Derive expression of power with non sinusoidal voltage and current
Answers
Explanation:
In our tutorial about the AC Waveform we looked briefly at the RMS Voltage value of a sinusoidal waveform and said that this RMS value gives the same heating effect as an equivalent DC power and in this tutorial we will expand on this theory a little more by looking at RMS voltages and currents in more detail.
The term “RMS” stands for “Root-Mean-Squared”. Most books define this as the “amount of AC power that produces the same heating effect as an equivalent DC power”, or something similar along these lines, but an RMS value is more than just that. The RMS value is the square root of the mean (average) value of the squared function of the instantaneous values. The symbols used for defining an RMS value are VRMS or IRMS.
The term RMS, ONLY refers to time-varying sinusoidal voltages, currents or complex waveforms were the magnitude of the waveform changes over time and is not used in DC circuit analysis or calculations were the magnitude is always constant. When used to compare the equivalent RMS voltage value of an alternating sinusoidal waveform that supplies the same electrical power to a given load as an equivalent DC circuit, the RMS value is called the “effective value” and is generally presented as: Veff or Ieff.
In other words, the effective value is an equivalent DC value which tells you how many volts or amps of DC that a time-varying sinusoidal waveform is equal to in terms of its ability to produce the same power.
For example, the domestic mains supply in the United Kingdom is 240Vac. This value is assumed to indicate an effective value of “240 Volts rms”. This means then that the sinusoidal rms voltage from the wall sockets of a UK home is capable of producing the same average positive power as 240 volts of steady DC voltage.
When a sinusoidal voltage instantaneous value, ν is applied to a pure inductor L, the sinusoidal current i lags the voltage by 90°. The power at any instant, p = vi. During the first quarter of a cycle, energy is taken from the supply and stored in the magnetic field. During the next quarter of the cycle as the magnetic field collapses, this energy is returned to the supply.
It is now necessary to find the amount of energy stored in the magnetic field during the first quarter of a cycle.
When the power is instantaneously at its maximum value, the voltage and the current have instantaneous values of Vm/2 and Im/2.
Maximum instantaneous power=VM√2×IM√2=VMIM√2Average value of power=2π×VMIM2.Time of one-quarter of a cycle=14fsec.∴Energy stored in one-quarter of a cycle=14f×2π×VMIM2=14πf×VMIM.Substitute forundefinedVM=2πfLIM.
Energy stored in one-quarter of a cycle=14f×IM×2πfLIM=12LIM2joules
where L is the inductance in henrys and IM is the maximum value of the current in amperes.