Alternating Current
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ALTERNATING CURRENT Powerloss due to resistance of wire is I2R. DC loss can be reduced by transmitting at high voltage and low current levels. Transmission of DC at high voltage is not practical because loads would be required to work at the high voltage levels. Unlike DC, AC may be stepped up or down in voltage using a transformer. AC voltage (amplitude) varies over time. It reverses polarity every half cycle. Many practical electronic circuits require DC. A rectifier circuit is used to convert AC to DC. AC is used for power transfer and information transfer. Electronic equipment manages the flow of information. Electrical equipment manages the flow of power. The three common attributes of AC are amplitude, frequency, and phase quantities. THE SINE WAVE The sinusoidal waveform, or sine wave is the most basic and most used form of AC. It is produced by an AC generator or electronic oscillator. Peak and PeaktoPeak Values The peaktopeak value is twice the maximum or peak value. Average Value The true average value of a sine wave is zero. For AC the average value is taken as the average for one half cycle only. A DC voltmeter will read the true average or zero when measuring a sine wave. Effective (RMS) Value The effective value is the value of AC voltage that has the same heating effect on a resistance as an equal value of DC. Vrms=0.707Vp Vp = 1.414Vrms Instantaneous Values The value of voltage at a particular instant is the instantaneous value. This value may be any value between the positive peak and the negative peak. FREQUENCY AND PERIOD Frequency Frequency, measured in hertz, is the number of complete cycles in one second. 60 Hz is used for household power in the US. Some other countries use 50 Hz. Lower frequencies may cause lights to flicker. Higher frequencies cause more power loss. Period Period is the time it takes a waveform to complete one cycle. Period is measured in seconds. T = 1 / f f = 1 / T WAVELENGTH The distance traveled by a waveform during one cycle is the wavelength. d = v / f Electromagnetic (Radio) Waves Radio waves travel at the speed of light. The higher the frequency the shorter the wavelength. Shortwave radios are used to receive or transmit high frequency signals. Sound Waves Sound waves travel at approximatly 340 m/s (1100 ft/s). PHASE RELATIONSHIPS Phase is the number of electrical degrees that one wave leads or lags another wave. Phasor Diagrams A phasor is a quantity that has magnitude and direction. DC quantities are scalar, they have magnitude only. A phasor diagram shows the magnitude of a waveform relative to phase. NONSINUSOIDAL WAVEFORMS A waveform that is not a sine wave (or cosine wave) is a nonsinusoidal waveform. Pulse Wave A pulse waveform is comnonly used to convey digital information. The duty cycle is the ratio of pulse width to the period of one cycle. DC = PW / T DC = duty cycle PW = pulse width (s) T = period (s) Duty cycle can be expressed as a percentage by multiplying times 100. Square Wave A pulse waveform with a duty cycle of 0.5 or 50% is a square wave. Sawtooth Wave Triangular Wave HARMONIC FREQUENCIES Exact wholenumber multiples of the fundamental frequencg all called harmonics. The second harmonic of a 100 Hz waveform is 200 Hz. The third harnonic is 300 Hz. The second harnonic has one half the amplitude of the fundamental frequency. The third harmonic has onethird the fundamental frequency. Units for Frequency Multiples The octave is a common unit for frequency multiples. 