Power quality is the ability of electric utilities to provide un-interrupted electrical power. In order to measure power quality, it’s important to examine any deviation from a perfect sinusoidal waveform or “sine wave”. In a sinusoidal wave, the flow of power moves in a smooth and repetitive oscillation, however it’s almost impossible to keep wave quality this pure in modern electronic systems and motors. In fact, it’s common to see distortions in the sine wave which can have dangerous consequences – such as harmonics.
Harmonics are the electrical currents and voltages of a distorted waveform, which can be responsible for causing issues with power quality. High harmonic levels can indicate distortion in electrical motors that lead to problems like increased overheating, misfiring variable speed drives, and detrimental torque pulsation. In other words, it’s important to reduce harmonics wherever possible for efficiency and productivity.
Experts define harmonics as a component of a periodic wave or frequency within an electrical system. In systems with a single-frequency sine wave, there should be no harmonic content at all, which may be the case if it weren’t for the presence of nonlinear components. In electrical motors, nonlinear components disrupt the clean power of sinusoidal waves by drawing currents that are disproportionate to the source voltage.
Up to the 1960s, most electric loads were linear — capable of producing a perfect sine wave. However, the introduction of innovative modern appliances, and the development of electric motors to meet new functional requirements has increased the presence of non-linear loads. Instead of loads drawing proportional currents into a sinusoidal voltage, electrical devices now change their impedance by alternating between off and on modes at the peak of the waveform — resulting in abrupt nonsinusoidal pulses. The short pulses introduce reflective currents, known as harmonics, back into the distribution system, causing distortion.
In electrical motors and other systems, “Total Harmonic Distortion” measures interference to a power signal, and issues with the sine wave. The “THD” of any system is defined as the ratio of the power in the supply as a result of harmonics, and the amount of power in the fundamental supply.
How Harmonics Impact Electrical Motors
While devices causing harmonics are present in all commercial, residential, and industrial installations, their presence doesn’t always mean that an office or factory cannot run properly. Similarly to other phenomena regarding power quality, the result depends on the susceptibility of the equipment and the stiffness of the power distribution system.
Because harmonics represent a distortion in clean, linear waves for energy supply, their presence in an electrical motor can wreak havoc in the distribution system. Motors generally require largely linear loads, but when the voltage supply is rich in harmonics, the motor draws on the harmonic current, leading to a higher operation temperature, an increased risk of failure, and a shortened service life.
What’s more, the presence of harmonic currents can lead to additional rotating fields within a motor, forcing counter-torque rotation, or torsional oscillations in the motor shaft. If the oscillation frequency resonates with the natural mechanical frequency of the motor shaft itself, it can maximize vibrations within a motor, leading to damage at a devastating level. In order to ensure the longevity of their electric motors and maintain regular function, businesses need to conduct harmonic analysis to determine the functionality of a motor.
Harmonics and Power Quality
The fact that we rely so heavily on access to consistent power, modern motors are more dependent on good power quality for optimum operation. While deviations from ideal power quality are common, and sometimes acceptable, even the smallest amount of interference can have an impact on the power system as a whole. Most industries are able to withstand exposure to nonlinear loads that make up 15% of the complete electrical system, but lowering the harmonics in electric motors as much as possible can act as a form of preventative maintenance. Removing harmonics from a power system wherever possible may be the best way to achieve higher efficiency and longer life within electrical motors.