3 phase ?

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Old 08-25-02, 06:24 PM
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Question 3 phase ?

Just curious. What is the difference between 3 phase 220 and 1 phase 220? And when a building has 3 phase coming in how does 1 phase equipment run? Is htere a second set of wires bring the 1 phase in? Thanks Tim
 
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Old 08-25-02, 08:19 PM
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Maybe someone can refer you to a site that has some drawings on it. I am trying to come up with a good explanation here and so far I have trouble wording it.
 
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Old 08-26-02, 05:28 AM
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Take a look at this and see if it helps. You are probably talking about 120/240 center tapped delta.

http://home.att.net/~benmiller/elecsys.htm
 
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Old 08-26-02, 09:21 AM
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Overly simplified:
3 phase power has each hot conductor, 120 degrees offset from each other. Typical voltages are 208/120 or 480/277V.
Residential has two hots that might be originilly derived from portions of 3 phase, but goes through transformers to result in 2 hots, each 180 degrees offset from each other.

208V single phase motors work somewhat similar to 120V single phase motors. 208V three phase motors have different type of windings that permit them to take advantage of the 3 simultaneous hots and make them more efficient.
 
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Old 08-26-02, 09:23 AM
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If you want to learn about 3-phase systems it's best to start with a 120/208 3-phase 4-wire system.It's a"flexible" system for both lite and power.There are three "phase wires, A,B,& C, and one Neutral. There is 208 volts across any 2 phase wires and 120 volts across any phase wire and the Neutral. The Neutral wire is Grounded, usualy to an undergroung metal water-service pipe, and is ALWAYS identified with the color White.The three "phase" wires are "live" wires because they read 120 volts-to - Ground and are NEVER White wires of Green wires. The three "phase" wires are connected to either fuses of circuit-breakers. The Neutral (White wire) is NEVER connected to a device that would open the circuit. A three-phase motor is the simplest of machines because the motor-shaft is the only moving part. The three external phase wires connect directly to windings in the motor frame. The 3-phase 3-wire windings form a "rotating magnetic field" which causes the motor-shaft to rotate.This is because of a "time-displacement"- Phase "A" has a mamxium "Positive" voltage value 120 time-degrees before Phase B which inturn leads Phase C by 120 time-degrees. If you were connecting 120 volt lighting loads to the system you could connect a 120 volt 1500 watt load across Phase "A" and the Neutral-a 2-wire branch-circuit using a Black wire and a White wire. Next you could connect a similiar1500 watt load to Phase "B" using a Red wire and a White wire.Next would be the same 1500 watt load connected to the "C" phase with a Blue wire and a White wire- 6 wires for 3 circuits. But you could accomplish this with only 4 wires by using only 1 White wire-the Neutral -if you "balance" the loads. One Neutral can connect to three or two 120 volt loads if each load is connected to a different phase. 2 loads on three wires could be one Red (phase B),one Blue (phase C) and one White (the Neutral)-this is one of three combinations for 3-wire circuits connected to two 120 volt loads-the others are Black-Blue-White and Black-Red-White. Color-coding the phase leads is very important in muti-wire branch-circuits connections.
 
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Old 08-26-02, 10:01 AM
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Ichbod lnked a drawing of wiring configs,,, anyone know where there is one of a wave or cycle?
 
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Old 08-26-02, 02:13 PM
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Old 08-26-02, 04:03 PM
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Here's my expanation of why Phase A = 120 volts + Phase B = 120 volts = 208.----Phase A is a Positive vector, 120 volts at 0 degrees. Phase B is a Positive vector, 120 volts at 120 degrees. We have two 120 volt coils in series, and if A is Positive then B is Negative for "additive" polarity. This means we add the "back-vector" of B to A .The "back vector" is 120 + 180 = 300 degrees, so B is 120 volts at 300 degrees,which is 60 degrees from A. The sum of A + B is a vector that bisects the 60 degree angle, a vector 30 degrees from A. 1/2 the lenth of this vector is 120 volts X cosine 30 degrees = 120 volts X .866. The full lenth is 120 volts X 2 X .866 = 120 volts X 1.73 = 208 volts.
 
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Old 08-26-02, 07:20 PM
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Brain Overload :-)

Thanks everybody for your explainations & links.
 
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