Rectification in Motors and Generators

Ideally, the motors used in an electric vehicle will be able to function as generators when during braking, with the current generated going to recharge the battery. Sounds like a simple enough concept for an electrical engineer to be dealing with, but frankly, I doubt I ever understood the basics of how an electric motor worked when I was at university. It seems to me there was a senior year course elective called "Power Engineering" that wasn’t in my concentration, and maybe that’s where they taught basic electromotive concepts, but I doubt it. I remember friends telling me that it was all math, like most of our courses.

I searched around the library today for a nice basic text on motors, and came across "Electrical Machines" by Kostenko and Piotrovsky. It’s a fun read because it was written as a textbook during the communist era in the former USSR, and therefore, Russian inventors are much lauded and capitalist exploiters are blamed for the early lack of progress in that country. However, it does have an excellent explanation of the basic concepts of a motor/generator right in the first chapter, using a two magnetic poles and a Faraday loop. My ability to produce illustrations while I’m traveling is pretty limited, so I’ll stick to a text commentary.

If you picture two permanent magnets with a nice cylindrical channel cut out of each, oriented across from each other so that the North pole of one faces the South pole of the other, you have set up a nice continuos magnetic field. If you introduce into that field a loop shaped conductor, the armature, you’ve essentially placed two conductors in the magnetic field, which happened to be made from one piece of wire that forms a "U" on one end. As that armature is spun between the two magnets, a current is induced in the conductors (Faraday’s Law). The free ends of the wires are attached to a split cylinder, or commutator, where the current is drawn off each half on the commutator by a brush, forming a circuit with whatever load is put between them. The function of the split cylinder commutator and brushes is to rectify the current so that it always flows in the same direction, though the magnitude will be very sinusoidal. The brushes are fixed in position, one so that it always draws the current off from the conductor near the South Pole, the other from the North. The whole arrangement is rather magical, and proves there is beauty in electrical engineering.

The design formula for electric motors, which are far more complex in practice than our simple loop with two magnets, are highly idealized using rules-of-thumb and cookbook methods. The reason is that the motor components themselves have geometrys that just aren’t describable with continuos functions. I suppose you could do something with numerical methods to describe the flux through the odd shaped windings and magnets, but since motors and generators are built for practical purposes, practical solutions built upon existing data rule the roost. If instead of turning the armature with an external mechanical force (generation) you cause a current to flow through it from an external electrical force, it causes the armature to spin between the polls and you have a motor.

The reading has me itching to build a couple crude DC motors of my own when I get back. I wouldn’t dream (at this point) of actually designing and winding my own motors and generators to build an electric power train for a car. It wouldn’t be practical or cost effective, even if I knew what I was doing, and would add a couple years of lead-up to my first go. It does bring back memories of a few weeks I spent winding transformers some twenty years ago. They were custom transformers with a special ferrite core, a split secondary with multiple taps and a bifilar wound primary, meaning the primary winding consisted of two identical length conductors wound the same number of turns, side by side. I was actually pretty good at it, when the inductance (or was it reluctance:-) was checked by test equipment.