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EQUIPMENT basics |
Motor/magnet magic
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A typical electric motor has three major parts: the stator (tubular piece at top), the armature (the long piece at right), and the brushes and tension springs (assembly at left). While they differ in details, almost all electric motors have this same basic structure. Photo by Scott Nesbitt |
Electric motors appear in a wide variety of places on the machines that maintain golf courses. Motors start the engines and power the cordless and corded hand tools; they power the pumps and spin the fans and generally make life simpler and more productive.
For all the differences in shape, size and application, all these electric motors operate on the same basic principles. Understanding the basic physics and mechanics of these motors can help you keep motors operating properly and troubleshoot problems if they erupt.
Motors are based on magnetism. All magnets, no matter how they are shaped, have two opposite “poles,” labeled north and south, in keeping with the magnetic poles that are at the top and bottom of planet Earth.
Similar poles repel, while opposites attract. The stronger the magnetic fields, the more power you see when the magnets try to push apart or move together. An electric motor rotates because it contains magnets that are arranged to make its magnets repel and attract.
Editor's note: This is the first in a series of three articles about electric motors in golf course equipment. |
Lining the inside of a motor’s outer shell are two or more magnetic blocks that comprise what’s called the “stator” or “field.” On battery-operated “direct current” machines, ranging from vehicles to cordless tools, these stator magnets are made of a special form of iron oxide. Unlike rust, another form of iron oxide, this special material has the unusual property of permanently giving off a magnetic field. This material, magnetite, is found in natural deposits in the earth. There are also man-made materials that comprise what are generically called “permanent magnets.”
Electromagnetic personality
There’s a second kind of magnet, called an “electromagnet.” This gadget relies on the fact that an invisible force field appears on the outside of any wire that has electricity flowing through it. If you wrap a long strand of insulated wire around a hunk of iron, this force field gets concentrated and the wire-wrapped iron becomes a magnet. Cut the electric current, and the magnetism goes away. The electromagnet “core” is made of a specialized iron alloy that doesn’t retain its magnetism. Think of it as a “temporary” magnet. In a motor that runs on the “alternating” current that comes from a wall outlet, that stator magnets are also electromagnets. With that major exception, the principles of both types of motors are similar.
In a motor, long, thin electromagnets are arrayed side-by-side down the length of a shaft that’s supported on bearings at each end, so the whole unit is free to spin. This assembly is called the “armature.”
To make the armature spin, you need to have one of the armature segments positioned next to one of the permanent stator magnets. Feed electrical current to the armature segment so it has the same polarity as the adjacent stator magnet, and the armature will naturally push away. It can only go sideways, so it rotates the entire armature.
Each segment of the armature has a mate on the other side of the armature. Together, they form a single magnet with one north and one south pole. The pairs of stator magnet segments are also arranged with the opposite poles, so each arm of the armature magnet moves.
For an even stronger movement, a stator magnet segment that has a pole is positioned so it attracts the armature at the same time it’s being repelled by its neighboring stator magnet.
Look inside an electric motor and you may see four, six or more permanent magnet segments inside the stator. You’ll also find a large number of segments on the armature. The more segments, the more powerful the motor and the more it costs to build.
The great commutator
Once the temporary magnet has moved away from the repulsive pole and is aligned with the attractive pole, the armature would naturally want to stop spinning. You overcome this tendency by shutting off the electricity to the armature segment that just moved. Then you energize the next armature segment so it moves as its neighbor did.
The job of turning the power on and off for each armature segment is handled by the “commutator” and the “brush set.” These are found in all DC motors and many AC motors, though some AC motors are “brushless.”
The commutator is found at one end of the armature. It’s a series of narrow, slightly curved segments sitting side-by-side, wrapped around the armature shaft. Electric power comes into one of these segments, travels through the wires wrapped on the armature segments, then comes out of the commutator segment on the opposite side. Electricity enters and leaves through the brushes, which are typically made of carbon. Each brush has a spring that lightly holds it against the commutator, which is made of a wear-resistant copper alloy.
Electric motors are reliable in part because there is only one major moving part — the armature. But there are a lot of things happening in the area of the brush-and-commutator arrangement. Next month we’ll take a look at common causes and cures for electric motors that don’t want to run.
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