Phasor control

While all these paramclcrs are extremely essential for a process line, with the R D in the field these limitations have been overcome with the use of phasor controls. To implement the.se controls different manufacturers have tidopled different control and feedback systems to monitor and control the torque and field components. They have also given these controls different trade names. The basic technological concept remains the same but process implementation may vary from one manufacturer to another. Below we attempt to identify the more common phasor controls introduced by a few leading manufacturers. 

I herel orc a nornial W/can also be transformed into a phasor control, and/, being torque-and flux-producing components respectively. These components arc represented only theoretically. In fact they arc not. separate and hence the dilTiculty in controlling each of them more precisely. 

Figure 6,8 Speed-torque characteristics by flux (/ ,) control (single phasor control)...

For field-oriented controls, a mathematical model of the machine is developed in terms of rotating field to represent its operating parameters such as /V 4, 7, and 0 and all parameters that can inlluence the performance of the machine. The actual operating quantities arc then computed in terms of rotating field and corrected to the required level through open- or closed-loop control schemes to achieve very precise speed control. To make the model similar to that lor a d.c. machine, equation (6.2) is further resolved into two components, one direct axis and the other quadrature axis, as di.sciis.sed later. Now it is possible to monitor and vary these components individually, as with a d.c. machine. With this phasor control we can now achieve a high dynamic performance and accuracy of speed control in an a.c. machine, similar to a separately excited d.c. machine. A d.c. machine provides extremely accurate speed control due to the independent controls of its field and armature currents. 

This is an alternative to FOC and can provide a very fast response. The choice of a static drive, whether through a simple V7/control, field-oriented phasor control or direct torque control with open or closed-loop control and feedback schemes, would depend upon the size of the machine, the range of speed control (whether required to operate at very low speeds, 5% and below), the accuracy of speed control and the speed of correction (response time). The manufacturers of such drives will be the best guide for the most appropriate and economical drive for a particular application or process line. 

Figure 6.12 Block diagram for a flux-oriented phasor control...

Table 6.2 Comparison ol conventional V7/control with dillerent modes ol phasor control drives...

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

These drives are normally open loop (sensor-less) without encoder. For higher regulation, it is better to adopt a two-phasor control, such as a field-oriented control (FOC) or a direct torque control (DTC) drive. 

V and/of the fixed a.c. input supply or 4 and (phasor control) in the machine s parameters and use these to perform a required duty cycle with very precise speed 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. 

Ficld-oricntcd control (FOC), commonly known as doublc-phasor or phasor (vector) control and... 

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... 

This is commonly known as double phasor or phasor (vector) control. If we analyse equation (I. I) in Chapter I we will observe that 0 is a function of stator magnetizing current, /, and / is a transformation of the stator active current, /. ... 

The phasor /, and /, are separated and then controlled separately as discussed later. For more precise speed control a pulse encoder feedback device can also be employed. The characteristics now improve to Figure 6.10. The torque can now be maintained constant at any speed, even at zero speed. 

The microprocessor plays the role of an electronic controller that transforms electrical quantities such as V. / and N etc. into space flux phasors, to be compared with the pre-set data. It then creates back V. I and N etc.. 

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. 

Flux (/, ) control Double-phasor (vector) or Direct torque control (single-phasor afield-oriented control (DTC)... 

The total VA level of a control or an auxiliary circuit is the phasor sum of the VA burdens of each individual component and device connected in the circuit, and... 

Figure 16.7 Phasor diagrams for QDC, referred to the secondary side of the control transformer...

This part-also deals w ith static controls and drives, soft starting and process control through solid-state technology (phasor and field-oriented controls) using IGBTs as well as energy conservation,... 

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