POWERQUALITY BY RICHARD P. BINGHAM
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 cur-
rents on other ballasts. High harmonic
currents cause higher losses in elec-
tromagnetic 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 cir-
cuits. 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 cur-
rents 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.
Has This Ever Happened to You?
Reviewing some common PQ issues
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.”
BINGHAM, a contributing editor for power quality, can be reached at 732.248.4393. PH
Capacitors that blew up
from a power supply failure.