Electric Machine Design using SPEED and Motor-CAD
1
Introduction
1.1
Rules of thumb
There are many “rules of thumb” for designing electric machines, but no such rule applies in all cases. As an example, we have the idea that the pole-face of a synchronous machine should be roughly square.
Page 1
This idea has deep theoretical roots. It can sometimes be observed in production, though there are many more exceptions. It is an old idea, as we can see in Fig. 1.1.
Fig. 1.1 Crompton alternator, Walmsley
However, a square pole-face would not be suitable in a down-hole drilling motor, which must produce a huge torque with a severely limited diameter. Such motors may have a rotor diameter of a few inches, with an axial length measured in metres, so that with two poles the ratio of the axial length to the pole-arc is of the order of ten times the value that would give a “square pole”.
1.2
Fundamental laws
At the same time, there are fundamental physical laws that do apply rigorously in all cases, though not necessarily in an obvious way. One of the most basic laws of electric machines is Faraday’s law. A general form of it is expressed in terms of flux-linkage 5: v
05 , 0t
(1.1)
but an engineer designing an AC machine is more likely to use
E 4#44 kw1 Tph 0m1 f
[V
RMS] .
(1.2)
Even the units of eqns. (1.1) and (1.2) are not the same. Eqn. (1.1) is in instantaneous volts, whereas eqn. (1.2) is in RMS volts. Moreover, eqn. (1.1) is always true in all circumstances, whereas eqn. (1.2) is true only when the flux-linkage of the winding is varying sinusoidally with time. The form of eqn. (1.1) is of limited use to the practical design engineer, even though eqn. (1.2) is derived from it. Conversely eqn. (1.2) is utterly useless outside the AC machine, or when the time variation of the winding flux-linkage is not sinusoidal. SPEED must use both forms: (1.1) for time-stepping simulation and for the underlying theory, and (1.2) for calculating E in a form that can readily be checked by the expression 4#44 * kw1 * Tph * PhiM1 * Freq1 which can be entered exactly in that format in the CALCULATOR, [F4].