You recently installed some variable frequency drives at the pump station to save energy by better matching your motor’s load profile to its application. You also decided to use these VFDs as power monitors, enabling you to record several new power quality metrics than you previously captured. But minutes after going live on the new system, you get an email from Motors@Work: “High total harmonic distortion detected on Pump #1.”
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 [see post on power factor]. However, modern electronics and inductive loads are non-linear, generating harmonics throughout the power distribution system.
High harmonic content causes significant heating in your motors. Here’s a quick primer on why harmonic distortion affects your motors, and how Motors@Work helps our clients identify, troubleshoot, and resolve harmonic issues that otherwise would have gone undetected without Motors@Work’s timely condition-monitoring alerts.
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 Hertz in North America; 50 Hertz in Europe), or the first harmonic — producing a non-sinusoidal shape, or distorted waveform [see figure below].
This distorted waveform affects your motor in five ways. First, harmonics reduce the efficiency of your motor. Harmonic content makes it harder to magnetize the copper and iron in your 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 your motor experiences. It degrades winding insulation, causes bearing grease to lose lubricity, and reduces your motor’s life. Depending on the level of harmonic content, the heat generated may cause nuisance tripping of thermal protection systems in your motors.
Third, harmonics can trigger bearing currents. Bearing currents cause arcing between the bearing raceway and journal or balls, creating a much rougher surface, increasing your friction losses, and potentially causing the bearing to seize. The arcing also accelerates breakdown of the lubricant. All in all, bearing currents cause your 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 your power factor — raising your power bills and reducing your motor’s efficiency [read more on power factor’s effects here].
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 your motors than positive-sequence harmonics.
Positive-sequence harmonics — the seventh, thirteenth, nineteenth orders and so on — help your 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 your 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.
You notice that your motors are a bit louder now that they’re operating on drives and decide to look further into what types of harmonics are affecting your system to make sure torque pulsations won’t damage your motor or pump. You schedule a consultation with a power quality engineer, who brings an oscilloscope to classify your harmonic content and other power quality issues.
The engineer finds your new drives are injecting significant harmonics. She sizes and recommends harmonic filters to remove harmful negative sequence harmonics. She also recommends purchasing drives with integrated harmonic filtration in the future.
Motors@Work’s continuous, near-real-time condition monitoring alerts catch potentially motor-damaging conditions that would’ve gone unnoticed otherwise. Additionally, our help content and content-rich alerts reduce the time required to troubleshoot the issue.
How will condition monitoring benefit your organization? Email Nicole at firstname.lastname@example.org to learn more.