Phasor control motors

The performance of the drive would also depend upon the accuracy of the motor s mathematical model u.sed for the phasor control. 

With the availability of phasor control technology, by which one can separate out the active and magnetizing components of the motor s stator current and vary them individually, it is now possible to achieve higher dynamic performance and accuracy of speed control in an a.c. machine similar to and even better than a separately excited d.c. machine. 

In the above schemes the two quantities (/ and / ,) are not separated. Initially it w as not easy to separate them and the whole phasor ( was varied to achieve a speed variation. Yet close speed control was possible but the motor s basic parameters were essential to achieve more accurate speed control. Since it may not be practical to obtain all the parameters of each motor promptly, the drive software is designed so that the name plate particulars ofa motor (V, N, and kW) alone arc enough to determine the machine s required parameters through the motor s... 

Such a control is good for machines that are required to operate at low speeds with a high accuracy. Now the phasor /, in terms of /, , is varied according to the speed required. Figure 6.2 now changes to Figure 6.8, which is a marked improvement on the earlier characteristics. The torque variation with speed is now almost constant, except at very low speeds. The reason for poor torque at low speeds is the method of speed variation which is. still based on Vlf. Now a motor s mathematical model is used... 

Since the motor s fixed parameters can now be varied to suit a particular load requirement, there is no need to pre-match a motor with the load. Now any motor can be set to achieve the required characteristics to match with the load and its process needs. Full-rated torque (TJ at zero speed (during start) should be able to pick up most of the loads smoothly and softly. Where, however, a higher 7 s, than is necessary, a voltage boost can also be provided during a start to meet this requirement. (See also Section 6.16.1 on soft starting.) The application of phasor (vector) control in the speed control of an a.c. motor is shown in a block diagram in Figure 6.12. 

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