VECTOR (FIELD-ORIENTED) CONTROL
Where very rapid changes in speed are called for, however, the standard inverter-fed drive compares unfavourably with d.c. drive. The superior- ity of the d.c. drive stems Wrstly from the relatively good transient response of the d.c. motor, and secondly from the fact that the torque can be directly controlled even under transient conditions by controlling the armature current. In contrast, the induction motor has inherently poor transient performance.
For example, when we start an unloaded induction motor direct-on- line we know that it runs up to speed, but if we were to look in detail at what happens immediately after switching on we might be very surprised. We would see that the instantaneous torque Xuctuates wildly for the Wrst few cycles of the supply, until the Xux wave has built up and all three phases have settled into a quasi-steady-state condition while the motor completes its run-up. (The torque–speed curves found in this and most other textbooks ignore this phenomenon, and present only the average steady-state curve.) We might also Wnd that the speed oscillated around synchronous before Wnally settling with a small slip.
For the majority of applications the standard inverter-fed induction motor is perfectly adequate, but for some very demanding tasks, such as high-speed machine tool spindle drives, the dynamic performance is extremely important and ‘vector’ or ‘Weld-oriented’ control is warranted. Understanding all the ins and outs of vector control is well beyond our scope, but it is worthwhile outlining how it works, if only to dispel some of the mystique surrounding the matter. Some recent textbooks on electrical machines now cover the theory of vector control (which is still considered diYcult to understand, even for experts) but the majority concentrate on the control theory and very few explain what actually happens inside a motor when operated under vector control.
Transient torque control
We have seen previously that in both the induction motor and the d.c. motor, torque is produced by the interaction of currents on the rotor with the radial Xux density produced by the stator. Thus to change the torque, we must either change the magnitude of the Xux, or the rotor current, or both; and if we want a sudden (step) increase in torque, we must make the change (or changes) instantaneously.
Since every magnetic Weld has stored energy associated with it, it should be clear that it is not possible to change a magnetic Weld instant- aneously, as this would require the energy to change in zero time, which calls for a pulse of inWnite power. In the case of the main Weld of a motor, we could not hope to make changes fast enough even to approxi- mate the step change in torque we are seeking, so the only alternative is to make the rotor current change as quickly as possible.
In the d.c. motor it is relatively easy to make very rapid changes in the armature (rotor) current because we have direct access to the armature current via the brushes. The armature circuit inductance is relatively low, so as long as we have plenty of voltage available, we can apply a large voltage (for a very short time) whenever we want to make a sudden change in the armature current and torque. This is done automatically by the inner (current-control) loop in the d.c. drive (see Chapter 4).
In the induction motor, matters are less straightforward because we have no direct access to the rotor currents, which have to be induced from the stator side. Nevertheless, because the stator and rotor windings are tightly coupled via the air-gap Weld (see Chapter 5), it is possible to make more or less instantaneous changes to the induced currents in the rotor, by making instantaneous changes to the stator currents. Any sudden change in the stator MMF pattern (resulting from a change in the stator currents) is immediately countered by an opposing rotor MMF set up by the additional rotor currents which suddenly spring up. All tightly coupled circuits behave in this way, the classic example being the transformer, in which any sudden change in say the secondary current is immediately accompanied by a corresponding change in the primary current. Organ- ising these sudden step changes in the rotor currents represents both the essence and the challenge of the vector-control method.
We have already said that we have to make sudden step changes in the stator currents, and this is achieved by providing each phase with a fast- acting closed-loop current controller. Fortunately, under transient condi- tions the eVective inductance looking in at the stator is quite small (it is equal to the leakage inductance), so it is possible to obtain very rapid changes in the stator currents by applying high, short-duration impulsive voltages to the stator windings. In this respect each stator current control- ler closely resembles the armature current controller used in the d.c. drive.
When a step change in torque is required, the magnitude, frequency and phase of the three stator currents are changed (almost) instantaneously in such a way that the frequency, magnitude and phase of the rotor current wave (see Chapter 5) jump suddenly from one steady state to another. This change is done without altering the amplitude or position of the resultant rotor Xux linkage relative to the rotor, i.e. without altering the stored energy signiWcantly. The Xux density term (B) in equation (5.8) therefore remains the same while the terms Ir and wr change instantaneously to their new steady-state values, corresponding to the new steady-state slip and torque.
We can picture what happens by asking what we would see if we were able to observe the stator MMF wave at the instant that a step increase in torque was demanded. For the sake of simplicity, we will assume that the rotor speed remains constant, in which case we would Wnd that:
(a) the stator MMF wave suddenly increases its amplitude;
(b) it suddenly accelerates to a new synchronous speed;
(c) it jumps forward to retain its correct relative phase with respect to the rotor Xux and current waves.
Thereafter the stator MMF retains its new amplitude, and rotates at its new speed. The rotor experiences a sudden increase in its current and torque, the new current being maintained by the new (higher) stator currents and slip frequency.
We should note that both before and after the sudden changes, the motor operates in the normal fashion, as discussed earlier. The ‘vector control’ is merely the means by which we are able to make a sudden stepwise transition from one steady state operating condition to another, and it has no eVect whatsoever once we have reached the steady state.
The unique feature of the vector drive which diVerentiates it from the ordinary or scalar drive (in which only the magnitude and frequency of the stator MMF wave changes when more torque is required) is that by making the right sudden change to the instantaneous position of the stator MMF wave, the transition from one steady state to the other is achieved instantaneously, without the variables hunting around before settling to their new values. In particular, the vector approach allows us to overcome the long electrical time-constant of the rotor cage, which is responsible for the inherently sluggish transient response of the induction motor. It should also be pointed out that, in practice, the speed of the rotor will not remain constant when the torque changes (as assumed in the discussion above) so that, in order to keep track of the exact position of the rotor Xux wave, it will be necessary to have a rotor position feedback signal.
Because the induction motor is a multi-variable non-linear system, an elaborate mathematical model of the motor is required, and implementation of the complex control algorithms calls for a large number of fast computations to be continually carried out so that the right instantaneous voltages are applied to each stator winding. This has only recently been made possible by using sophisticated and powerful signal process- ing in the drive control.
No industry standard approach to vector control has yet emerged, but systems fall into two broad categories, depending on whether or not they employ feedback from a shaft-mounted encoder to track the instantan- eous position of the rotor. Those that do are known as ‘direct’ methods, whereas those which rely entirely on a mathematical model of the motor are known as ‘indirect’ methods. Both systems use current feedback as an integral part of each stator current controllers, so at least two stator current sensors are required. Direct systems are inherently more robust and less sensitive to changes in machine parameters, but call for a non- standard (i.e. more expensive) motor and encoder.
The dynamic performance of direct vector drives is now so good that they are found in demanding roles that were previously the exclusive preserve of the d.c. drive, such as reversing drives and positioning applications. The achievement of such outstandingly impressive performance from a motor whose inherent transient behaviour is poor represents a major milestone in the already impressive history of the induction motor.