Power Electronics Dr. Imtiaz Hussain Assistant Professor URL : Lecture-2 Definitions and Terminologies 1

Measuring a Sine Wave 2 Peak value The PEAK value of the wave is the highest value the wave reaches above a reference value. In a voltage waveform the peak value may be labelled V PK or V MAX (I PK or I MAX in a current waveform). c c c c c

Measuring a Sine Wave 3 Peak to Peak value The PEAK TO PEAK value is the vertical distance between the top and bottom of the wave. It is measured in volts on a voltage waveform, and may be labelled V PP or V PK−PK. In a current waveform it would be labelled I PP or I PK−PK as I is used to represent current.

Measuring a Sine Wave 4 Amplitude The AMPLITUDE of a sine wave is the maximum vertical distance reached, in either direction from the centre line of the wave. As a sine wave is symmetrical about its centre line, the amplitude of the wave is half the peak to peak value.

Measuring a Sine Wave 5 Periodic Time & Frequency The PERIODIC TIME is the time, in seconds taken for one complete cycle of the wave. Thus if the periodic time of a wave is 20ms then there must be 50 complete cycles of the wave in one second (50Hz). c

Measuring a Sine Wave 6 Average Value The average voltage (or current) of a periodic waveform whether it is a sine wave, square wave or triangular waveform is defined as: “the quotient of the area under the waveform with respect to time”. In other words, the averaging of all the instantaneous values along time axis with time being one full period, (T).

Measuring a Sine Wave 7 Average Value In a pure sine wave if the average value is calculated over the full cycle, the average value would be equal to zero as the positive and negative halves will cancel each other out. Then the average value is obtained by adding the instantaneous values of voltage over one half cycle only.

Measuring a Sine Wave 8 RMS Value Effective DC Value: RMS value gives the same heating effect as an equivalent DC power.

Measuring a Sine Wave 9

10 INSTANTANEOUS VALUE The INSTANTANEOUS value of an alternating voltage or current is the value of voltage or current at one particular instant. The value may be zero if the particular instant is the time in the cycle at which the polarity of the voltage is changing. It may also be the same as the peak value, if the selected instant is the time in the cycle at which the voltage or current stops increasing and starts decreasing. There are actually an infinite number of instantaneous values between zero and the peak value.

Measuring a Sine Wave 11 Form Factor The form factor of an alternating current waveform is the ratio of the RMS value to the average value. For a pure sinusoidal waveform the Form Factor will always be equal to 1.11.

12 Courtesy of Wikipedia

Measuring a Sine Wave 13 Crest Factor Crest Factor is the ratio between the R.M.S. value and the Peak value of the waveform. For a pure sinusoidal waveform the Crest Factor will always be equal to Crest factor indicates how extreme the peaks are in a waveform. Both Form Factor and Crest Factor can be used to give information about the actual shape of the AC waveform.

14 Courtesy of Wikipedia

Measuring a Sine Wave 15 Power Factor In an AC circuit, power is used most efficiently when the current is aligned with the voltage. However, most equipment tend to draw current with a delay, misaligning it with the voltage. What this means is more current is being drawn to deliver the necessary amount of power to run the equipment. And the more an equipment draws current with a delay, the less efficient the equipment is.

Measuring a Sine Wave 16 Power Factor The power factor is the ratio of the real power that is used to do work and the apparent power that is supplied to the circuit. The power factor can get values in the range from 0 to 1. When all the power is reactive power with no real power (usually inductive load) - the power factor is 0. When all the power is real power with no reactive power (resistive load) - the power factor is 1. The power factor is equal to the real or true power P in watts (W) divided by the apparent power |S| in volt-ampere (VA):

Importance of Power Factor 17 A power factor of one or "unity power factor" is the goal of any electric utility company since if the power factor is less than one, they have to supply more current to the user for a given amount of power use. Industrial facilities tend to have a "lagging power factor", where the current lags the voltage (like an inductor). This is primarily the result of having a lot of electric induction motors Capacitors have the opposite effect and can compensate for the inductive motor windings. Some industrial sites will have large banks of capacitors strictly for the purpose of correcting the power factor back toward one to save on utility company charges.

A Sinusoidal Waveform 18

Formulae 19

Sinusoidal Waveform Conversion Table 20 Convert FromMultipy ByOr ByTo Get Value Peak2(√2) 2 Peak-to-Peak 0.51/2Peak /(√2)RMS Peak0.6372/πAverage 1.570π/2Peak Average1.111π/(2√2)RMS 1.414√2√2Peak RMS0.901(2√2)/πAverage

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