Basic Three Phase AC Theory Understanding

Another subject that often baffles electrical apprentices is in the use of three phase AC. At technical college they are given the theory and little else by the lecturers. Confusion thus remains which can for some inhibit or at least slow down further learning on the subject.

Now at this point I have to mention that most of my experience is in industry and thus these apprentices are likely to be familar with the equipment found there. For those of you reading this that wishing to gain a better understanding I should point out that electrical motors form most of the direct use of three phase AC since lighting is single phase.

I ask the apprentice to demonstrate their understanding of single phase AC by having them draw the sinusodial waveform. I then see if they can draw the three phase sinusodial waveforms which hopefully they can and show that there is a 120 degree displacement between the three. Most have no problem in drawing out the waveform. A question is asked to the apprentice "When one phase is at a maximum in the postive direction what is the value of the other two at that instant." With a little bit of help they point out or can see that this happens

With two magnets it is demonstrated for the purpose of refreshing their knowlegde that two like poles repel each other and that two differing poles attract each other. From this I draw a circle and put on three windings and in the centre is a magnet. I then ask them to look at the waveform and show the direction on the circle that each phase would become postive. I then ask them what happens with the magnet and most indicate that this would turn in the direction that they indicated on the circle.
Have you ever seem the light bulb on someones head glow bright because for most apprentices in my experience it is at this moment that you can almost hear them shout "Eureka".