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The Foundations and Fundamentals of Electricity: Part 1

The ability to generate and transmit electricity is the foundation of modern civilization.  Without electricity, the world would still be in the dark ages.  The simplified sketch of a three-phase AC generator in Figure 1 shows how electricity is produced.

Figure 1.

The generator has three windings, or coils, located 120 degrees apart in a stationary magnetic structure, or housing.  A two-pole magnet is the rotor.  The magnet is spun around, spouting lines of force from its north pole and sucking them back into its south pole.

The revolving magnet creates a magnetic field sweeping around the stationary structure, once for every rotor revolution.  The lines of magnetic force (flux) sweep across the coils; first coil A, then B, followed by coil C.  Whenever lines of magnetic force cut across a conductor (coil) or when a conductor is moved through a stationary magnetic field, a voltage is induced in that conductor.    So, as the magnetic field sweeps across each coil, it generates voltages in each one.

There is an interval of time between the instants when maximum voltage is being induced in each coil.  Figure 2 shows the voltage put out by the generator.

Figure 2.

As the rotor turns 360 degrees, the lines of magnetic force are inducing voltages in each coil.  The maximum voltage of each coil occurs at a different time, as shown by the peaks in Figure 2.  The maximum voltage of coil B lags that of coil A by the time it took for the magnet to revolve 120 degrees.  As the rotor turns 360 degrees, the lines of magnetic force are inducing voltages in each coil.  The maximum voltage of each coil occurs at a different time, as shown by the peaks in Figure 2.  The maximum voltage of coil B lags that of coil A by the time it took for the magnet to revolve 120 degrees.  Coil C lags coil B by an equal amount.  At any particular moment, the values of the voltages induced in the coils differ, the voltages rising and falling, depending on the position of the revolving magnet.

Figure 3 represents a three-phase, four wire, 120/208 volt system used in buildings having a large portion of the load 120 volts.  Small stores, office buildings, and some small industrial plants will use this distribution system.

Figure 3.

C, A, B, and N are the four wires.  The line to neutral voltages is 120 volts; the line-to-line voltages are 208 volts.  Is there some strange kind of math being used here?  120 plus 120 equals 208? Not 240?

Most quantities, such as gallons, pounds, dollars, etc., can be described by specifying their unit of measure and ordinary math used to manipulate these measures.  Other quantities, such as forces and AC generators, have attributes of direction or time.  These factors have to be considered when such quantities are to be manipulated.

In a generator, voltages vary with time.  A type of arithmetic that uses size and time relationship called “vector analysis” needs to be used to discover why 120 + 120 = 208.

To illustrate a vector, let’s use an example using forces.

When adding two forces, the direction of the forces, as well as their sizes, must be considered in order to obtain a total force.  Constructing a parallelogram based on the size and direction of the forces represents this.  The diagonal obtained from the constructed parallelogram represents the resultant (total) force vector.

Figure 4 illustrates a vector where there are two forces, one of 10 pounds, the other 20 pounds, acting 60 degrees apart on point A.  Find the single source to replace them.

Figure 4.

Vectors AB and AC represent the forces 60 degrees apart.  Each is scaled to size.  Dotted lines BD and CD are drawn parallel to AC and AB, forming a parallelogram.  The diagonal, AD, is the direction and value of the resulting force.  By measuring vector AD, we find this force to be 28-1/2 pounds.

In a like manner, the individual coil and total output voltages of a generator can be drawn.

A vector, drawn 12 units in length represent each coil’s voltage of 120 volts.  1/4” square graph paper was used, so each vector is 3” long.  Each 1/4” unit has a value of 10 volts.  Each vector, CO, OA, and OB are located 120 degrees apart.  See Figure 5.  Look at Figure 3 again.  In order to find the resultant voltage from line A to B (or A to C, C to B), a parallelogram is constructed as shown by the dotted lines OO, OB, and BB.  Measuring the resultant voltage line AB, we see it is 20.8 units long.  Since each unit represents 10 volts, the resultant voltage is 208 volts.  (Mathematically the value 208 equals 120 multiplied by the square root of 3; 1.732.)

Figure 5.

Figure 6 combines Figure 3 and Figure 5.  Now you know how 120 + 120 can equal 208.

Figure 6.

Transmission & Distribution Systems

The outstanding advantage of AC current over DC current is that AC can be easily stepped up or down with transformers.  Electric utilities use many transformers to transmit current long distances.  Lines to transmit 120 or 240 volts long distances would need to be very large to prevent voltage drop.  For transmission lines from city to city, high voltages are used, up to 500,000 volts.  Within a city, 2,400-volt lines called “primaries” are used.  By using these high voltages, power losses are kept to a minimum, and comparatively small wires can be used.  When voltage is increased, amperage is decreased for the same amount of KVA.

Most AC current is produced at 13,800 volts, distributed by 2,400-volt primaries, and then further reduced to “secondaries” of 600, 480, 240, 208, and 120 volts for final use.

Figure 7 shows a typical residential area single-phase transformer that would be on a pole, and the “drop” into a residence.

Figure 7.

The transformer primary coil has 10 times as many turns as the secondary coil.  Voltage across the secondary is 2,400 divided by 10, or 240 volts.  A center tap is taken off the secondary, called neutral, and is connected to earth ground.  The three wires go to an entrance switch and fuse box in a home as shown.  The neutral should not be fused or opened by any switch.  Any switches, fuses, or circuit breakers are in the hot lines only.  This is the 3-wire single-phase 120/240-volt system found in all residential applications.

For factories and large buildings, three-phase current is required.  Three wires are led to the areas using three-phase current and connected to three-phase transformers.

Three-phase current can be thought of as three, single-phase currents.  Figure 1 showed how the generator, more accurately called an alternator, is built.  Three sets of coils generated three circuits or phases of electricity.  The coils were spaced so the three-phases followed one another at equal intervals.  Figure 2 showed the phases A, B, & C.  Each is a single-phase current with identical characteristics.

A is one phase; B is the second phase 1/180th of a second later, and C is the third phase 2/180 or 1/90th of a second later.  The three-phases are 120 degrees apart or 1/3 of a cycle apart.

While there are three-phases, each must have two wires to make a complete circuit.  It is done using only three wires instead of six.

Figure 8 illustrates how this is done.  Wires 1 and 2 are phase A, 2 and 3 are B, 1 and 3 are phase C.

Figure 8.

Near, or even in, factories, the high voltage primaries of 2400, 4800, or 7200 volts are stepped down to 208, 240, 480, or 600 volts, using three-phase transformers or three single-phase transformers connected together.

One method is called “Delta.”  Another is “Star” or “Y.”  Normally, the primary is connected in the Delta; the secondary is either star (Y) or Delta.

Figure 9 is a common “Delta-Star” transformer system.  It is popular because it can supply 208 volt 3-phase, and 208 or 120 volt single-phase, all off one three-phase transformer with four wires.

Figure 9.

Commercial facilities with a lot of air conditioning equipment, such as hotels, hospitals, department stores, you might say any building other than residential or a factory, utilize a 480Y/276 volt, three-phase, 4-wire system, as in Figure 10.

Figure 10.

Three-phase loads feed from the 480-volt lines A, B, and C.  Dry-type transformers feed the 240-volt and 120-volt loads.  Often the lighting loads will be the line to neutral 276-volt source.

Figure 11 is a system used in factories where the bulk of the loads are motors.  This system gets 120-volt circuits by using transformers as shown.  Large plants or buildings may have one or more, or even all of these systems for power.  Many factories even have their own substations.

Figure 11.

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