Has This Ever Happened To You?

Published On
Aug 1, 2017

In my 40-plus years in the industry, I have analyzed customers’ power quality monitor data files and heard their stories about hundreds of things that have happened in their facilities or homes: some mysterious, some catastrophic, some even comical. The following are just a few common types of PQ-related issues that don’t just happen to “the other guy.”

Failing insulation on outer motor windings

Damage to insulation on the outermost turns of electric motor windings is often caused by repetitive narrow transients with fast rise times. These can be caused by the voltage notches every cycle during the commutation period of three-phase, solid-state rectifiers, such as silicon controlled rectifiers or thyristors. The rectifier in one phase is still on when the next is told to switch on, leaving both of them conducting for a short duration until the current through the first one goes to zero and it shuts off. This effectively creates an electrical short between two phases, causing instantaneous large current flows and resulting in the corresponding voltage notch.

Adjustable speed drives (ASDs) trip for no apparent reason

A sudden decrease or increase in supply voltage to the power controller section of an ASD can cause it to trip offline to protect itself. A common source of this is the transient caused by the energizing of power-factor correction capacitors (PF caps) on the electrical distribution system. When the PF caps are energized, the negative transient is followed by a positive transient that can be twice the normal peak voltage, as the inductance in the wire “objects” to the sudden voltage change, confusing the controller.

Transient voltage surge suppression (TVSS) or surge suppression strips with blown fuse

The TVSS components inside of the popular outlet strips that are used to plug in computers, monitors, chargers, printers, etc., are designed to clamp the voltage from transients to a safe level for the connected equipment. But each time they do so, they give a piece of themselves from the absorbed and dissipated energy. Over time, they can have enough and fail catastrophically. When one of them fails in a short circuit, the fuse will blow, indicating that it’s time to get a new strip—perhaps one with a higher joule rating.

Damage to power supplies on electronic equipment

Often, the rectifiers and electrolytic capacitors in the rectified front ends of switch-mode power supplies will release some smoke, perhaps crack in half and leave burn marks on the PC boards. This is a good indication that some sort of PQ phenomena has claimed another victim. 

Transients, sustained swells or even severe momentary sags can be the culprits. The high voltage of transients and swells can exceed the breakdown voltage of semiconductor devices and their doom. Having a reduced voltage for a long duration can cause a power supply that produces constant wattage to make up the missing voltage with higher current, enough to cause overheating and failure in some components if not properly protected against such an event. Of course, the alternative to turning off the supply to protect itself will result in the equipment turning off as well.

Premature lamp or ballast burnout

Ironically, some electronic ballasts produce so much harmonic current distortion that they severely distort the voltage on the branch circuit as well, causing even higher harmonic currents on other ballasts. High harmonic currents cause higher losses in electromagnetic components, resulting in excessive heating, which shortens life.

Distortion on a hospital monitor after new equipment was installed on the floor below

A large horsepower adjustable speed drive being installed without proper shielding can generate significant voltage and magnetic fields. While the older CRT-type monitors are most susceptible to such fields, they can couple into communication and data circuits. Such distortion or data corruption is probably not good for hospital monitors.

Bearings on motors getting noisy; when disassembled, the bearings show severe damage

Harmonic currents can not only heat up the windings of motors, but they can also be conducted through the bearings to an electrical ground, eating away at the metal (called fluting) over time. 

In future columns, I will cover a number of other PQ-induced problems, including nausea, melting insulation on neutral conductors, exploding drywall, whistling transformers and more.

About the Author

Richard P. Bingham

Power Quality Columnist

Richard P. Bingham, a contributing editor for power quality, can be reached at 732.248.4393.

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