Re: 208V 1Ph vs. 208V 3Ph?
It?s hard to get into theory, without having a blackboard and the freedom to get deeply into math. But here?s a sniblet of theory.
Start with two things. (1) Three coils of wire so oriented that they have a common axis (i.e., the axis of the motor?s rotor) and are separated in space by 120 degrees of arc. (2) Three power connections so scheduled that their respective peak values are separated in time by 120 degrees of phase. You will get two extraordinary results. The first is that the magnetic field created in the three coils will rotate about the axis at a constant rate. Note that with no moving parts, we have a rotating magnetic field. That is what causes the rotor to move - it follows the motion of the rotating magnetic field. The second is that the power drawn by the motor is constant. I mean it does not vary from millisecond to millisecond; I am not discussing changes in mechanical load. By contrast, the power drawn by a single phase motor follows the ups and downs of a sine wave, with power being positive most of the time and negative a small part of the time (i.e., the motor is giving power back to the utility). (See the detailed discussion below).
I agree with spsnyder, in that motors above 1 hp should be three phase, rather than single phase, regardless of what voltage is available within the building.
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NOTE: WHAT FOLLOWS LOOKS LIKE MATH. IF YOU DON?T CARE FOR MATH, PLEASE FEEL FREE TO MOVE TO ANOTHER TOPIC.
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I believe that the biggest single advantage of three phase motors relates to power. By this, I specifically mean power in the sense of the rate of use of energy. We have all seen the formulas for single phase power (P=V x I) and three phase power (P = sqrt(3) x V x I) (NOTE: I?m talking about KVA, and not KW, so that we don?t have to consider that bizarre ?power factor thingy?).
I?ll not show the math here, but only talk about the math. It uses trigonometry, which I always enjoyed ? but it may prove distasteful to others. I can send it or post it, if there is an interest. I can also describe how to construct a spreadsheet that shows a clear picture of the results.
First look at single phase: The voltage and current will be out of synch with each other by some amount that has to do with inductive and capacitive loads. But both are sine waves. If you (using trigonometry) multiply the voltage sine wave by the current sine wave (which is generally around 30 degrees behind the voltage), the result will still look something like a sine wave. But it will not have half of the cycle as positive power and half as negative power. It will look like it was lifted up above the zero axis, so that most of the power is positive and only a small part of the bottom of the curve will be negative. What this means is that a single-phase generator is putting out a different amount of power at every moment in time, that the power varies as a smooth curve, and that it is mostly positive (i.e., power is used by the motor).
Now look at balanced three phase: Lets call Va, Vb, Vc, Ia, Ib, and Ic the three voltages and the three currents. The trigonometry would have you multiplying and adding sine waves as follows:
Va TIMES Ia (which is 30 degrees behind Va),
ADDED TO
Vb (which is 120 degrees behind Va) TIMES Ib (which is 30 degrees behind Vb),
ADDED TO
Vc (which is 120 degrees behind Vb) TIMES Ic (which is 30 degrees behind Vc).
This extraordinarily messy set of terms gives a result that is extraordinarily simple: the power is constant! At every moment in time, the power used by a three-phase motor is the same.