Εικόνες σελίδας
PDF
Ηλεκτρ. έκδοση

multiplied by the whole current equals apparent power. The actual power is the apparent power multiplied by the cosine of the angle of lag. The cosine of the angle of lag is the power factor. Expressed in percentage,

[graphic]

10° SOR 120°/150° 100° 210 240 270° 300° 3° 360°

FIG. 21.

the power factor is the cosine of angle of lag multiplied by 100. To find actual power in watts, multiply apparent power in watts by power factor or cosine of angle of lag:

True power volts X amperes X power factor.

=

The wattless current, or idle current, is the actual current multiplied by the sine of the angle of lag. Table XIV gives sines, cosines and power factors for angles from 0° to 90°.

Examples:

1. What is the idle current if the angle of lag is 40° and the actual current 30 amperes? Answer, 19.2 amperes.

2. What is the true power if the apparent power

is 2,000,000 watts, that is, 2,000 kilowatts, and the angle of lag 25°? Answer, 1,812.6 kilowatts.

3. If the voltage is 20,000, current 500 amperes, angle of lag 20°, what is the true power? - Answer, 9,397 kilowatts.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

There is the same difference between a kilowatt and a kilowatt hour that there is between a horse-power, and a horse-power hour.

The output of a 1 horse-power engine running 1 hour is 1 horse-power hour.

Q. If a 1 horse-power engine runs 10 hours, how much greater is its output than if it runs 1 hour? Answer, the output is 10 times as great.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

Q. If a 1 horse-power engine runs 10 hours, how many horse-power hours does it give out? Answer, 10 horse-power hours.

Q. Is it then a 10 horse-power engine? - Answer, no; it is still a 1 horse-power engine.

Q. What is the difference, then, between horse

power and horse-power hour? - Answer, horse-power is the capacity of the engine or the work it can do if it runs a specified time. The horse-power of an engine remains the same, no matter how long it runs. One horse-power hour is the output of a 1 horsepower engine running 1 hour. The output of an engine in horse-power hours depends both on its horsepower and on the length of time it runs. Power is measured in horse-power. Output or work done may be measured in horse-power hours.

Q. What is the difference between a kilowatt and a kilowatt hour?- Answer, a kilowatt is 13 horsepower. A kilowatt is a measure of power. A kilowatt hour is the energy delivered or work done by 1 kilowatt running for 1 hour.

Q. If power at the rate 1 kilowatt is delivered for 10 hours, how many kilowatt hours of energy are delivered? Answer, 10 kilowatt hours.

Q. If 10 kilowatts are delivered for 1 hour, how many kilowatt hours are delivered? Answer, 10 kilowatt hours; 10 kilowatts for 1 hour will do the same amount of work as 1 kilowatt for 10 hours.

Q. How can a 50 K. W. machine be made to have an output of 500 K. W. H.? Answer, simply run it 10 hours.

-

Q. Why do you pay for electricity in kilowatt hours and not in kilowatts?-Answer, because if we paid for a kilowatt of power we could use that power for any length of time. In that case we would pay the same, no matter how long we used the power. One man might get a hundred times as much electrical energy as another man for the same money. If you hire a team to do a job of work you pay for the length of time the team works.

Q. Why do we speak of the capacity of a plant in K. W. and not in K. W. H.? Answer, simply because

we do not know how long it is to be operated. Its power is given in kilowatts. It can deliver so many kilowatts of power. Its power is the same whether it runs a day or a year.

The Metric System.

The engineer and electrical worker should understand the metric system, because it is frequently used in accurate work. It is useful because it is a decimal system. Reductions are made by multiplying or dividing by 10 or some multiple of 10.

The following table gives all the metric units used in practical work.

Metric Weights.

=

10 milligrams (mg.) - 1 centigram (cg.).
10 centigrams = 1 decigram (dg.).

10 decigrams, or 100 centigrams, or 1,000 milligrams 1 gram (g.).

1,000 grams

1

1 kilogram (kg.).

Metric Units of Length.

10 millimeters (mm.) = 1 centimeter (cm.).
10 centimeters 1 decimeter (dm.).

10 decimeters, or 100 centimeters, or 1,000 millimeters = 1 meter (m.).

1 meter

1 inch

=

=

39.371 inches

=

3.281 feet.

2.54 centimeters or 25.4 mm.

1 kilogram = 2.2046 pounds.

1 pound 453.592 grams.
1 ounce = 28.349 grams.

The fundamental unit of weight is the gram, and the fundamental unit of length is the meter. The same prefixes are used for units of length and units of weight. The first three prefixes are from the Latin. Milli means one thousand; centi, one hundred; deci, ten. Kilo is Greek and means one thousand.

« ΠροηγούμενηΣυνέχεια »