The Golden State: Where & How to Live, Secure, Visit, Enjoy and Thrive in California

Practical Electricity And House Wiring

In a three-phase motor, which has three separate windings, the rotor gets 7,200 pushes every minute in each winding, or 21,600 pushes altogether considering all three windings. The motor will actually be much smaller and less expensive to build, and much simpler in construction than a single-phase motor.

A 3-phase generator consists essentially of three single-phase generators on a single shaft, so spaced that the maximum voltages of the three generators come at points spaced one-third of a revolution apart. (The three separate generators are wound so that they look like a single winding.) The leads from the three generators are so interconnected that only three wires are used in total; the current on any two of the wires is single-phase current; the power from all three collectively is known as three-phase current. Sometimes a fourth grounded wire is used. Three-phase as compared with single-phase current, may be likened to power from a 3-cylinder engine as compared with a 1-cylinder engine—it is smoother, less pulsating.

Watts in Alternating Current.—In Chapter 1 we learned that volts X amperes — watts. This is strictly correct for Direct Current, but is correct in Alternating Current only with lamps, heating devices and a few other devices. If the device under consideration is a motor, a transformer or any other device which falls into the classification of "inductive load," then it is no longer correct. In Alternating Current work, volts X amperes — volt-amperes or "apparent watts" (1,000 volt-amperes — 1 Kilo-volt-ampere or KVA).

For example, in measuring the power consumed by a motor we might find 115 volts, 6 amperes, equivalent to 690 volt-amperes. A watt-meter connected at the same time would probably indicate around 450 watts.

Power Factor.—The ratio between watts as measured by a watt-meter, and the volt amperes, is the "Power Factor" for that device. 450/690= 65.2; this particular motor has a Power Factor of 65.2%.

The theory and explanation of Power-Factor is beyond the scope of this book; it can be found in any book on Electrical Engineering.

Appliances-Home Repairs

If we connect a length of No. 18 copper wire exactly 2072 feet long, directly across a 115-volt line, and then measure the current flowing through this wire, we will find that exactly 1,000 watts are flowing.

If instead of copper wire we use aluminum wire of 97.5% purity, we will find that the watts have dropped from 1,000 to about 600. If we use zinc wire still the same size and length, the watts will have dropped to 277, with an iron wire to about 170, with nickel wire to about 130; if we substitute a column of mercury of the same length and thickness as a No. 18 wire, we will find only about 17 watts.