Electrical Machines and Drives

Undoubtedly, transformers are omnipresent in our society. In this chapter, the electromagnetic principles of transformers are explained in detail. Equivalent circuits are derived starting from the basic laws of Maxwell and discussed in detail. As such, the construction of transformers is also treated in some detail. Attention is also paid to numerous applications.

Although DC commutator machines are nowadays largely being replaced by rotating field machines, they remain an interesting study object. The basic principles for energy conversion are similar to those of other machines. Moreover, their excellent control properties (e.g. speed control, torque control) are the inspiration for modern drive control schemes for rotating field machines. Starting from the basic electromagnetic laws, the electromagnetic energy conversion in DC machines is explained. Attention is paid also to commutation and armature reaction. Motoring and generating characteristics are discussed in detail.

Rotating fields are the basis for most electric drives (induction and synchronous machines). First, the generation of a rotating field is discussed. As in the previous chapters, we start again from the basic electromagnetic laws. Both using a graphical depiction and a more mathematical method the rotating field generation is explained. Next, the emf is discussed. Finally, the torque on a (rotating) current layer in a (rotating) field is discussed.

Nowadays, the induction machine is by far the most commonly used electrical machine in electrical drives (for applications that require a highly dynamic behaviour or where energy efficiency or compactness is primordial the permanent magnet synchronous motor may be preferred, however). The main reasons for this are its straightforward and robust construction and its quite efficient energy conversion. Moreover, the last 30 or so years variable speed operation of induction machines using power electronic converters has ousted almost completely the DC commutator machine in variable speed applications. This chapter starts from the traditional transformer properties of an induction motor at standstill. Then, the operating principle of an induction machine is explained both intuitively and from a more mathematical point of view. In the subsequent sections we treat the energy conversion and torque, equivalent circuits and equations for an induction machine, the current locus and single-phase induction machines. Much attention is paid to per-unit values and scaling laws as these determine the behaviour of the machine to a great extent.

In this chapter we discuss the synchronous machine mainly from its traditional function as generator. Motoring, in particular with permanent magnet synchronous motors, is treated in a later chapter. The first section gives an overview of the main two rotor types and their properties. In the following section, the smooth rotor synchronous machine is treated in detail, including armature reaction, phasor diagrams, equivalent circuits, current diagram, torque but also a thorough discussion of the non-linear generator characteristics. In the next section, salient pole synchronous machines are treated. One of the last sections is devoted to the operation (mainly as generator) on an infinitely strong grid.

This chapter aims to give an overview of the main power electronic components for application in electrical drives and electrical grids. It is not at all intended as a power electronics components course, but rather as the basis for the later chapters on the main power electronic circuits like rectifier, chopper or inverter. In addition to the traditional components (for example: diode, thyristor, IGBT) also a section is devoted to new developments regarding Si-C and GaN components.

This chapter mainly discusses the classical rectifier. Starting from the diode rectifier, next the controlled rectifier is treated (both mainly for inductive loads). A section also is devoted to operation on capacitive loads. Much attention is paid to reactive power requirements and to harmonics in the grid (and load as well).

For AC, there is a straightforward and almost lossless way to transfer electrical energy from one (voltage) level to another level, i.e. the classical electromagnetic transformer. For DC such a standard solution is not available. Energy transfer from a higher to a lower level is indeed possible using a resistance network, but this comes at the expense of high losses. The chopper is an elegant power-electronic solution that does not lead to any significant losses. Starting from a fixed DC voltage, it adapts the load voltage by periodically switching the input DC voltage (or current), and as such adapting the average load voltage. Two advantages are that some chopper circuits also allow energy transfer to a higher voltage level, and that the chopper principle eases electronic control. Another way to control the average current is by using periodic switching of a series resistor, but this method is not energy-efficient. For low-power applications (e.g. telecommunications, computer power supply), yet another alternative using resonant circuits is quite common (see Sect. 8.5). In the first section, the basic chopper circuits will be discussed. The next section shortly discusses the power-electronic switches used in choppers. In other sections, chopper applications in traction and DC drives are illustrated. The final section gives a brief overview of the principles of resonant chopper circuits.

The aim of the AC chopper (also called AC line control or phase control) is somewhat analogous to the DC chopper, i.e. to obtain a variable voltage. Here, the output is an AC voltage with the same fundamental frequency as the mains supply. An important difference compared to the DC chopper is that also the current is now bi-directional. This chapter discusses the basic circuits and analyses in more detail a pure inductive load, both single-phase and three-phase.

Variable speed operation of rotating field machines requires a variable frequency supply. Nowadays, a variable frequency supply is usually realised by means of an inverter (see Chap. 11), which accomplishes this starting from a DC source (i.e. a DC battery or the rectified grid voltage). The cycloconverter offers another solution, however limited to rather low output frequencies. Nowadays, the cycloconverter is (or was) mainly used for high output power (and low output frequencies). In this chapter we review the operating principle, provide some examples of practical three-phase circuits and discuss the main control methods. Important aspects are the output voltage harmonic content and the input current harmonic content and reactive power requirements.

Rectifiers convert AC into DC. With a controlled rectifier, energy can also be transferred from the DC-side to the AC-side. However, the AC source (i.e. the grid) always remains necessary as it is responsible for the commutation of the switches (using the emf of the source or, put differently, using the reactive power of the source, to switch off the thyristors). Controlled rectifiers can therefore not be used to convert DC into AC with variable frequency. Nevertheless, the only energy-efficient way to obtain variable speed operation of rotating field machines (i.e. induction and synchronous machines) is by feeding them from a variable frequency source. Inverters are able to convert DC into AC with variable frequency and, in most cases, also variable amplitude. Contrary to controlled rectifiers, inverters require switches that can be turned on and off at will at any instant. Nowadays, the switches used in inverters are mainly Mosfets (for lower power), IGBTs or IGCTs (for very high power).

In the past, the DC machine was the only motor that could provide easy speed control. Nowadays, since the advent of power electronics, rotating field machines offer cheaper and more powerful possibilities for speed control. Nevertheless, in older industrial installations and environments, DC motor drives may still be used. As a result, and also because of their excellent characteristics, it remains instructive to shortly review the possibilities of DC machines for speed control, including braking in particular.

For rotating field machines supplied by a constant frequency supply, there are almost no acceptable means of speed control (and none whatsoever for synchronous machines). This section will mainly focus on starting, accelerating and braking, and speed control will be discussed in later chapter. Pure synchronous machines fed from a constant frequency supply cannot start and accelerate, as should be clear from Chap. 5 in Part 1. However, when starting and accelerating is required, the damper winding of a synchronous machine (which is usually required for stability) can be adapted to give a starting and accelerating torque, operating as an asynchronous machine. When approaching synchronous speed, the DC supply of the field winding can be switched on and the machine will synchronise when the load is not too large. The asynchronous starting, accelerating and braking discussed in the first section below is therefore also important for synchronous machines which are not too large. In the next section, speed control and cascade connections for slip-ring induction motors are reviewed. Other sections discuss the behaviour of rotating field machines at voltage variations and power-electronic voltage control for rotating field machines.

An ideal current supply (i.e. a current independent of the load) is not feasible. In fact, most sources come close to more or less ideal voltage sources. In the (distant) past, a DC current supply was sometimes used for the supply of a group of machines, as will be illustrated in the next section. In the subsequent two sections, a purely theoretical discussion will be given of the ideal current supply of induction and synchronous machines, respectively. Primarily, the aim of this discussion is to point out the practical issues that would arise if an ideal current source existed. In reality, modern controlled drives make use of a controlled current source inverter or a voltage source inverter with a current control loop, as will be discussed in the following chapters.

Traditional rotating field machines have no (for synchronous motors) or poor and limited (for induction motors) possibilities for variable speed operation when power is supplied from a fixed frequency source. In this chapter, we will discuss the open-loop variable frequency voltage supply of rotating field machines. These kinds of drives are commonly denominated as V/f drives, because both the voltage and the frequency are controlled together in open loop, so as to control both the speed and the flux. The aim of the open-loop variable frequency operation of rotating field machines is to obtain a cheaper and more reliable variable speed drive than the expensive and high-maintenance DC drive. Drives that are even more demanding in terms of speed, position or torque control will be discussed in later chapters.

In this chapter (which is mainly based on Novotny, Equivalent circuit steady state analysis of inverter driven electric machines, [30]), we will study a method for the fundamental harmonic modelling of inverter supplied rotating field machines. First, we derive fundamental harmonic equivalent circuits for the inverter. These fundamental harmonic circuits are then combined with the steady-state equivalent circuits of rotating field machines to derive the basic characteristics of these inverter-supplied machines. For the modelling of the inverters, the resistive voltage drop over the switches will not be taken into account. Commutation will also be simplified, as most transients will be disregarded.

As is clear from the previous chapters, DC commutator machines are ideal machines to obtain controlled torque and speed. To obtain similar properties, rotating field machines require more complicated supplies and control circuits. In this chapter we derive the basic control methods for rotating field machines, with as starting point the DC commutator machine characteristic properties.

Electric machines cover an extremely wide range of power ratings, from 1 mW ($$10^{-3}$$ W) or less, to 1 GW ($$10^{9}$$ W), which is a ratio of $$1:10^{12}$$. The power range of 1 kW may be considered as the boundary between small and large machines.

Single-phase AC commutator machines are nowadays mostly used in (small) household machines, such as coffee grinders, mixers, or vacuum cleaners. Larger AC commutator machines also used to be applied in traction, for example in trains, but are now replaced by inverter-fed rotating field machines. In this chapter we review the basic properties of these single-phase AC commutator machines.

For large synchronous machines, the excitation is usually provided by a DC-fed field winding in the rotor. For small synchronous machines, however, this is undesirable, on the one hand because of the complications for supplying DC to the rotor (i.e. slip rings, rectifier), on the other hand because the smaller the dimensions, the less efficient a field winding is in producing a magnetic flux.

In contrast with the continuous motion of usual induction or synchronous machine drives, stepping motors produce a controlled, stepwise motion without any need for position measurement and feedback. The main characteristic quantities and properties of stepping motors are described. Thereafter, the different types of stepping motors are discussed.

The switched reluctance machine (SRM or also SWRM) started to receive much attention at the end of the 20th century. Its operation principle had already been known since 1838 but had not been able to find a practical use because no (fast) power electronic switches were available. Nowadays, the switched reluctance motor is applied in many industrial applications like washing machines or looms and even in more demanding applications like the starter-generator of jet turbines in airplanes. The most important advantages of the SRM are its simple and rugged construction, its inherent redundancy, and its suitability for high speeds. Its most negative aspect is the high level of noise and vibrations that it brings along.

In this chapter we shortly review the well known principles and definitions of stability and dynamics of systems.

In this chapter we discuss some transient phenomena in simple electrical circuits (resistive, inductive, capacitive), as an introduction to the later chapters on (local) stability and dynamics of electrical machines and drives.

For many induction machines with pulsating loads, the pulsation frequency is rather low. Therefore, a quasi-stationary approach can be used to study the behaviour of the induction machine. In this chapter it is shown that a simple mathematical averaging cannot be used for dimensioning the machine.

As an introduction to the later chapters on the dynamics of induction and synchronous machines, in this chapter we study the modelling and dynamic behaviour of DC commutator machines. The traditional machine model, using a simplified modelling of the main field saturation, permits to derive the basic dynamic properties. A more accurate model for main field saturation is also presented. It is shown that the dynamic properties derived using this model are slightly different from those using the traditional model, but also somewhat more realistic.

With the advent of variable frequency supply of rotating field machines in the second half of the twentieth century, some cases of hunting of induction machines fed with a low frequency supply became apparent. This was the starting point of research into the causes of these instabilities. In this chapter we analyse the stability behaviour of induction machine drives for variable frequency supply. A traditional model with constant saturation is used to analyse the dynamic behaviour. Using well-chosen dimensionless parameters, the characteristic dynamic behaviour of induction machines can be represented in a handy way. Because induction machines obey scaling laws quite narrowly, it is possible to predict the dynamic behaviour of a typical machine.

Not only induction machines, but synchronous machines as well may exhibit dynamic problems when fed by a variable frequency supply. In this chapter we analyse and represent the dynamic behaviour of synchronous machines in an analogous way as we have done for the induction machine. The results are quite similar. However, for synchronous machines such scaling laws do not exist to the same extent as for induction machines.

In Chap. 17 of Part III, vector control and field orientation have already been presented. However, the dynamics have not been analysed. In this chapter we discuss vector control and field orientation more thoroughly, including an analysis of their dynamic behaviour.

While the previous chapters analysed the local stability of electrical machines, here we study a typical case of large transients, i.e. the sudden short-circuit of a synchronous generator. The model used is a simplified constant saturation machine model.

Windings of transformers and electrical machines are often subject to voltage surges, either by switching in the grid or by atmospherical phenomena. The resulting voltage waves may propagate into the windings of transformers and rotating machines, causing voltage stresses in the winding. In this chapter we present two simplified models to calculate these voltage stresses. Although nowadays computer models exist to rather accurately predict these voltage stresses, the simplified models offer valuable insight into the physical causes of the localised voltage stresses.