Main airgap flux and leakage flux
Broadly speaking the motor designer shapes the stator and rotor teeth to encourage as much as possible of the flux produced by the stator to pass
- Figure 5.7 Resultant air-gap flux density wave produced by a complete 3-phase, 4-pole winding at three successive instants in time
right down the rotor teeth, so that before completing its path back to the stator it is fully linked with the rotor conductors (see later) which are located in the rotor slots. We will see later that this tight magnetic coupling between stator and rotor windings is necessary for good running performance, and the field which provides the coupling is of course the main or air-gap field, which we are in the midst of discussing.
In practice the vast majority of the flux produced by the stator is indeed main or 'mutual' flux. But there is some flux which bypasses the rotor conductors, linking only with the stator winding, and known as stator leakage flux. Similarly not all the flux produced by the rotor currents links the stator, but some (the rotor leakage flux) links only the rotor conductors.
The use of the perjorative-sounding term 'leakage' suggests that these leakage fluxes are unwelcome imperfections, which we should go out of our way to minimise. However, whilst the majority of aspects of performance are certainly enhanced if the leakage is as small as possible, others (notably the large and unwelcome current drawn from the mains when the motor is started from rest) are made much worse if the coupling is too good. So we have the somewhat paradoxical situation in which the designer finds it comparatively easy to layout the windings to produce a good main flux, but is then obliged to juggle the detailed design of the slots to obtain just the right amount of leakage flux to give acceptable all-round performance.
The weight which attaches to the matter of leakage flux is reflected in the prominent part played by the associated leakage reactance in equivalent circuit models of the induction motor, and is discussed in Chapter 7. However, such niceties are of limited importance to the user, so in this and the next chapters we will limit references to leakage reactance to well-defined contexts, and in general, where the term 'flux' is used, it will refer to the main air-gap field.
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