2-Phase or 4-Phase Power

I think 3 phases leads to the most economical design of motors and generators, for long distance power transmission 3 phase bi-phase is used the voltage on one set of three cables at 180° to the other three.

I noticed when I did an instalation of a machine in America the voltages on the three phase were not symetrical to ground so that you got 220v between two phases that puzzled me!

In our 3-phase system (USA) The three phases are offset by 120° each, so that in a 3-phase motor, some winding is always near peak voltage/current. This reduces the power required to avoid stalling. The three phases ARE symmetrical to ground, as long as you take phase into account.

Murphys,

I'm not sure what you meant by "not symmetrical to ground .... that puzzled me".

It may have had to do with the way the service transformer was tapped, or whether the secondary was a delta or wye configuration, unbalanced loads on the phases, or perhaps the way you measured it, and with what. As I'm sure you know, measuring AC voltage with high impedance voltmeters will often yield readings that are completely unrelated to any available current capacity and are therefore spurious for any ordinary purpose; the voltage representing merely trivial "leakage" or, capacitive or inductive coupling in the circuit.

Greg

From Murphys/syhprum

The asymetrical supply I encountered was in a small print shop in Philidelphia as far as I could see the supply was phase to phase 220v but to enable a lower voltage supply for single phase applications one phase was tapped at 17v from the wye point and brought in as a neutral/ground, the machine I was installing was a HELL scanner/recorder that was supplied with a multi tapped 6KVA transformer with a 400/230v output (Delta-Wye).

I connect the transformer with a 220 v phase to phase delta input and found the output to quite normal.

Murphys,

Interesting.

Thanks for the reply and explanation.

I too have encountered many "puzzling" situations in various plants. Sometimes there was a good reason for them that was not apparent to me. In other cases it was the result of something that was done expediently or even improperly.

Greg

There are a few facilities around that are unable to provide three phase power that has a reference to ground; one of my customer's older facilites is such a case. It plays hell with servo equipment if not adequately designed for. We end up having to add a three phase transformer (usually not required) and then setting up the proper reference on the secondary.

I'm glad I'm not the only one who has encountered this problem my buddies back in the UK thought me mad when I tried to convince them of the reality of this problem

I too had an "interesting" experience with servo "grounding".

I was called in to finish a half completed project involving an automated wire forming machine after the original engineer quit. The servo power supply called for a particular transformer type (which was located several feet away from the control cabinet)with a tap on the secondary to which a heavy wire was to be connected to the power supply: it looked for all intents and purposes to be a ground, and was connected to ground at the transformer, but the literature made clear that the heavy wire to the power supply was not a ground but was rather a current carrying return that could exceed 200 amps at times. I followed the instructions and it worked fine. I carefully made a wiring diagram for connection at the customer's site with a bold warning about the nature of that connection. The machine arrived in Canada and was wired appropriately until the electrical inspector said it violated the code and he changed the grounding scheme. (Codes do in fact allow for exceptions related to electronic equipment, and/or single point grounding systems but many inspectors are unaware of this). To get to the point, when I flew up there and found the change out I was dumbstruck. The steel building frame had become part of the circuit and during operation there would be intermittent sparks where metal objects came in contact with the girders. Pieces of wire scraps on the floor were literally welded together and to parts of the machine ... and were continuing to spark in places as I watched in amazement. A pair of pliers touching a piece of equipment 20 feet away was welded to a girder! The customer had wondered why that was happening, yet had never contacted me about the inspector having him change the circuit by grounding the power supply at a point other than that secondary tap. Now that was a "ground loop".

A few points to consider. 1. Given a three phase delta theoretically you can tap it anywhere and establish a different voltages to ground. 2. Less common today but not uncommon in the past was the corner grounded delta. Drawing a quick schematic will illustrate it's usefullness under certain applications. 3. A midpoint grounded delta with a "high leg" is another less common but at one time not uncommon installation. In industrial installations you can run into many different variations depending on the original supply, design, and usage. In the US you can probably run into a countless number of situations where the end user transforms the standard three phase balanced service into something unbalanced and asymetrical especially in very old installations.

Unfortunately in our case its a floating ground (actually a floating supply); only referencing that occurs if a neighboring piece of equipment (usually old) was "influencing" it away from a balanced condition. Usually one leg measures about 35V to ground. Have seen a few pieces of equipment let out the magic smoke if it couldn't handle it. Still one of our primary customers so we just cost in the extra transformer.

the power setup that you first refer to is called "B" phase ground it was and still is used in a lot of places ... wisconsin is real big on it .... and there are still pockets of it within the city of chicago it is a real pain in the a$$ when you are troubleshooting machine tools especially if you haven't seen it before ... hwne you measrue phase to phase you will get 480 or close to it but when you measure phase to ground it will look like B phase is dead... high leg 240 is still also very common especially in places with older systems you can load the hell out of the high leg when you start big motors and still not drop out the other two as much... servo controls especially FANUC and Mazak just detest that voltage ... they run so much longer and nicer on 208

This reference to a high leg rings a bell this is what a local electrician spoke of.

I see now how this arises if an additional 17v is added to the centre point of two phases relative to ground one of the phases will be augmented and making the high leg while the other will be reduced to approximately to that of the third leg.

It is good after 30 years to get an explanation of this problem.

Back in the UK the transformers that were supplied with the HELL scanners sometimes had delta inputs and sometimes Wye often when I went out to service scanners that I had not installed I found that the centre point of the wye on the input had not been connected leading to some odd voltages around the system!

you can obtain two phase by link in way of scott-t. you can get the transformer easily

Different numbers of phases other than 1 and 3 were/are used but except for very specialized applications became obsolete.

Part of it goes back to the inventor of the AC motor: Nikola Tesla. His best designs were 2 and 3 phase. While the original Niagra Falls AC generators he designed were 2 phase, at 25 Hz (The largest AC generators in the world at the time.), and later on 2 phase power was used in some rural areas for distribution to farms because it eliminated the need for a third wire, the advantages of 3 phase power made it the world standard. 3 phases, each 120° apart is the most efficient way to generate, distribute and finally provide to end users electric power in either single or 3 phase service.

Using 3 phases, motors achieve a rotating magnetic field with just basic windings, and the direction of rotation can be changed merely by switching any two of the phases. The added complexity of a greater number of phases would only increase the number of wires needed, and add to the expense of every component in the generation and distribution system while adding little or no benefit except for very specific applications. Less than 3 phases will not produce a rotating field in a straight induction motor. (Rotating fields are achieved in single and 2 phase motors typically by using a more complicated winding and rotor, the inclusion of inductors and/or capacitors to produce phase shifts, etc.) Single phase motors are less efficient and are therefore rare in ratings much above 10 HP.

Check the link below for a reasonably good history and explanation of AC power, the number of phases, and why.

http://en.wikipedia.org/wiki/Three-phase_electric_power

Another link that explains the history of electric power from its beginnings up to the introduction of AC is also well worth reading.

http://americanhistory.si.edu/powering/past/prehist.htm

yes, it is a correct ans.

It offers a smooth load to the generator and uses the minimum amount of materials to get the power from where it's made to where it's used.

The phases must always be of odd numbers to induce a phase shift for motor operation. Even what is commonly called "2-Phase" in American homes is really 2-Pole 120/240 single phase. The voltages and currents from the 120/240 are in phase and add with no difference in phase angle. This is accomplished by using a center-tap utility power transformer on the power pole. This is usually 13.8kv to 120/240v single phase done with one transformer.

Occasionally in medium sized commercial operations, you might encounter a 120/208 3-phase "Open Delta" system where a three phase delta is established with only two stepdown transformers. This type of three phase system is uniquely identifiable by the presents of one high leg. The high leg will generally measure 10% higher voltage (with respect to ground) than the other two legs. The Open Delta can provide the same efficiency gain as pure 3-phase power for small motors. However the open Delta is limited to 58% of the total power that would normally be available to a 3-phase system.

Am I correct in believing both these configerations are used I do not recal the high leg being as high as 196v but the second one giving 142v seems really eccentric!

Excuse the rather poor art work but you can spend hours getting this sort of thing just right