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2018-03-26   Plant Engineering

Prior to the advent of the transistor, electrical loads were linear: their consumption of current resembled voltage’s sine wave.

By: Nicole Dyess, Motors@Work

Prior to the advent of the transistor, electrical loads were linear: their consumption of current resembled voltage’s sine wave. Resistive loads consumed current in sync with voltage, inductive loads’ current consumption followed voltage, and capacitive loads’ current consumption led voltage. However, modern electronics and inductive loads are non-linear, generating harmonics throughout the power distribution system.

Waveform distortion

The Institute of Electrical and Electronic Engineers (IEEE) defines harmonics as voltage or current waveforms at integer multiples of the fundamental frequency at which the power system operates. These harmonic waveforms combine with the fundamental frequency—also called line frequency (e.g., 60 Hz in North America; 50 Hz in Europe), or the first harmonic—producing a non-sinusoidal shape, or distorted waveform.

This distorted waveform affects a motor in five ways. First, harmonics reduce the motor's efficiency. Harmonic content makes it harder to magnetize the copper and iron in the motor’s stator and rotor, causing higher eddy current and hysteresis losses. If harmonic frequencies exceed 300 Hertz, the skin effect compounds these losses.

Second, all these extra losses manifest as additional heat. Heat, as we’ve discussed previously, is perhaps the most damaging stress the motor experiences. It degrades winding insulation, causes bearing grease to lose lubricity, and reduces the motor’s life. Depending on the level of harmonic content, the heat generated may cause nuisance tripping of thermal protection systems in the motor.

Third, harmonics can trigger bearing currents. Bearing currents cause arcing between the bearing raceway and journal or balls, creating a much rougher surface, increasing friction losses and potentially causing the bearing to seize. The arcing also accelerates breakdown of the lubricant. All in all, bearing currents cause the bearings to fail sooner.

Fourth, harmonics with high rates of change in voltage (high dV/dt), such as notching and ringing, may cause partial-discharge arcing in windings, accelerating degradation in the winding insulation.

Finally, high harmonic content lowers power factor, which raises power bills and reducing the motor’s efficiency.

Positive vs. negative sequence harmonics

Because even-order harmonics alternate direction in amplitude and cancel each other out, we only consider odd-ordered harmonics when assessing power quality. However, all harmonics are not equal: negative-sequence harmonics cause significantly more damage to motors than positive-sequence harmonics.

Positive-sequence harmonics—the seventh, thirteenth, nineteenth orders and so on—help the motor turn in the direction of the fundamental frequency, increasing torque production. However, negative-sequence harmonics (fifth, eleventh, seventeenth orders, etc.) cause braking—they try to turn the motor in the opposite direction of the fundamental frequency. As a result, these negative-sequence components cause torque pulsations. These torque pulsations can cause shaft torsion—even shaft breakage—as well as vibration issues.

Nicole (Kaufman) Dyess has nearly 20 years’ experience optimizing the performance of motor-driven systems. She began her career at Advanced Energy testing thousands of motors, consulting with motor & appliance manufacturers on their designs, and documenting motor management best practices for the US Department of Energy. Subsequently, she managed statewide energy efficiency programs at the NC Department of Commerce and facilitated sustainability and process improvement projects for the City of Raleigh. Now, as Motors@Work’s director of client solutions, Nicole focuses on client implementations and user experience while providing technical support to the sales and development teams. Nicole holds master’s degrees in mechanical engineering and public administration. This article originally appeared on Motors@Work, a CFE Media content partner.

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