Published February 2000

MOTORS: A LOST ART

by Bob Perrin

Start • A Few Words on Words • The DC Motor • Polyphase AC Motors • Single-Phase AC Motors • Winding Down • Sources and PDF

POLYPHASE AC MOTORS

In the 1880s, Nikola Tesla developed a two-phase AC motor. This was followed shortly by Tesla’s three-phase motor. Tesla also designed generators to supply the multiple-phase (polyphase) AC for his inventions. The invention of the AC motor legitimized Tesla’s (and Westinghouse’s) AC approach to power distribution and eventually caused Edison’s DC approach to power distribution to be relegated to footnote status in the history books.

Tesla was an interesting character, an inventor and a man the world owes a lot to. His biography is interesting reading.

Three-phase induction motors have been in existence for more than a century. Given the abbreviated life expectancy technological inventions normally suffer from, the three-phase induction motor is an incredibly long-lived invention. Such an accomplishment is deserving of a second look.

The first thing to understand about induction motors is that the rotor need not have windings, although some do. The windings on the stator are used to introduce currents in the rotor. The currents in the rotor set up magnetic fieldsthat interact with the stator’s changing magnetic field. The end result is that a force (torque) is produced on the rotor.

Fractional horsepower motors are generally considered "small motors." These induction motors most often have a squirrel-cage rotor (see Figure 3). The steel laminations are insulated from each other, preventing unwanted eddy currents from developing.

Figure 3—A squirrel cage rotor is mechanically simple and inexpensive to manufacture.

Squirrel cage rotors get their name from their geometry, which resembles the little exercise wheel found in hamster or gerbil cages. At the turn of the century, squirrels were common pets and had similar exercise wheels. To people at the turn of the century, the rotor geometry resembled their squirrel cages.

Larger induction motors are more likely to have windings on their rotors. These windings are used to control the torque and speed characteristics of the motor. Donald Richardson’s text on motors describes wound rotor induction motors in detail. [3] Of the references listed, this text is the most in-depth. It is used at several universities as a textbook for rotating machinery classes.

The usual three-phase motor will have a stator that looks similar to the one shown in Photo 1. The number of slots will vary from motor design to motor design. The winding’s geometry will also vary from design to design. But, if you crack open a three-phase motor, you’ll generally find a stator resembling that shown in Photo 1.

In Photo 1a, you can see that the windings are very organized and uniform. This is typical of high-quality machine-wound stators. Photo 1b shows another feature of modern motors, a thermal switch. If the stator overheats, the thermal switch will open and shut down the current to the motor.

The stator used for Photo 1 is actually not wound as a three-phase stator. It is wound as a "capacitor-run single-phase" stator. Other than wire count and the simplicity of having only one thermal switch, cosmetically, the stator in Photo 1 resembles a three-phase stator.

The concept most central to the operation of polyphase induction motors is also the one that can be the most difficult to visualize. The idea is that, in an unmoving stator, a polyphase motor sets up a rotating magnetic field.

Figure 4 shows the time domain voltage waveforms for a three-phase power system. For simplicity, the magnitudes are normalized to 1 V. Notice, the three phases are separated by 120^°.

Figure 4—In a three-phase AC system the phases are offset by 120°.

Figure 5 illustrates the rotating magnetic field of a simple three-phase stator. The arrow shows how a compass needle would point. From the compass’s point of view, the magnetic field is rotating even though the stator is mechanically static.

Figure 5—It’s almost magic the way a mechanically fixed stator can produce a rotating magnetic field. (Click here for chart)

The notation on the poles of A, A’, B, B’, C, and C’ indicate how the current is applied to the winding. The primed letters indicate that side of the winding is attached to zero volts. The non-primed letters indicated that the end of the winding is attached to the voltage source.

For the stator shown in Figure 5, when a positive voltage is applied to a winding, a north pole will be developed by the non-primed letter. Conversely, when a negative voltage is applied to a winding, a south pole will be developed by the non-primed letter.

In Figure 5, the magnitude of the voltage applied to each phase is shown by each snapshot. By carefully examining Figure 4 and Figure 5, it should be fairly easy to see the pattern that results in a rotating magnetic field inside the stator.

Donald Richardson’s text does a nice job of explaining why the magnitude of the magnetic field inside the stator is constant. This text also explains in great detail how real-world three-phase stators are wound.

If you compare the stator shown in Photo 1 with the one depicted in Figure 5, you can see that Figure 5 is clearly a simplified geometry.

Another characteristic of induction motors is slip. Slip is defined as the difference between the rotational speed of the stator’s magnetic field and the rotor’s mechanical speed.

Slip is necessary for torque to be developed in an induction motor. If there were no difference in speed between the rotational speed of the stator’s magnetic field and the rotor’s rotational speed, the rotor would simply see a static field, and no currents would be induced in the rotor. Without currents in the rotor, no magnetic field would be produced by the rotor to interact with the stator’s magnetic field. And therefore, no torque would be produced.

Slip is specified as a percentage of the synchronous speed. This is simply the speed at which the magnetic field generated by the stator is moving.

The stator in Figure 5 rotates its magnetic field one revolution per cycle. Assuming the three-phase power is 60 Hz, the magnetic field in Figure 5 has 60 revolutions per second. Converting to revolutions per minute yields a synchronous speed of 3600 RPM.

Other stator-winding geometries will produce different synchronous speeds. Joe Kaiser’s book has some fairly good examples of three-phase windings. [4] Kaiser’s book is less rigorous than Richardson’s text, but Kaiser’s book is also quicker to read if you are just trying to acquire a qualitative understanding of motors and transformers.

Slip is usually specified at the motor’s unloaded speed and is usually between 2% and 5%. Slip can be expressed as either a percentage or a decimal. To get a decimal representation, the percent is just divided by 100.

Slip will change with load, as will torque and speed.

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