Control of Electrical Drives

Energy is the basis of any technical and industrial development. As long as only human and animal labour is available, a main prerequisite for social progress and general welfare is lacking. The energy consumption per capita in a country is thus an indicator of its state of technical development, exhibiting differences of more than two orders of magnitude between highly industrialised and not yet developed countries.

Since electrical drives are linking mechanical and electrical engineering, let us recall some basic laws of mechanics.

The equations derived in Chap. 1 , , describe the dynamic behaviour of a mechanical drive with constant inertia in steady state condition and during transients. Stiff coupling between the different parts of the drive is assumed so that all partial masses may be lumped into one common inertia. The equations are written as state equations for the continuous state variables ω, ε involving energy storage; only mechanical transients are considered. A more detailed description would have to take into account the electrical transients defined by additional state variables and differential equations. The same is true for the load torque m _L which depends on dynamic effects in the load, such as a machine tool or an elevator.

With the assumptions introduced in the preceding section the motion of a single axis lumped inertia drive is described by a first order differential equation (Fig. 3.1) , which upon integration yields the mechanical transients. Several options are available for performing the integration.

So far our considerations have only dealt with mechanical phenomena and the pertinent steady-state and dynamic conditions, but suitable torquespeed curves and adequate power are not the only criteria for designing electrical drives.

Direct current (DC) motors have been dominating the field of adjustable speed drives for over a century; they are still the most common choice if a controlled electrical drive operating over a wide speed range is specified. This is due to their excellent operational properties and control characteristics; the only essential disadvantage is the mechanical commutator which restricts the power and speed of the motor, increases the inertia and the axial length and requires periodic maintenance. With alternating current (AC) motors, fed by variable frequency static power converters, the commutator is eliminated, however at the cost of increased complexity. This is one of the reasons why controlled AC drives could not immediately supplant DC drives, once the semiconductor-technology had sufficiently advanced.

This motor differs from the one previously discussed only by the design and the connection of the field winding which, according to Fig. 6.1, now carries part or all of the armature current. R _{a 1} is the armature resistance, possibly increased by an external resistor, R _p is an adjustable shunt resistor for field weakening.

In Chap. 5 the steady state and dynamic behaviour of a separately excited DC machine with adjustable armature and field voltage has been explained; this discussion is now extended by considering the machine as part of a feedback control system. The reason for this is that in practice the choice of a DC drive is normally motivated by the possibility of operating over a wide speed range with low losses and matching the behaviour of the motor to the needs of the load. In order to achieve the desired operating characteristics in the presence of supply- and load-disturbances, feedback control is usually necessary. Another reason why DC drives are normally contained in feedback loops is that the armature of a larger motor represents a very small impedance which — when supplied with rated voltage — would result in an excessive current of up to 10 times rated value. Under normal operating conditions this is prevented by the induced armature voltage e, which cancels most of the applied voltage u _a so that only the difference determines the armature current i _a . It is these two quantities which are performing the actual electromechanical energy conversion.

A characteristic feature of the rotating converter discussed in Sect. 7.4 is the consecutive power conversion from electrical (constant AC line voltage) to mechanical (speed of motor-generator) and back to electrical form (variable direct voltage) from which it is eventually transformed into controlled mechanical power in the drive motor. These conversions constitute the advantages of the control scheme (separate electrical circuits, decoupling by rotating masses) as well as its drawbacks (cost of machines, foundations, power losses, servicing, limited speed of response).

Static converters are ideal electronic actuators for DC drives because of their virtually unlimited output power and excellent controllability. The speed of response is usually adequate to handle the electromechanical transients occurring in drives. Line-commutated converters or, as they are also called, converters with natural commutation, are the most frequent choice for industrial applications, where a three-phase supply is available; this is due to the simplicity of the circuits requiring a minimum number of active and passive components.

The asynchronous or induction motor is the most widely used electrical drive motor; its invention at the end of the last century has given a strong impetus for the transition from DC to AC in the field of generation, transmission and distribution of electrical energy. Its main advantage is the elimination of all sliding contacts, resulting in an exceedingly simple and rugged construction. Induction machines are built in a variety of designs with ratings from a few watts to many megawatts.

An important result of the preceding chapter is that the application of AC motors in continuous duty adjustable speed drives calls for static inverters of adequate power, generating three-phase voltages of variable amplitude and frequency. This is necessary in order to maintain at all speeds a low rotor frequency, which is a precondition for acceptable overall efficiency of the drive. Inverters of this type are available today employing thyristors, including gate turn-off thyristors (GTO), or switched power transistors, but the complexity and cost of the converter equipment and control still exceeds that of line commutated converters of similar rating. While part of the increased cost for the inverter can be recovered by the savings on the AC motor which is considerably less expensive than a DC machine of comparable rating, the overall cost of the AC drive may still be somewhat higher than that of a standard DC drive.

When comparing the dynamic model of a separately excited DC machine, Eqs. (5.1–5.4), Fig. 5.4, with that of an AC induction machine, Eqs. (10.38–10.41), Fig. 10.16, it is obvious that the latter represents a much more complex control plant. This is caused by the fact that the main flux and the armature current distribution of a DC machine are fixed in space and can be directly and independently controlled while with an AC machine these quantities are strongly interacting and move with respect to the stator as well as the rotor; they are determined by the instantaneous values of the stator currents, two of which represent independent control variables. An additional complication stems from the fact that the rotor currents cannot be measured with ordinary cage rotors. Hence the AC motor is a highly interacting nonlinear multi-variable control plant that kept control engineers puzzling for a long time.

The speed of a synchronous motor with constant rotor excitation is determined by the stator frequency and the number of poles. As long as an efficient, variable frequency power supply was not available this meant constant speed operation at fixed frequency. There are drive applications, where constant speed is desired or where the reactive power that can be generated with line-connected synchronous motors is an important feature. These are, apart from electric clocks, mainly high power drives, such as for compressors in the chemical industry. Another field of application exists in pumped storage power plants, where the synchronous generators are used as motors in periods of low demand for electrical power to drive pumps, feeding water into elevated reservoirs for later use during hours of peak demand. The type of motor is, of course, not a free choice in this instance but the synchronous machine is very well suited for this duty; it is, in fact, the only one that could be used at a power level of, possibly, several hundred MW. Problems with large synchronous motors operating on a constant frequency supply may be caused by the inherent oscillatory response and the required start-up procedure. Asynchronous starting at full or reduced line voltage with the help of the damper winding and the short circuited field winding as well as special starting motors are common practice; more recently, large synchronous machines are also started with variable frequency supplied from static inverters.

The preceding chapters were dealing mainly with the different types of electrical drives and their control; applications were only mentioned as they affected the operation of the machine and the associated equipment. Also, the specifications for a mechanical power supply are normally not met by just one type of electrical drive and the variety of applications can be bewildering. In this chapter the problems of applying controlled electrical drives will be explained in more detail. For this we begin with a 4-quadrant drive, be it DC or AC, the basic structure of which is contained within the dashed lines of Fig. 15.1. The moving masses are at first assumed to be rigidly coupled, represented by a lumped inertia. The inner loop which comprises the power converter and part of the electrical machine assures fast torque control; with an integrating controller it exhibits unity gain and serves for linearisation. Once the torque loop with the equivalent lag T _e is closed, there is little difference between a DC and an AC drive. By limiting the torque reference, protection of the power converter and the mechanical load is achieved. Torque control is mandatory on high performance drives, except for the smallest power ratings, because it serves as the controlling input to the mechanical system. Whenever load torque must be counteracted or the speed is to be changed, it is only possible by acting on the torque reference; hence the response of the torque control loop limits the control bandwidth of the complete drive.