The Amazing, Mysterious, Marvelous, and Humble Electric Motor
We all know what electricity is right? You come home from somewhere, flip a switch and lights come on. We pick up a remote control, press a button and our TV screen comes alive with moving images and sound. A twist of a knob or push of a button brings heat to cook meals, and another gizmo, when adjusted keeps our living space comfortable. We all know this, and we all probably know as well what an inconvenience it is to be without it when something happens. But what do we really know about it? Do we have any idea really, how this marvel we know as electricity accomplishs all these tasks, and all we do is press, turn, pull, twist, or push switches? Oh yeah, that and write a check each month to the power company. The purpose of this hub is to provide a baseline, or starting point for the information, to answers some "whys" and "hows." Hopefully it will make for some interesting reading, and at the end, you will be a little better informed.
One of the most common uses of electricity is to drive motors. In fact, next to lighting it may be the largest use of electricity in the nation. That's because they require more power in many cases than a light bulb and there are so many of them in use. How many, you might ask? Well while researching for this hub, I did a quick estimate and found there are approximately a hundred in our house alone. Now that may sound like a lot, but consider a large side by side refrigerator can have three or four motors in use. The compressor holds one, and the fan exhausting the warm air out is driven by another. Add one for the freezer compartment, and maybe another if the thing is a frost free design and you get to four motors on a single unit. An ice maker has one, and if there is a dispenser in the door then you have another. Computers, at least the older models have one to vent the power supply, and maybe another to cool the processor. A motor powers the CD/DVD drive too, and likely the hard drive as well. Then you can add up the air conditioners, circulating fans, personal care equipment, kitchen appliances, and tools and you might be surprised how many of them are in your own home. And if a single one stops working, it can range from an inconvenience to a near disaster. Replacing the one on my wife's washing machine last summer cost nearly three hundred dollars and was totally necessary. But as I'll explain a little later, the ones you use daily are just a few of the ones that affect your life.
Most of us know electricity in two forms. There are receptacles on the walls that will accept a two pronged plug and provide power to vacuum cleaners and coffee pots, and then there are batteries for watches, flashlights, cameras, and so forth. These two sources represent very well, the two types of electrical current, DC or direct current and AC or alternating current. Batteries supply DC and since its the simpler of the two, I will cover it first.
If you've ever seen a sine wave, the graph that shows the oscillations of electrical current (it looks a little bit like waves on the water) you have an idea of how alternating current is shown on paper. It cycles from 0 to full power, back to zero and then full power the opposite direction. There's a better explanation for that and I'll cover it later. Meanwhile, the sine wave for a direct current is simply a flat line at full power. Whatever power is available from the battery, runs through the light bulb, motor, or other device and then back to the battery via the other terminal end. Engineers drawing electrical schematics illustrate the flow going from positive (+) to negative (-) but the sources I checked for this hub state it's actually the reverse. It doesn't really make any difference as long as the job gets done. Now batteries come in different voltages, from 1.5 volts in the smaller AAA, AA, C, and D's up to 12V in automotive ones. There are others as well, including 9V which are box shaped, and then special application batteries for computers, watches, hearing aids, and many more which are specifically designed for the product they power. These voltages can be unusual like 13.4V, 18V, or 5.3V but are all still relatively low numbers.
Speaking of voltage, it is one of three terms that are used when talking about electricity and electrical power. The other two are amps and watts. For a lot of people, these are almost a foreign language and discussing them a foreign idea, but a few years ago, I came upon a simpler way of understanding them.
To a degree, we can compare electrical flow and current to water in our garden hose. When we turn on the faucet, water comes out the end of the hose in a specific amount, at a specific pressure, and at a specific speed. We can twist the handle and measure how long it took to fill a bucket. We can attach a spray nozzle to the end, and turn the water on and control the flow to an extent. We can increase the pressure to wash the car, or decrease it to mist the flower beds. We can fill tanks, or waterbeds, or open the valves wide and listen to the water as it speeds through the hose.
Electricity has some similarities. Volts can be thought of as electrical "pressure." a 12V car battery has more pressure than a 6V tractor battery. Watts are how electricity use is measured. Your power bill shows the number of kilowatt hours used, and the amount per KWH you are being charged. The comparison for the water hose would be gallons and/or gallons per minute. Amps measure the electrical flow of current and are similar to the speed at which the water flows through the hose. With these analogies, we can begin to see that electricity has some constants and all three terms fit together within those constants. The equation below explains more.
Volts(V) = Watts (W) / Amps (A)
So, if your car heater motor is running on 12 volts, and going through a 10 amp fuse, then it's consuming 120 watts or less of electricity. An increase of even 1 watt would overload the fuse and it would fail.
Now what with math working like it does, the constant shown above can be modified to apply to any of the measurements of electricity. So conversely, W x A = V, or A = W/V. You can learn more about this here. But you can see how adjusting any one of the three controlling units will affect the others and that is the primary thing you need to know.
You may have done something like this in your elementary science classes, but if you coil a piece of insulated copper wire around a nail, and hook the ends of the wire to separate terminals on a battery, the resulting current flow will create a magnetic field around the nail. In fact, a sufficiently strong battery will power a field capable of picking up other nails. The relation between magnets and electricity is a subject of study, and probably isn't totally understood, and I won't even attempt to broach it here. All we need to know at this point is that electricity can generate magnetic fields.
Also from science class, we learned that magnets have poles. These are usually called the north (N) and south (S) like the directions on a compass. We also know that the opposites attract while the likes repel. In other words, take two magnets off the refrigerator, and note that while both stick to it, they will only stick to each other if turned in the correct position. Flip them around and you can feel the force between them pushing them apart. I recall watching a demonstration where like poles were matched up on small magnets, and sliding one toward the other on a table top would cause the other to slide away until it by chance spun around and the opposites stuck together.
Since electricity can create a magnetic field around a conductive metal, then it would naturally follow that the field could be turned off or on depending on one's choice. And that is what makes an electric motor possible. The motor itself isn't a complicated machine consisting of only two major components. The part you see when you look at the outside of it is called the stator because it is stationary. It's the housing that has the mounting feet and protective structure that keeps dirt and moisture out and holds hazardous electricity inside. The illustration below shows this in better detail.
As you can see in the view above, the exterior of the motor is somewhat barrel shaped with removable end caps. The end caps are either cast or machined with a hole in the exact center for the mounting of the armature or rotating part. This rotating member sometimes called the rotor is the component that drives or applies power to the driven equipment, ie appliance, etc.
The DC motor has magnets mounted on either the inner circumference of the stator or the outer surface of the rotor. The magnets are placed at exact intervals around that location, and then opposite part is fitted with electro-magnets. When power is applied, the opposite poles in the motor pull toward each other while the like poles push against one another. This causes the rotor to rotate. Now logically, when the opposites come close to each other, they would stop, and the motor would cease to spin. Engineers designing DC motors in the late 1800's saw this and developed an ingenious little device called the commutator. It was wired into the electrical system and mounted on the rotor is such a manner as to cause the polarity (charge of the poles, positive or negative) to switch. So then the rotor would be driven by a new set of poles farther around the circumference of the stator and would continue to rotate to the new point. By keeping the commutator switching the polarity, a continuous rotation was achieved. And by pouring enough power into the motor, the rotation was strong enough to accomplish something desirable.
DC motors are handy and versatile. They're found on a wide variety of applications, but as mentioned earlier, DC is the sort of current that comes from a battery, so logically one of the largest uses would be automotive and similar types of equipment. But since DC is so readily available from AC with a simple transformer or power supply, DC motors have found a niche in applications requiring miniaturization like inside computers, where the primary current is household 120V.
Aside from the simplicity of design and easy miniaturization, DC motors are relatively inexpensive and easy to control. Since voltage is "electrical pressure," the speed of the motor can be varied by adjusting the input voltage again with a transformer. That allows multiple speed settings on windshield wipers, heater and air conditioning systems, and industrial equipment. In fact, until a relatively recent development in AC current, variable speed drives either had to be powered by DC or use a mechanical speed adjuster between the AC motor and driven load.
AC Current and Motors
During the days when Edison and Tesla argued the relative advantages of AC versus DC, a number of points were made. DC is simpler, and easier to generate and control. But AC is easier to transmit over long distances and that was the winning factor when America electrified during the last century. If you look at the electrical outlet in your home, you will probably see two receptacles, one on top of the other. Each one will have two vertical slots a half inch or so apart and one will likely be a bit taller than the other. Some may also have a U shaped hole above the vertical slots. This is a safety grounding feature and doesn't affect the operation of the electrical equipment.
Now earlier we discussed the polarity of the DC current, positive and negative. AC current has polarity as well and polarized is the term used to describe the difference in the sizes of the plug slots. But in truth, AC current has no fixed polarity like DC. That is the reason its called alternating. The sine wave pictured above shows the oscillations that alternating current makes as it flows. The polarity actually changes from positive to negative and back at a rate of sixty times each second on power supplied here in the United States. A simple way of understanding this it to look at the slots in an electrical outlet, and realize that they trade back and forth between positive and negative sixty times in a single second.
This oscillation is generally invisible in lighting applications, and doesn't affect the operation of toasters or televisions, but it does offer a whole new dimension to electric motors. As seen in the picture below, the AC motor is similar in design to a DC motor but it doesn't have a commutator. It has no need for it because AC current reverses polarity by itself. That means it's possible to wind the electrical coils in such a way that when current is applied to them, the reversal of polarity creates a rotating magnetic field that drives the rotor and produces power.
This makes AC motors also simple, economical to manufacture and operate and well suited to virtually any application where AC current is available. Still they have some disadvantages when compared to DC motors. First, they are built for a specific speed and changing that speed in the same way as a DC motor isn't possible. Remember that to slow down a DC motor only requires a reduction in voltage and that's easily done through a transformer. Hobbyists have been doing it for years with model railroads. But reducing the voltage on an AC motor will only damage it because the speed is controlled by the cycle frequency instead of voltage and while DC motors are easily reversed, AC motors cannot be without a special adaptation.
That adaptation came in the form of an Inverter. Inverters that change DC to AC and back have been around for years, but the inverter that simply modified or adjusted the frequency of AC current made its debut in the last decade or two of the twentieth century. This Adjustable Frequency control converts the AC current to DC, and then back to AC but with a changed frequency. That means that the input current is running at sixty cycles per second, but when it reached the motor it may only be running at thirty cycles per second. Cutting the frequency in half cuts the motor speed in half and a good inverter can vary the speed from maximum, all the way down to a stop and any point between.. Also possible with these inverters, is over speeding which is accomplished by increasing the frequency to a point beyond sixty cycles per second. In addition, these marvels can provide a reverse feature, a "ramp up or ramp down" feature and allow an AC motor to work in almost any place a DC motor will. "Ramping" up or down means the speed and be increased or decreased gradually over a short preprogrammed time and allows for even more flexibility of applications. However, inverters are expensive and only used when necessary.
In conclusion, electric motors are both interesting and necessary, but how necessary is often overlooked. In truth, they affect every one of us on a daily basis here in the more advanced areas of the world. On your last trip to the supermarket, did you place your items on the conveyor belt at the check out? It's powered by a motor. Electric motors run the coolers and freezers where fresh meat, vegetables, dairy products and frozen foods are stored. In some places that is a third of their offering. Was the store comfortably warm or cool? Again, motors in use. And that's just at the retail level. The production of food uses many more. Chillers, processors, conveyors, mixers, grinders, all are powered by an appropriate electric motor. Motors often run some of the equipment on the farms where food is produced including milking or grain handling equipment.
They sew our clothes, stamp out parts for appliances, cut wood, and make possible the micro electronics that operate computers, cell phones, cameras and so forth. Nearly everything in our homes from the books we read to the the house's very construction involves the use of electric motors at one or more operations.
In fact if electric motors had not been invented, life and society probably would never have developed beyond the late nineteenth century. Imagine a world where steam was the only large volume power supply. Even when steam trains progressed from wood and coal to petroleum, the the refinery depends on motors to power pumps and without them wood or coal fired trains would still be the norm with belching clouds of smoke and burning embers.
Cars would still be hand made, one at a time and probably in small quantities. Modern electronics would not exist, and even electrical lighting on a large scale would need mechanized factories where the bulbs are manufactured. Most food would be grown by those who eat it with only the basic staples available at small stores a fraction the size of a Kroger or Safeway. And of the automobiles that did exist, all would be started with a manual crank like the Model T, because the starter is actually a DC motor. No electric windows, no forced air ventilation or air conditioning, and windshield wipers if even in existence would be powered otherwise. Electric seats? Nope. And no recorded music beyond the Victrola The increase of productivity and development of new products has been largely because of the motor.
So next time you load your laundry equipment, get out your vacuum cleaner, turn on the music, open your refrigerator door and hear it "kick" on, or leave for an errand in your car, think about the role the electric motor played in it. And should your washer suddenly stop with a load of clothes and water in the tub, or your refrigerator give up with a week;s worth of food inside, while you're bemoaning your bad luck, take a moment and marvel at how many other of these selfless little mechanical servants are maintaining your life. The electric motor is truly amazing, mysterious, marvelous and humble.