List the prartical application of single phase with operating
Answers
In essence, most equipment is by nature single-phase, so end-point delivery of single-phase power is thus required. Virtually everything is manufactured assuming 110 VAC single phase.
That said, long-distance power transmission such as in urban areas cannot do this because of the inherent inefficincy, thus both kinds of equipment come into play.
Typically, in urban areas three-phase power is delivered where 110V is available from any of the three with respect to neutral [not to be confused with safety ground, which should be close but usually not quite identical]. Often, each building is provided two of the three to allow larger appliances to work based on 208 V or more.
In less and less areas, there is an older form of distribution past the really primitive single-phase service that is virtually non-existent these days, and that is two-phase distribution, which is merely single phase with a mandatory center-tap and the voltage thus twice as much end to end on the output winding. Thus, many people would refer to this as “220 volt” power and in some areas they may even be right. But others will be wrong because they are actually delivered somewhat less and nominally called 208 volts. Why the discrepancy?
In anything other than three-phase, the voltages are perfectly out of phase and thus off both ends they are measured as twice either, thus the common numbers. [Note: local conditions can adjust these numbers, some areas it’s closer to 120v than 110v, but the common parlance makes a lot of assumptions. The explanation of the rarios doesn’t change.]
In a three-phase generator, the voltages are not perfectly out of phase with each other, thus there is a slightly smaller terminal voltage but still the ability to draw the same range of power, all other things the same. As such, today most equipment is rated across the entire gamut of say 200–240 volts and reasonable operation throughout.
It is the job of the local power company to balance the load, and thus which two gets to which building is guestimated and in extreme circumstances may be rewired. Even though one tap may not be used as heavily as the other two, this excess capability would be wasted, thus creating needless bulk and expense, etc. A good company should check periodically and plan well and be prepared to be wrong and fixup later, but the better the guess the less troubles in the future. Unfortunately, in some places, it takes a transformer fire to get some lazy foreman to figure out his proper job, etc.
In some areas, especially if in former days there actually was 220 volt service, they use the alternate three-phase wiring arrangement [wye versus delta] so that there is an “extra” lead that none of the customers can use. This is then off-limits to them and used only by the occasional business customer. The reason is that this third leg is 177 volts and not 110V. This is often known as the “wild” phase and is used in various ways, including a new breed of commercial lighting that actually embraces 177 volts, but mainly the commercial customer actually has three-phase equipment and is the only customer s in the area actually using the wild phase along with the two “tame” ones.
The larger and older the area, the less this “special” status transition matters and you just don’t find it the “wild” way because it is not practical, too many who need 110V and can tolerate 208V and the true three-phase customer is coincidental. Think of the way towns develop and you’ll understand how these decisions are made. At first they cater to the large businesses because they are there first and the primary reason power was brought into the area.
They usually have outdoor poles distributing the power to say a row of stores with three-phase available in every building, and then add-ons of much higher voltage are needed later, at the top of the poles with large open-frame disconnect knife switches to do maintainene, especially if there are downed wires in a storm, etc. In some better planned areas, there is underground vault wiring, but much of the USA is in a state of implementation along the entire gamut of what is possible. Thus, if you want some form of electrical engineering career where knowing this applies, you have to know all the variations and all the reasons that were used, whether short-sighted or not.
Thus, when speaking to the notion of primary distribution to an area, single-phase is virtually gone. Of course for smaller equipment within a structure, anything goes. Small application of transformers comes up a lot, truly an humongous call for small stepdown transformers that are the “brick” power supplies of billions of devices everywhere. Some include rectifiers to produce DC, but virtually all of them have a transformer to reduce the voltage to something such as 5V. There are “tiers” to the distribution as such.