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What do Harmonics Do?

Harmonics cause interference with Audio communications equipment

Harmonics are voltages and currents that are multiples of the line frequency.
These can be well up into the audio band and can cause interference on audio communications circuits such as telephone circuits and HiFi systems. If the harmonics are of a high amplitude, then there can be cross coupling between adjacent cables due to electromagnetic coupling or capacitive coupling. Good separation between circuits will help to reduce this. The harmonics can also pass through the power supply of electronic equipment and then be amplified by that equipment.

Harmonics cause increased currents in capacitors

The current trough a capacitor is a function of the voltage across the capacitor and its impedance. As the frequency of the applied voltage increases, the current will increase due to a falling impedance. The impedance of the capacitor is inversly proportional to frequency.
If the aplied voltage waveform comprises 50Hz plus 10% 7th harmonic, then the current through the capacitor will be the 50Hz current (proportional to the applied voltage plus the seventh harmonic current. As, in this case, the seventh harmonic voltage is 10% of the applied 50Hz voltage, the seventh harmonic current will be 1/10 x 7 times the 50Hz current. The RMS current through the capacitor will increase by 70% due to the 10% 7th harmonic voltage present. This wiould damage a power factor correction capacitor.

Harmonics cause increased heating in induction motors

The losses in induction motors are primarily copper loss and iron loss.
The presence of harmonics in the applied voltage will cause an increase in the copper losses and more particularly, in the iron losses of the motor. The result is that the operating temperature of the motor will rise and in order to keep the winding temperature down, the motor should be derated due to the harmonic presence.
The impedance of the induction motor is low to frequencies other than synchronous frequency where the spped of the torque field is equal to the speed of the stator.
Harmonics create rotating magnetic fields at speeds well away from synchrouns speed, some forwards and some backwards.
The rotor dissipates slip energy where the energy dissipated is equal to the slip (difference in speed between the rotor sped and the torque speed) times the torque. There can be a) significant harmonic currents flowing into the motor due to the low impedance to those harmonic voltages and b) significant rotor losses causing the motor to heat up significantly.
It is not uncommon for the THDi to be more than twice the applied THDv.

Harmonics cause increased heating in Transformers and transmision equipment

The losses in transformers are primarily copper loss and iron loss. The presence of harmonics in the applied voltage will cause an increase in the copper losses and more particularly, in the iron losses of the motor. The result is that the operating temperature of the transformer will rise and in order to keep the winding temperature down, the transformer should be derated due to the harmonic presence.
The losses in the lines and cables are "copper" loss and this is affected by non sinusoidal wave forms. Harmonics can require that cable be derated.