INTRODUCTION
Judged in terms of fitness for purpose coupled with simplicity, the induction motor must rank alongside the screwthread as one of man- kind’s best inventions. It is not only supremely elegant as an electro- mechanical energy converter, but is also by far the most important, with something like one-third of all the electricity generated being converted back to mechanical energy in induction motors. Despite playing a key role in industrial society, it remains largely unnoticed because of its workaday role in unglamorous surroundings driving machinery, pumps, fans, compressors, conveyors, hoists and a host of other routine but vital tasks. It will doubtless continue to dominate these fixed-speed applications, but thanks to the availability of reliable variable-frequency inverters, it is now also the leader in the controlled-speed arena.
Like the d.c. motor, the induction motor develops torque by the interaction of axial currents on the rotor and a radial magnetic field produced by the stator. But, whereas, in the d.c. motor the ‘work’ current has to be fed into the rotor by means of brushes and a commutator, the torque-producing currents in the rotor of the induction motor are induced by electromagnetic action, hence the name ‘induction’ motor. The stator winding therefore not only produces the magnetic field (the ‘excitation’), but also supplies the energy that is converted to mechanical output. The absence of any sliding mechanical contacts and the consequent saving in terms of maintenance is a major advantage of the induction motor over its d.c. rival.
Other diVerences between the induction motor and the d.c. motor are:
firstly that the supply to the induction motor is a.c. (usually 3-phase, but induction motor rotates relative to the stator, whereas in the d.c. motor it is stationary and thirdly that both stator and rotor in the induction motor are non-salient (i.e. eVectively smooth) whereas the d.c. motor stator has projecting poles or saliencies which define the position of the field windings.
Given these diVerences we might expect to find major contrasts be- tween the performance of the two types of motor, and it is true that their inherent characteristics exhibit distinctive features. But there are also many aspects of behaviour which are similar, as we shall see. Perhaps most important from the user’s point of view is that there is no dramatic diVerence in size or weight between an induction motor and a d.c. motor giving the same power at the same base speed, though the induction motor will almost always be much cheaper. The similarity in size is a reflection of the fact that both types employ similar amounts of copper and iron, while the diVerence in price stems from the simpler construc- tion of the induction motor.
Outline of approach
To understand how an induction motor operates, we must first unravel the mysteries of the rotating magnetic field. fie shall see later that the rotor is eVectively dragged along by the rotating field, but that it can never run quite as fast as the field. fihen we want to control the speed of the rotor, the best way is to control the speed of the field.
Our look at the mechanism of the rotating field will focus on the stator windings because they act as the source of the flux. In this part of the discussion we will ignore the presence of the rotor conductors. This makes it much easier to understand what governs the speed of rotation and the magnitude of the field, which are the two factors that mostly influence the motor behaviour.
Having established how the rotating field is set up, and what its speed and strength depend on, we move onto examine the rotor, concentrating on how it behaves when exposed to the rotating field, and discovering how the induced rotor currents and torque vary with rotor speed. In this section, we assume – again for the sake of simplicity – that the rotating flux set up by the stator is not influenced by the rotor.
Finally, we turn attention to the interaction between the rotor and stator, verifying that our earlier assumptions are well justified. Having done this we are in a position to examine the ‘external characteristics’ of the motor, i.e. the variation of motor torque and stator current with speed. These are the most important characteristics from the point of view of the user.
In discussing how the motor operates the approach leans heavily on first building up a picture of the main or air-gap flux. All the main characteristics which are of interest to the user can be explained and understood once a clear idea has been formed of what the flux wave is, what determines its amplitude and speed and how it interacts with the rotor to produce torque.
The use of mathematics has been kept to a minimum, and all but the simplest equivalent circuit have been avoided in favour of a physical explanation. This is because the aim throughout this book is to attempt to promote understanding not only of what happens, but also why.
The alternative approach, which is favoured in most textbooks
on electrical machinery, is to move quickly to a position where the machine is represented by an (fairly complicated) equivalent circuit model, which can then be used for performance prediction. The danger of this for newcomers is that they can easily be daunted by the apparent complexity of the circuit, and as a result lose sight of the key messages that ought to emerge. And although equivalent circuits can provide qualitative answers to some of the questions we will be addressing, there are other matters (such as the fact that the rotor frequency is diVerent from the stator frequency) that are disguised in the circuit approach.
Experience has shown that to get the most benefit from an equivalent circuit, a good grasp of why the machine behaves as it does is an essential prerequisite. Armed with this knowledge the power of the equivalent circuit can be properly appreciated, so readers are urged to come to grips with the material in this chapter before exploring to Chapter 7, which can be regarded as an ‘extra’ for those seeking a diVerent viewpoint.
The fundamental aspects we have explored so far (magnetic flux, MMF, reluctance, electromagnetic force, motional e.m.f.) will be needed again here, just as they were in the study of the d.c. machine. But despite their basic similarities, most reader will probably find that the induction motor is more diYcult to understand than the d.c. ma- chine. This is because we are now dealing with alternating rather than steady quantities (so, for example, inductive reactance becomes very significant), and also because (as mentioned earlier) a single winding acts simultaneously as the producer of the working flux and the supplier of the converted energy. Readers who are unfamiliar with routine a.c. circuit theory, including reactance, impedance, phasor diagrams (but not, at this stage, ‘j’ notation) and basic ideas about 3-phase systems will have to do some preparatory work before venturing further in this chapter.