You get an urgent call. The thermal sensor tripped a critical motor off-line because the motor was running too hot!
You inspect everything carefully. You restart and everything seems to work fine. What could it be?
Maybe ghosts really do exist!
What allows these “Ghost Anomalies” to enter into your power system? A more scientific explanation for the symptom may be that you’ve got harmonics problems in your system. Generally speaking, “Ghost Anomalies” start to happen when the Total Harmonic Distortion (THD) of your power system reaches about 30-35%.
What is a harmonic? IEEE standard 519-1992 defines a harmonic as, “A sinusoidal component of a periodic wave or quantity having a frequency that is an integral multiple of the fundamental frequency.” Well, what is a sinusoidal wave? The picture below gives you a taste how to generate a sinusoidal wave, and the relationship between the sinusoidal wave and the circle. (When you are drawing a circle with a constant speed, while moving the center of the circle along the t-axis with another constant speed, you can get a sinusoidal wave.)
In the United States, our electricity has the fundamental frequency of 60 Hz. The illustration above describes how many cycles per second, or how many circles you can draw in one second. The first harmonic is 60 Hz, the 2nd harmonic is 120 Hz, the 3rd harmonic is 180 Hz, and so on.
Ideally, voltage and current waveforms are perfect sinusoidal waves. When you hook up an oscilloscope to show the voltage waveform, you will not see the pure sinusoidal wave. Why? It’s because of the existence of harmonics and because of the non-linear load in your facility. The picture below shows a slight distortion of 460 VAC voltage waveform when the voltage contains its 3rd harmonic.
The utility tends to deliver a pure sinusoidal voltage to your facility, not always perfect, but close. However, because of the increased popularity of power electronic equipment and devices, the waveform can easily be distorted. Your computer and your copy machine, for example, contain power transistors, rectifiers, etc. that are non-linear loads. Even the fluorescent lamps are non-linear loads because they are arc discharge devices, not pure resistive devices.
When you start adding Variable Frequency Drives (VFD) to drive your equipment, you do so with the expectation of reducing your utility bills. However, the down side of this energy saving project is that it might bring you the harmonics and dreaded “Ghost Anomalies”. Your motor running across-the-line (but near to a VFD) might run hotter than normal. Your motor driven by the VFD might suffer a shortened life due to the partial discharge (small arcs across a void in the windings) which leads to the insulation failure.
How do you evaluate your power system harmonics and avoid the nuisance tripping? How do you protect your drives from input power line disturbances? How do you protect your motor from running too hot?
We’ll be addressing some of these questions in future issues when we discuss Ghost Anomalies Exorcism (Remedy of the Harmonics).
Call Brithinee Electric at 909-825-7971. We’ll help you engineer the right package for your controls. We will refurbish your motor with extra protection of the windings and bearings.
Keep the “ghost” out of your business. Save it for your Halloween Party!