تبلیغات
سوپر مارکت اینورتر - مقدمه ای بر الکتریسیته بخش هفتم

امروز:

مقدمه ای بر الکتریسیته بخش هفتم

» نوع مطلب : آموزش‌ ،

Alternating Current

 

The supply of current for electrical devices may come from

a direct current (DC) source or an

alternating current (AC)

source. In a direct current circuit, electrons flow continuously

in one direction from the source of power through a conductor

to a load and back to the source of power. Voltage polarity for

a direct current source remains constant. DC power sources

include batteries and DC generators.

By contrast, an AC generator makes electrons flow first in one

direction then in another. In fact, an AC generator reverses its

terminal polarities many times a second, causing current to

change direction with each reversal.

 

AC Sine Wave Alternating voltage and current vary continuously. The graphic

representation for AC is a sine wave. A sine wave can represent

current or voltage. There are two axes. The vertical axis

represents the direction and magnitude of current or voltage.

The horizontal axis represents time.

When the waveform is above the time axis, current is flowing in

one direction. This is referred to as the positive direction. When

the waveform is below the time axis, current is flowing in the

opposite direction. This is referred to as the negative direction.

A sine wave moves through a complete rotation of 360

degrees, which is referred to as one cycle. Alternating current

goes through many of these cycles each second.

Basic AC Generator A basic generator consists of a magnetic field, an armature,

slip rings, brushes and a resistive load. In a commercial

generator, the magnetic field is created by an electromagnet,

but, for this simple generator, permanent magnets are used.

An armature is any number of conductive wires wound in loops

which rotates through the magnetic field. For simplicity, one

loop is shown. When a conductor is moved through a magnetic

field, a voltage is induced in the conductor. As the armature

rotates through the magnetic field, a voltage is generated in the

armature which causes current to flow. Slip rings are attached

to the armature and rotate with it. Carbon brushes ride against

the slip rings to conduct current from the armature to a resistive

load.

An armature rotates through the magnetic field. At an initial

position of zero degrees, the armature conductors are moving

parallel to the magnetic field and not cutting through any

magnetic lines of flux. No voltage is induced.

Generator Operation from As the armature rotates from zero to 90 degrees, the

 

Zero to 90 Degrees conductors cut through more and more lines of flux, building up

to a maximum induced voltage in the positive direction.

Generator Operation from The armature continues to rotate from 90 to 180 degrees,

 

90 to 180 Degrees cutting fewer lines of flux. The induced voltage decreases from

a maximum positive value to zero.

Generator Operation from As the armature continues to rotate from 180 degrees to 270

 

180 to 270 Degrees degrees, the conductors cut more lines of flux, but in the

opposite direction, and voltage is induced in the negative

direction, building up to a maximum at 270 degrees.

Generator Operation from As the armature continues to rotate from 270 to 360 degrees,

 

270 to 360 Degrees induced voltage decreases from a maximum negative value

to zero. This completes one cycle. The armature continues to

rotate at a constant speed causing the cycle to repeat as long

as the armature rotates.

Four-Pole AC Generator An AC generator produces one cycle per revolution for each

pair of poles. An increase in the number of poles, causes an

increase in the number of cycles completed in a revolution. A

two-pole generator completes one cycle per revolution and a

four-pole generator completes two cycles per revolution.

Frequency The number of cycles per second of voltage induced in the

armature is the frequency of the generator. If a two-pole

generator armature rotates at a speed of 60 revolutions

per second, the generated voltage have a frequency of 60

cycles per second. The recognized unit for frequency is hertz,

abbreviated Hz. 1 Hz is equal to 1 cycle per second.

Power companies generate and distribute electricity at very low

frequencies. The standard power line frequency in the United

States and many other countries is 60 Hz. 50 Hz is also a

common power line frequency used throughout the world. The

following illustration shows 15 cycles in 1/4 second which is

equivalent to 60 Hz.

Amplitude As previously discussed, voltage and current in an AC circuit

rise and fall over time in a pattern referred to as a sine wave.

In addition to frequency, which is the rate of variation, an AC

sine wave also has amplitude, which is the range of variation.

Amplitude can be specified in three ways: peak value, peak-topeak

value, and effective value.

The peak value of a sine wave is the maximum value for each

half of the sine wave. The peak-to-peak value is the range

from the positive peak to the negative peak. This is twice the

peak value. The effective value of AC is defined in terms of an

equivalent heating effect when compared to DC. Instruments

designed to measure AC voltage and current usually display the

effective value. The effective value of an AC voltage or current is

approximately equal to 0.707 times the peak value.

The effective value is also referred to as the RMS value. This

name is derived from the root-mean-square mathematical

process used to calculate the effective value of a waveform.

Instantaneous Value The instantaneous value is the value at any one point on the

sine wave. The voltage waveform produced as the armature of

a basic two-pole AC generator rotates through 360 degrees is

called a sine wave because the instantaneous voltage or current

is related to the sine trigonometric function.

As shown in the following illustration, the instantaneous

voltage (e) and current (i) at any point on the sine wave are

equal to the peak value times the sine of the angle. The sine

values shown in the illustration are obtained from trigonometric

tables. Keep in mind that each point has an instantaneous

value, but this illustration only shows the sine of the angle

at 30 degree intervals. The sine of an angle is represented

symbolically as sin q, where the Greek letter theta (q)

represents the angle.

جهت دریافت فایل ترجمه شده، ثبت نام نمائید.


نوشته شده در : چهارشنبه 22 آذر 1391  توسط : جواد خانی.    نظرات() .

silvasirmon.hatenablog.com
جمعه 13 مرداد 1396 02:32 ب.ظ
Hi i am kavin, its my first occasion to commenting
anywhere, when i read this piece of writing i thought i could
also create comment due to this sensible article.
BHW
جمعه 1 اردیبهشت 1396 03:38 ق.ظ
It's great that you are getting thoughts from this piece of writing as well as from our argument
made at this time.
 
لبخندناراحتچشمک
نیشخندبغلسوال
قلبخجالتزبان
ماچتعجبعصبانی
عینکشیطانگریه
خندهقهقههخداحافظ
سبزقهرهورا
دستگلتفکر