View Full Version : 3 phase power
04-26-2001, 09:55 AM
Anyone want to take a whack at explaining 3 phase power to a beginner? Despite tinkering with electrical items for years, I have never really understood what 3 phase power actually is. If normal 110v is single phase, is normal 220v double phase?
Oh, boy. I'll take a whack at it, but I don't guarantee the results. No, 220V is not "double phase." Technically, it's one single phase of 3phase power, and 110V is gotten with a center-tap neutral off a 220V transformer, so you get two 110V legs, i.e. 220V between the "outer" wires and 110 between either "outer" wire and the neutral.
For 3phase power, you have 3 wires. Between any two of them there is 220V. Between any two pairs, the AC sine wave is shifted by 120 degrees. And, in fact, at any instant the voltage of all three pairs adds up to zero. (This becomes more clear with a diagram: draw three superimposed sine waves, with one starting at 0, one at 120 degrees, and one at 240 degrees. Take the value of the the three curves at any point, add them up, and the result is zero.)
This shift among the phases is what makes 3phase motors so simple. They don't need any starting circuit. The phase shift that's put on the 3 sets of windings in the motor by the 3 phases makes a 3-phase motor self-starting.
I'm sure that is all clear as...mud.
04-29-2001, 10:02 PM
Here's some more mud. It's true that at any instant the three voltages will add to zero algebraically (observing positives and negatives.) Power, however, is proportional to the square of the voltage (P=V^2/R). Squaring the negative half-cycles results in positives. When all these positive half-cycles are added together you get constant power, just as you would with DC. (Still with me?)
Here's how one-phase and three-phase compare in motors. As you know, the familiar one-phase voltage (and current) goes from zero to a positive maximum, back to zero, on to a negative maximum, back to zero,and so on sixty times each second, smoothly and continuously. When applied to a coil, the current produces a magnetic field that goes from zero to a maximum strength in one direction, back to zero, on to a maximum in the other direction, and so on. In other words, a reciprocating magnetic field. Put a rotating thing with its own magnetic field inside this reciprocating field and you have a motor. Each half-cycle gives the rotor a boost, much the same way that a piston moving up and down makes a crankshaft go round and round. The reciprocating field is a crude but useful approximation of a rotating field. With three-phase motors there are three sets of windings equally spaced around the axis (120 degrees) which matches the "phase angle" spacing of the three supply voltages. When one winding is at maximum field strength, another is at half and decreasing, the third at half and increasing, and the sum field is aligned with the first winding. As the supply voltages vary (in sync with each other of course) the magnetic field orientation moves to the next winding, and the next, and so on. You have a rotating magnetic field. And just as the power is constant as described above, the magnetic field has constant strength. A rotating magnetic field of constant strength; ideal for driving a motor.
In a one-phase motor a reciprocating field is an adequate substitute once the rotor is spinning. But when the rotor is stationary you must have a rotating field momentarily to get the rotor to turn, hence start windings. The start winding is set at an angle to the main winding, and some way to apply a current which is out of phase with the supply voltage must be provided. The winding's own magnetic inductance may do (split phase) But a capacitor is much more effective at producing phase difference between voltage and current. (that's why capacitor start motors are used on machines with high starting loads, like compressors.)And of course some sort of relay, either current sensing or centrifugal, must be used to cut out the start winding once the rotor is near full speed. These are the complexities that add expense and reduce reliability to one-phase motors. Three-phase motors naturally have a rotating field at all times, thus have no need for start circuit. They have only one moving part, the rotor (and bearings last a long time.) Plus they have continuous power throughout the electrical cycle, a one-phase has to coast through the field strength zero, just as momentum carries a crankshaft through the piston's top dead center and bottom dead center. (By the way, did you know that your car's alternator is a three-phase AC generator?)
04-26-2002, 09:24 PM
Short and real simple explination- think of two generators sitting side by side. Both have rotating magnets mounted on the center shaft(rotor) and windings(insulated wire wrapped around iron poles) but the frist one has only has one single pole and the other has three equally spaced poles. Assume everything is the same so when the shaft rotates power is induced into the windings of the poles. In the first generator, the one with the single pole or a single phase generator, the same voltage(say 110 because voltage produced depends on the concentration of lines of magnetic flux and how fast they are going accross a conductor)will be created as the next generator but the last generator will produce more power because it has three poles or three phases. The big difference will be felt by the engine pushing the shaft, a motor for a three phase generator will work three times harder than for a single phase generator.
Motors work greatly oppisite of what generators do but are basicly poled the same. Three phase motors, like generators produce three times the hp as a single phase motor and use three times the power(not necessarly three times the current http://bbs.homeshopmachinist.net//rolleyes.gif juice http://bbs.homeshopmachinist.net//rolleyes.gif or voltage). In reality single phase motors have two windings so wiring for a motor will have 4 wires inside the motor but two wires going to it. With three phase motors six or nine wires will be in the motor and it will have three wires going into the motor. The wires may be encased in a cable so investagation may be necessary to see how many wires are really going to the motor. Most homes DO NOT have three phase power any where near them but if you do the montly service charge may be around $200 -$250 for the cheapest option(don't forget to multiply the meter reading x 3). so if you own or just bought a three phase do-hicky a three phase converter is probley the most economical answer. Hope this helps.
Q. I have heard power out west is real cheap T or F?
04-27-2002, 04:36 PM
Much like the Hudson's Bay Co. and its long, proud history of hosing Indians for furs, the bastich Government has abandoned and sold western Canada out for the benefit of two whining, sniveling leach provinces. No, we get screwed for power just like everyone else does. Power companies screw everyone, they enjoy it.
My Mitsubishi VFD can be run from single or three phase power. On single phase it gets derated 50% in hp.
04-27-2002, 08:35 PM
The power company screws me so well and so often, that I sometimes think my name is Lolita. Mike
04-27-2002, 10:39 PM
Almost all commercial generating plants produce 3ph. As one person pointed out, the alternator in your car is 3 ph. 3 hots, 120 deg out of phase with each other.
It is the most effecient way to build an electric motor. The magnetic field goes in circle, rotor tries to catch it. No starting circuit as in single phase motors, also more effecient as to power in, power out as compared to single phase motors.
Actually it is easy to work with, if motor turns the wrong dirrection, switch 2 wires, any 2.
Here's you a site to look at, even has pictures, they are worth a thousand words, or so they say.
04-28-2002, 06:53 AM
Best make that statement any two of the three incoming power wires. Do not swap the motor leads around.
04-28-2002, 10:31 AM
To sum it up, the power company delivers one leg of the 3-phase power it generates and distributes, to each of its residential customers, stepped down to 240 volts, which is split at the house in half to provide the standard 120 volts used for lighting and most plug-in appliances. Commercial customers can get 3-phase power delivered to take advantage of the greater effieciency of 3-phase electric motors.
As residential customers, we have to recreate the 3 phases from the single phase power that we get. We can use the single phase power to run a motor to turn a 3-phase generator, or, increasingly, take advantage of solid-state conversion with a VFD, which also provides an easy way to control the speed of 3-phase motors.
Like Thrud, I installed Mitsubishi VFD's in my shop (when 'last year's models' became available at surplus prices!). And he is right, the power output of the units is only half what it is with 3-phase input when driven with only single phase power, very important to keep in mind when selecting the size of unit to purchase. And there are significant advantages to the VFD's infinitely variable speed control of belt-driven machines over gear-head machines, especially when just starting out (as opposed to retrofitting).
Now if I could only find a good use for those single-phase motors . . .
[This message has been edited by Indexer (edited 04-28-2002).]
[This message has been edited by Indexer (edited 04-28-2002).]
04-28-2002, 02:12 PM
As JCHannum says, to reverse switch input wires, any 2, not any wire coming out of motor. Some have 6, some have 9 coming out of motor, connections can be changed for high or low voltage.