cmb said:
I don't see what you are saying.
A 'neutral' is a line with [nominally] constant charge on it (hopefully 'net neutral charge', if earthed).
A 'hot' AC line is one with periodic time-varying charge on it.
Two phases are two lines with periodic time-varying charge on them, but of different phase angles.
A two phase system is one with two lines with periodic time-varying charge on them at 180 degree apart. You get this from one phase feeding into a transformer with a grounded center-tap secondary. The two outputs will be at 180 deg out of phase = two phase.
No, you cannot tap any old number of phases off a simple transformer. If you feed one phase in, you get one phase out of one winding, or two phases out of two windings which might be 180 translated.
So what do I have if I connect the three secondary windings of a three phase transformer or transformer array in a delta configuration so as to deliver three phase two hundred forty volt power? That is three phase is it not?
I am allowed to ground one of the phase conductors but I am not required to do so. Ungrounded Delta secondary three phase transformers are used to supply loads were the failure of the power to the load may introduce a greater hazard than the electric fault would. Some of the loads that are supplied from such systems are: certain types of cranes; dust collectors in cotton mills, graineries, and explosive manufacturing plants; ventilation systems in flammable liquid handling facilities, refrigeration in pharmaceutical manufacturing, fire pumps in buildings that present a high hazard fire risk, and the list goes on. What may be installed on such systems but is again not required is a fault detection system the function of which is to alert facility personnel of the fault so it can be cleared before a second fault can occur on another phase of the supply that would cause the conductor over current protection to open.
Now I will center tap the A - C phase in order to obtain one hundred twenty volt power for receptacle outlets. Do I still have three phase or is it something else? The National Electric Code in the United States requires that I ground the center tap connection when I use it as part of a premise wiring system but I digress. This arrangement of a secondary with three windings with one of those windings center tapped to provide one hundred twenty volts for receptacle outlets is very common were the utilities transformer array will be used to supply such premises as a small manufacturing plant, a large farm, or a multi tenant building were only some of the tenants need three phase power. In that application each of the windings is often contained in a separate enclosure with the external winding terminals connected to form the Delta secondary with the primary often connected in a wye configuration. What the utilities install is three ordinary pole mounted transformers that could be used to provide 240 / 120 volt service if used as stand alone units. Often the transformer that is to supply the 120 volt loads is larger than the two that will only supply the other two phases of the thee phase premises supply. That larger transformer may also be used to supply 120 / 240 volt services to other premisses located in the same area. The transformer bushings that are at the end of each winding are connected to the next transformers winding at each end of the secondary winding. The center taps of the two smaller transformers are not connected to anything. The primary windings are connected together at one end of each winding to form a wye connected primary. The other end of the primary windings are connected to one of each of the three phases in the utility distribution network.
So without the center tap on one of the secondary windings such as in ungrounded three phase delta it is three phase. Again I ask what does it become when I use the center tap of one of the secondary windings to supply 120 volt loads?
By the way this system of three secondary windings with one secondary winding center tapped is called high leg delta by utility linemen and electricians. The phase conductor with the higher voltage to ground is customarily called the wild or stinger leg to remind everyone that it will have a higher voltage to ground than the other two phase conductors so it cannot be used to supply single phase loads against ground. In actual practice the three phase loads are supplied from power panels that have three buss bars but no neutral buss. The buss bar that is supplied from the phase conductor with the higher voltage to ground will be configured physically so that a single phase breaker will not connect to it. That phase conductor that has the higher voltage to ground is required to be coded orange at every place that it is accessible in order to warn of its higher voltage.
The one hundred twenty volt and single phase two forty volt loads will be supplied from lighting panels that only have the two phase conductors that are connected to the ends of the center tapped winding. These panels will also have three buss bars but one of them will be for connections to the grounded current carrying conductor and will not be configured physically to have breakers of any description mounted to it. The phase conductor with the higher voltage to ground is not brought to the lighting panels.
Obviously I think that this arrangement is still three phase. Additionally I cannot see how the tenants or adjacent premises that are supplied from that same array off of the larger of the three transformer windings without the third phase even being brought to those premises are two phase when they are connected to a single phase of a three phase secondary which is delta connected.
I have been wrong before and I will be wrong again but I just cannot buy a split phase being two phases.
--
Tom
