AC Motors: Magnetism and the DC Motor


Written by:
Joe Evans, Ph.D

I received many comments on my four-part series on AC Power, and most of them were very positive. Several readers, however, thought they were too elementary. I
reminded them that the title of my column is “Pump Ed101” not 201 or 301, and its purpose is to introduce people to new and unfamiliar topics. Why do I use this approach? 

Things have changed quite a bit since I attended high school in the late 1950s and early 1960s. Back then, serious science started in the fourth grade, and there was no opting out of physics and chemistry in high school. The physics education I received in high school was conceptual and did not delve into complex mathematical relationships. That made it interesting and understandable—even to a teenage boy with lots of other things on his mind. Had it not been, I doubt that I would have pursued it in college and grad school.

High school education changed substantially in the 1970s and 1980s, and I am not sure that our current system achieves what was achieved back then. Because of this, a lot of us did not learn how simple physics can allow us to simplify many of the complex topics that we have to deal with on a day-to-day basis. That is the purpose of “Pump Ed 101,” so this introduction to AC motors will start with the basics. I will also provide several references if you would like to further your understanding of this interesting
and essential topic.

Magnetism

One of the more neglected subjects taught in science class is magnetism, and like many science topics, it is almost always presented in a boring manner. It is at the heart of both AC and DC motors and is a primary reason that AC power became the dominant power source in the world. The rudimentary definition of a magnet is an object that attracts iron. Although undoubtedly discovered in prehistoric times, it was not until 600 BC that the Greek philosopher Thales reported its properties. He studied a sample of loadstone (iron ore) from the town of Magnesia on the Aegean coast and because of its attractive properties, called it Magnesian rock.

Thales also discovered that amber (a fossilized resin known as elektron), when rubbed, also exhibited an attractive force. It was different, though, because its attractive forces were not limited to iron but would attract any number of objects including feathers and parchment. In this latter case, he had unknowingly discovered what we call electrostatics or electricity at rest.

An object that exhibits magnetism without the aid of electricity is called a permanent magnet. These magnets have two areas of maximum attraction, referred to as their north and south seeking poles. Although a number of rules apply, the most basic is that opposite poles attract and like poles repel one another.

A moving electric charge can also give rise to a magnetic field, and a magnetic field, regardless of how it was produced, exerts a force on a moving electric charge.

Before the 19th century, electricity and magnetism were thought to be independent forces. In 1819, however, Hans Christian Oersted performed an unplanned experiment that demonstrated that they are intimately related.

This experiment occurred during a lecture when he accidentally placed a wire that was connected to a battery over the face of a compass and noted that the needle moved to the right. Upon reversing the battery connections, the needle swung to the left. He had accidentally discovered the interaction between electric current and a magnet.

The French physicist Andre Ampere, for whom the unit of current intensity is named, went on to demonstrate that a magnetic force generated by an electric current is indistinguishable from that of a permanent magnet. His simple experiment consisted of two parallel wires that were connected to separate batteries (electromagnets). One wire was fixed while the other was free to slide toward or away from the fixed one.

Components and magnetic field relationships

Figure 1. Components and magnetic field relationships.

When current traveled in the same direction in both wires, the movable wire slid toward the stationary one. When current traveled in opposite directions, the movable wire slid away. His experiment demonstrated that an electric current could exhibit the same attractive and repulsive forces as a permanent magnet. It also showed that reversing the connections to the battery reversed the polarity of the magnetic field by causing current to flow in the opposite direction. The changing polarity of the electromagnet, when combined with permanent magnets, is the basis for the simple DC motor.

The DC Motor

It is difficult to credit any one person with the development of the DC electric motor. Obviously, it started with Oersted'sdiscovery of electromagnetism, but many others—including  Sturgeon, Henry, Ampere, Faraday and Davenport—contributed to the development process. Unfortunately, none of these early designs had any practical value due to their low output power.

In 1873, and once again purely by accident, one appeared on the scene. In 1871, Belgian inventor Zénobe Gramme developed a high output DC generator that used 34 poles and produced a waveform that was nearly constant. During a demonstration in Vienna in 1873, his assistant accidentally connected a generator to one that was already running, and its shaft began to rotate. The same machine that produced high electrical power as a generator also produced high mechanical power as a motor.

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