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BASICS OF ELECTRICITY

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ALTERNATING CURRENT (AC):

The supply of current for electrical devices may come from a direct current source (DC), or an alternating current source(AC).

 In direct current electricity, electrons flow continuously   in one direction from the source of power through a conductor to a load and back to the source of power. The voltage indirect current remains constant. DC power sources include batteries and DC generators. In alternating current an AC generator is used to make electrons flow first in one directionthen in another. Another name for an AC generator is analternator. The AC generator reverses terminal polarity manytimes a second. Electrons will flow through a conductor from the negative terminal to the positive terminal, first in onedirection then another.

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 currentor voltage. The horizontal axis represents time.

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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.The unit of measurement of cycles per second is hertz. In general it is 50Hz or 60 Hz depending upon the country.

SINGLE PHASE AND THREE PHASE AC POWER:

Alternating current is divided into single-phase and threephase types. Single-phase power is used for small electical demands such as found in the home. Three-phase power is used where large blocks of power are required, such as found in commercial applications and industrial plants. Single-phasepower is shown in the above illustration. Three-phase power,as shown in the following illustration, is a continuous series of three overlapping AC cycles. Each wave represents a phase, and is offset by 120 electrical degrees.

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AC GENERATORS:

BASIC GENERATOR:

A basic generator consists of a magnetic field, an armature,slip rings, brushes and a resistive load. The magnetic field is usually an electromagnet. An armature is any number of conductive wires wound in loops which rotates through the magnetic field. For simplicity, one loop is shown. When aconductor 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 thearmature and rotate with it. Carbon brushes ride against the slip rings to conduct current from the armature to a resistive load.

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BASIC GENERATION OPERATION:

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.

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GENERATION OPERATION FROM 0 TO 90 DEGREES:

The armature rotates from zero to 90 degrees. The conductors cut through more and more lines of flux, building up to a maximum induced voltage in the positive direction.

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