, Rs, only. However, the vector of derivative of the rotor flux, pXp is separated from that of stator flux, pXs, by the stator and rotor leakage inductances, L,s and L^. Therefore, reaction of the rotor flux vector to the stator voltage is somewhat sluggish in comparison with that of the stator flux vector. Also, thanks to the low-pass filtering action of the leakage inductances, rotor flux waveforms are smoother than these of stator flux.

The impact of stator voltage on the stator flux is illustrated in Figure 8.1.

At a certain instant, t, the inverter feeding the motor switches to State 4, generating vector v4 of stator voltage (see Figure 4.23). The initial vectors of stator and rotor flux are denoted by ks(t) and Xp respectively. After a time interval of At, the new stator flux vector, Ks(t + At), differs from Xs(0 in both the magnitude and position while, assuming a sufficiently short At, changes in the rotor flux vector have been negligible. The stator flux has increased and the torque angle, ®SP has been reduced by Asr Clearly, if another vector of the stator voltage were applied, the changes of the stator flux vector would be different. Directions of change of the stator flux vector, Xs, associated with the individual six nonzero
Illustration of the impact of stator voltage on the stator flux.
FIGURE 8.1 Illustration of the impact of stator voltage on the stator flux.
vectors, v1 through v6, of the inverter output voltage are shown in Figure 8.2, which also depicts the circular reference trajectory of \s. Thus, appropriate selection of inverter states allows adjustments of both the strength of magnetic field in the motor and the developed torque.
Illustration of the principles of control of stator flux and developed torque by inverter state selection.
FIGURE 8.2 Illustration of the principles of control of stator flux and developed torque by inverter state selection.

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