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At first glance motors appear to be complicated machines, and in truth they are. But the principle of operation, electromagnetism, is relatively simple, understood even by high school students. Differences aside, today’s various motor technologies are at heart quite similar and quite understandable.

The origin of the earliest motors — machines that convert electrical energy to mechanical power — can be traced to designs conceived by Michael Faraday. In 1831, Faraday formulated the fundamental concepts of electromagnetic induction, noting that a current-carrying conductor in a magnetic field sees a force proportional to the strength of the field and the current passing through it.

Electric motor design, both then and now, centers on the placement of conductors in a magnetic field. The conductors, of course, are in the form of windings with many turns of wire, each contributing to the intensity of the electromagnetic action. The more current, Faraday pointed out, the more force (torque) you can expect. Motion, the ultimate goal, is thus the result of two magnetic fields (one on the rotor, the other on the stator) pulling on each other. This concept is the basis of all dc and ac motor designs, and the starting point for modern motion engineering.

ABC’S of ac

Alternating-current (ac) motors are the most widely used motors in the world. They are essentially constantspeed devices, as determined by the number of magnetic poles and input frequency. In general there are two types of ac motors — induction and synchronous.

Induction motors may be viewed as a type of transformer, the primary winding corresponding to the stator and the secondary to the rotor. Applying a voltage to the “primary” does two things: It forces current through the stator while inducing current in the rotor. In other words, it sets up a magnetic field in the stator, while inducing a second field on the rotor. The interaction of these two fields is what makes the rotor move.

The speed of the magnetic field going around the stator determines rotor speed. The rotor will try to follow the stator field, but will “slip,” particularly when a load is applied. Induction motors, therefore, always run slower than the rotating stator field.

The stator in an induction motor consists of steel laminations and turns of copper wire. The rotor, on the other hand, is typically made from stacked laminations with large slots on the periphery. In a “squirrel cage” rotor, the slots are filled with copper or aluminum bars short-circuited by conductive end caps. This “one-piece” casting usually includes integral fan blades to circulate air for cooling.

Standard induction motors operate at a “constant” speed determined by standard line frequency. There are ways, however, to control speed. Microprocessor- based drives using vector control technology, for example, manipulate the magnitude of the magnetic flux in the rotor and stator fields, achieving a sort of variable-slip response. With an appropriate feedback sensor this control method is viable even in positioning applications.

Although very demanding jobs — such as rapid start-stop positioning — would be out of the question, some indexing applications are, nevertheless, feasible. The limiting factor, though, is heat. As motor size goes up to keep temperatures in check — larger motors cool better — the torque-to-inertia ratio become prohibitive to speed.

The advantages of induction motors are well known, including low initial cost, availability of standard sizes, reliability, and quiet, vibration-free operation.

Synchronous motors are similar to induction motors, differing mainly in rotor construction. The rotors are designed to rotate at the same speed as the stator field, hence the name “synchronous.” There are basically two types of synchronous motors: self-excited (like induction motors) and directly excited using permanent magnets.

Self-excited synchronous motors (sometimes called reluctance synchronous motors) employ a rotor with notches, or teeth, on the periphery. The number of notches corresponds to the number of poles in the stator. Often the notches or teeth are called “salient poles,” reflecting the fact that they create an easy path, a handle almost, for the magnetic flux field, thereby letting the rotor lock in and run at the same speed as the rotating field.

Directly excited synchronous motors (sometimes called hysteresis synchronous or ac permanent-magnet synchronous motors) employ a permanentmagnet alloy rotor. The permanent poles are, in effect, the “salient poles” and therefore prevent slip.

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