A pure sinusoidal voltage is a conceptual quantity produced by an ideal AC generator built with finely distributed stator and windings that operated in a uniform magnetic field. Since neither the winding distribution nor the magnetic field are uniform in a working AC machine, voltage waveform distortions are created,and the voltage- time relationship deviates from the pure sine function. The distortion at the point of generation is very small (about 1% or 2%) , but nonetheless it exists. Because this is a deviation from a pure sine wave, the deviation is in the form of a periodic function, and by definition, the voltage distortion contains harmonics.
When a sinusoidal voltage is applied to a certain type of load, the current drawn by the load is proportional to the voltage and impedance and follows the envelope of the voltage waveform. These loads are referred to as linearloads(loads where the voltage and current follows one another without any distortion to their pure sine waves). Examples of linear loads are resistive heaters, incandescent lamps, and constant speed induction and synchronous motors.
In contrast, some loads cause the current to vary disproportionately with the voltage during each half cycle. These loads are classified as nonlinear loads, and current and voltage have waveform has numerous additional waveform superimposed upon it, creating multiple frequencies within the normal 60 Hz sine wave. The multiple frequencies are harmonics of the fundamental frequency.
Normally, current distortion produce voltage distortion. However, when there is a stiff sinusoidal voltage source ,one need not be concerned about current distortions producing voltage distortions.
Examples of nonlinear loads are battery chargers, electronic ballasts, variable frequency drives, and switching mode power supplies. As nonlinear current flows through a facility's electrical system and the distribution - transmission lines, additional voltage distortion are produced due to the impedance associated with the electrical network. Thus ,as electrical power is generated, distributed and utilized, voltage and current waveform distortions are produced.
Power system designed to function at the fundamental frequency, which is 50 Hz in india, are prone to unsatisfactory operation and, at times, failure when subjected to voltage and currents that contains substatial harmonics frequency elements. Very often, the operation of electrical equipment may seem normal , but under a certain combination of conditions, the impact of harmonics is enhanced, with damaging result.
When a sinusoidal voltage is applied to a certain type of load, the current drawn by the load is proportional to the voltage and impedance and follows the envelope of the voltage waveform. These loads are referred to as linearloads(loads where the voltage and current follows one another without any distortion to their pure sine waves). Examples of linear loads are resistive heaters, incandescent lamps, and constant speed induction and synchronous motors.
In contrast, some loads cause the current to vary disproportionately with the voltage during each half cycle. These loads are classified as nonlinear loads, and current and voltage have waveform has numerous additional waveform superimposed upon it, creating multiple frequencies within the normal 60 Hz sine wave. The multiple frequencies are harmonics of the fundamental frequency.
Normally, current distortion produce voltage distortion. However, when there is a stiff sinusoidal voltage source ,one need not be concerned about current distortions producing voltage distortions.
Examples of nonlinear loads are battery chargers, electronic ballasts, variable frequency drives, and switching mode power supplies. As nonlinear current flows through a facility's electrical system and the distribution - transmission lines, additional voltage distortion are produced due to the impedance associated with the electrical network. Thus ,as electrical power is generated, distributed and utilized, voltage and current waveform distortions are produced.
Power system designed to function at the fundamental frequency, which is 50 Hz in india, are prone to unsatisfactory operation and, at times, failure when subjected to voltage and currents that contains substatial harmonics frequency elements. Very often, the operation of electrical equipment may seem normal , but under a certain combination of conditions, the impact of harmonics is enhanced, with damaging result.
Motors
There is an increasing use of variable frequency drives(VFD) that power electric motors. The voltage and currents emanating from a VFD that go to a motor are rich in harmonic frequency components. Voltage supplied to a motor sets up magnetic fields in the core, which creates iron losses in the magnetic frame of the motor. Hystersis and eddy current losses are part of iron losses that are produced in the core due to the alternating magnetic field. Hystersis losses are proportional to frequency , and eddy current losses vary as the square of the frequency. Therefore, higher frequency voltage components produce additional losses in the core of AC motors,which in turn , increase the operating temperature of the core and the windings surrounding in the core. Application of non sinusoidal voltage to motors results in harmonic current circulation in the windings of motors.
The net rms current is
Irms = √[(I1)2 + (I2)2 +
(I3)2 + …], where the subscripts 1,2,3, etc. represent the different harmonics
currents . The I2R losses in the motor windings vary as the square of the rms currents.
Due to skin effect , actual losses would be slightly higher than calculated values. Stray motor losses , which include winding eddy current losses , high frequency rotor and stator surface losses, and tooth pulsation losses, also increase due to harmonic voltage and currents.
Due to skin effect , actual losses would be slightly higher than calculated values. Stray motor losses , which include winding eddy current losses , high frequency rotor and stator surface losses, and tooth pulsation losses, also increase due to harmonic voltage and currents.