Question: what is meant by AC?
Electric current moves often like a water stream. The only difference here is that electric charges do not always move in the same direction. When they do, it is called Direct Current or DC. On the other hand, AC or Alternating Current, as the name suggests, occurs when charge carriers in a conductor, or semi conductors, reverse their direction of movement in a periodic manner.
With AC, it is possible to run many types of electrical equipment like generators, motors, power distribution systems etc, and also kitchen appliances, TVs, electric lamps etc. Hence, for almost all high power applications, it is a global trend to use AC instead of DC because of its higher efficiency. Besides these advantages over DC, AC power is believed to be less expensive. One can also use high voltages with small currents to reduce losses, with AC power.
Alternating current (AC) is an electric current which periodically reverses direction. The usual waveform of alternating current in most electric power circuits is a sine wave, whose positive half-period corresponds with positive direction of the current and vice versa. In certain applications, different wave forms are used, such as triangular or square waves.
Audio and radio signals carried on electrical wires are also examples of alternating current. These types of alternating current carry information such as sound (audio) or images (video) sometimes carried by modulation of an AC carrier signal.
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.
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.
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.
As 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.
The armature continues to rotate from 90 to 180 degrees, cutting fewer lines of flux. The induced voltage decreases from a maximum positive value to zero.
As the armature continues to rotate from 180 degrees to 270 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.
As the armature continues to rotate from 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.
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.
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.
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-to peak 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.
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.
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 θ, where the Greek letter theta (θ) represents the angle.