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Alternating current electricity switches direction 120 times per second, going through a full cycle from zero to a peak voltage, back to zero, then to a peak in the other direction, and back to zero, 60 times per second, or 60 hertz (Hz) (In some countries it is 50 Hz rather than 60 Hz.) If the voltage were plotted over time, it would be the sine wave we encountered in school. The voltage (pressure) is averaged by a fancy technique (RMS - Root Mean Square) so that the subsequent computations are simplified. The only time you normally have to worry about the actual peak voltage is in the insulation on the wires... the 120 volt (RMS) electric service in most homes actually around 300 volts peak.
The normal power distribution system draws electricity from the generators at three different points in the generator's rotation, and is distributed throughout the country on three separate wires, called phases. This is why you always see three wires on the giant towers between cities (or to different parts of cities). The peak voltage on the second phase is slightly later than the peak on the first phase, corresponding to the 1/3 of the way through the generator's cycle. Likewise the third phase is 2/3 of the way through the generator's rotation.
When the power is brought to a building, a transformer brings the voltage down to the 120 or 240 volt level normally used. For three phase power, three transformers are required. For residential use, a transformer is required for each few houses, historically atop a nearby power pole. To simplify, one phase is used a group of homes, with the other phases serving other homes.
Single phase power is ample for lighting. But for motors, single phase power is like a bicycle with a single pedal - at least the expert type bike where the rider's shoes clip to the pedals so he can both push and pull. (You can think of it as a standard two pedal bike if you prefer, where you only push the pedals, but that doesn't make as good an analogy, since the electrical power pushes and pulls). When starting a bike, you need a little kick to start the pedals turning before the normal hard downward pushing does any good. In a single phase motor, the starting capacitor gives it the little kick that starts it moving, but it still is a very hard push to get going. Very hard push, translates to high starting current, which becomes heat in the motor. Smaller motors (up to a few horsepower - hp) can dissipate the heat from their lower starting current, but as you get to the larger single phase motors, the starting heat can build up, which is why some experts recommend not starting a larger single phase motor more than 6 times per hour, and other vendors recommend as few as 3 times per hour. And over about 10 hp, single phase isn't practical at all.
Large (industrial) users receive all three phases. Three phase power is like a three pedal bike (or 6 pedal if you want a separate push-push pedals rather than push-pull). Now with the power applied at different parts of the rotation, starting is not such a big deal. If you have a big motor, or one that must be started frequently (like an elevator), this is the way to go. If you don't have three phase power in your area, the power company may charge you to bring it to your property - sometimes the cost is modest, but I have heard of as much as $50,000 per mile. I know somebody who built a house in the country. I noticed that he had 3 phase power at the meter, but the contractor had only installed a single phase meter and run single phase power to his home. If he wanted three phase power, he would only have to bring it from the meter (I should be so lucky!)
Back in the dark ages (when I was an electrical engineering student), it was common to convert any type of power to any other type of power by connecting a large motor to a generator that created the desired power. A single phase motor could be connected by a mechanical shaft to a three phase generator. Since starting a large single phase motor is hard, we sometimes would start the motor without a load, then gradually add the load of the generator using a clutch. Even with a "hard" connection, we would certainly start the motor without an electrical load on the generator. If your generator needs to handle a large starting current, a large flywheel on the motor generator can help - it helps keep the generator spinning as it has to push the extra starting load. The three phase power is quite "clean" - pretty sine waves in the graph of the voltage generated.
If you crank the theory long enough, you can modify the motor that would have run the motor-generator so that one of the three phases just comes from the primary power line, and the converter only has to generate the differences for the second and third phase - no separate generator. Or more specifically, some of the windings of the converter function as a motor and other windings in the same machine function as a generator, adding to or subtracting from the incoming power for the second and third phase output. The power is fairly clean, almost as good as from a regular generator. The better quality units have cleaner power, sometimes achieved by using electronic components to clean up the power, ultimately reaching the quality of the power from the big rotary generators used by the power company.
The technology of the rotary converter is well proven, but the rotary converters need to be fairly large (1.5 to 2 times the size of the largest motor it will have to start, or more if there is a large starting load, plus total running load of all the motors that could be used concurrently. The starting load is based on the sum of the motors that start at the same instant (so use separate switches for separate motors like a dust collector.) The rotary converter is noisy; it is fairly efficient under load, but uses a fair amount of power "idling." Think in terms of $3,000 and up to buy one. A well-known vendor is Kay Industries, who (unlike most companies) rates their units based on the largest motor to be started rather than the more common "running load" times a multiplier.
Once a three phase motor is started, if power continues to be provided to only one of the three phases, the motor will continue to run and produce up to 70.7% of full power. Many simple (cheap) static converters just use the "extra" windings of the motor to get it started, like a starting capacitor circuit on a single phase motor. Sometimes this cheap solution is adequate, and 2/3 of the power from the motor is sufficient. Although this concern is not widely discussed, I wonder how the motor will wear since all of the power is provided through one side of the physical motor, especially if it is used near the maximum available power.
The term "Inverter" has come to be used for this type device. Electronics can be used to "develop" 3 phase power. Early systems were very expensive, and produced "dirty" power (poorly shaped waves with extra harmonic frequencies, etc.), bad for the devices using the power (an old used unit may not be a bargain). Electrical systems, even three phase motors, were sometimes damaged by the dirty power from these early systems. The technology has improved dramatically, until the current units are quiet, efficient, and produce relatively "clean" power. (Solar and wind power have to be converted this way, so may have contributed to the maturing of the technology.) These inverters are quiet and efficient, and the cost of good units has dropped dramatically. They are now cost competitive with rotary phase converters, and may be making that technology obsolete. Today, this is what I would buy for my own use. Check out Phase Perfect as a vendor.
The rotational speed of a motor is largely determined by the frequency of the power provided - 60 Hz (or 50 Hz in some countries). A motor that can run on either 50 Hz or 60 Hz power will run 5/6 as fast on 50 Hz. If we are going to electronically generate the 3 phase power, the frequency could be varied also (VFD)... and suddenly we have a variable speed drive. Of course, if we start running the motor at half or twice the "normal" frequency, there may be unusual loads on the motor... the speed the bearings need to support, the heat generated in the windings, the effectiveness of internal cooling fans, and so forth. A motor designed to handle these variable speeds can work with a phase converter that is also a frequency converter, giving a variable speed system for a modest extra cost. Even most regular motors can be used for brief periods at a different speed and light loads, such a drill press. Motors with a "soft start" such as routers, use this technology.
If you buy three phase power from the electric company, you will have a special three phase breaker box, and your three phase wiring will require 4 or more wires. If you make your own three phase power you have the same issue... you will start with a large single phase circuit to run your converter, then have to distribute the three phase power. That requires a three phase breaker box to protect the wiring and machinery, and perhaps special three phase plugs. Hint - WalMart and Home Depot don't normally sell three phase boxes, breakers, plugs, or outlets. Wiring isn't hard, but you need to find a distributor who sells to commercial electricians who will sell you these components.
If you only have one three phase machine, it may be practical to connect the converter directly to the three phase machine, and control the single phase power going into the converter. Some converters have the protection for both the motor and the converter built into the converter. As the cost of converters goes down, it may pay to buy separate converters for each 3 phase machine, to avoid the cost of distribution (breaker panels, wiring, and plugs) for three phase wiring. Some apparently single phase machines have a three phase converter built into the machine, to take advantage of the three phase motors.
Rotation: The three "hot" wires in three phase are often called L1, L2, and L3. Any of the 3 wires goes to any of the three connections, BUT if the motor rotates the wrong way, switch any two of the wires.
As noted above, a single phase motor has little starting torque, so draws a large starting current - sucks up a lot of power to get moving. A lot of that power becomes heat in the motor, and with a larger motor, there are a lot of windings and iron. Therefore larger motors, especially single phase motors, "like" to be left running - or at least not started and stopped frequently. (Some experts recommend no more than 6 starts per hour for a large single phase motor, one dust collector vendor suggests starting their single phase dust collector no more than 3 times per hour.)
Less easily explained is how the motors warm up as they are used, and the bearings expand and lube differently. The bandsaw tires and power belts warm up as they run. Therefore when I start my big bandsaw after it has been idle for a while, I let it run 20-30 seconds or more before I start to use it - it is happier, and I seem to get a better cut, which makes me happier.
Since a three phase motor naturally has a starting torque, the starting issue is less important than with a single phase motor. The reasons to leave a three phase motor running between cuts are less important, but there is still an argument to leave it running for cooling and lubrication.
Note that you rarely see a single phase motor over 5-10 hp, because of the difficulty of starting it.
Most larger motors, single phase and three phase, have "magnetic" starters. These are relays that turn the power on and off, providing overload protection, and special contacts that aren't destroyed by the sparks that normally occur when starting and stopping a motor. That provides a safety factor as well - if there is a momentary power failure, the machine stops, and doesn't start when the power returns until you push the start button. If you want an extra start button, you aren't running the machine power to the start button, but only the power to momentarily connect the relay (sometimes called contactor). If you want multiple start buttons, no problem... just hook up another, and any one of them will do the job. The relay/contactor not only starts the motor but connects the power to hold the relay "on." How do you turn it off? All the time it is on, the power to the relay (not the power to the motor) goes through the off button, which is normally connected. When you push the off button, you interrupt the power that holds the relay connected, and the machine stops. If you want multiple off buttons, the power to the relay has to go through all of the stop buttons (connected in series), so any one of the stop buttons can interrupt the power to the relay and stop the machine. Start buttons are normally open, connected in parallel. Stop buttons are normally closed, connected in series.
If you happen to have three phase electric power available, you either will be treated as a commercial customer, or in some areas, have the option to buy power as a commercial customer. Your cost per kilowatt hour will be substantially lower. But remember that the power company has to maintain the generating capacity and a distribution system sufficient to handle your peak load. Therefore if you are a commercial customer you will have an additional meter that measures that demand (peak load). There is an extra charge for your "Demand." If your air conditioner starts at the same instant that you happen to start your saw and dust collector, you could have a very large peak. The power company has to respond to that, and charges extra for that peak demand (often for the next six months or more, not for the 10 seconds you used the peak). If you aren't careful, your "demand" charge can be larger than your "power" charge. But if you carefully manage your peak loads, your power company will be happier, and you can minimize the demand charge and benefit from the substantially lower rate per kilowatt hour of power used. Sometimes a way to keep the demand low is as simple as using separate switches to start the dust collector and each machine, so they don't start at the same instant. Don't use a master switch to turn on your shop lights, dust collection, ventilation, heating/air conditioning, compressed air, etc.
You may also have a charge for power factor. Some items, like the common induction motor, draw a significant current, but when they are idling they push the current back into the power grid each cycle (take and return 60 times per second). The peak current flow is not at the same instant as the peak voltage - they are "out of phase." The actual power used is the voltage times current at any instant, but if they are completely out of phase, the power could be zero. The power company still has to provide the distribution system to support your taking and returning the current, so sometimes has an extra charge for the "power factor." You can provide devices to improve the power factor and thus reduce the extra charge, or you can pay the power company if the power factor of your load is "bad," and they will provide the large capacitor on a power pole near you.
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