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How often do you encounter this problem in the field? The motor in
question continually nuisance trips. You look at the motor and the application:
the motor appears to be running properly; the driven load is working properly;
and yet the motor seems to keep overheating and tripping.
Many fractional horsepower motors come equipped with an internal overload
device that is sensitive to both current and temperature. This thermal-overload
protector (called a “thermo,” or “overload,” or
simply “OL”) may either require manual resetting or can reset
automatically. It is designed to protect the motor’s windings from
the damaging effects of too much heat or too much current.
In situations like this, where there appears to be no mechanical or electrical
problem, you may be tempted to blame the thermal device itself. Don’t
give into the temptation, however, or you may find yourself treating the
symptom, not the problem itself. Remember, thermos are extremely reliable,
and their job is to alert you to an unseen, but potentially catastrophic
problem with the application. As a good MD (Motor Doctor), you should
take a few moments to probe for answers.
For example, you may have inadvertently substituted the wrong motor in
the application. Have you matched the motor to the operating condition?
Do the voltages of the replacement motor and the original correspond?
One way to monitor all of these variables is to use a clamp-on ammeter
to determine that you did not exceed the motor’s nameplate amps.
A related consideration is determining that you have the right load for
the motor. Has the load changed from the original equipment load? You
may find that a larger diameter fan has been applied to the motor, or
a larger blower wheel. Or the pulley ratio is different from the original
specification. Or, in some cases, the original motor’s horsepower
was marginal for the driven load—and the replacement motor’s
horsepower could be even more marginal. Once again, a clamp-on ammeter
can help you reach a correct diagnosis.
If a motor in an older application suddenly begins to nuisance trip, you
may want to look for blockages in air flow caused by airborne debris.
Other air flow problems include applying a motor with excessive horsepower
for the load in an air-over application. Too much horsepower often provides
inadequate air flow to dissipate heat—even if the load is light.
You may find that added or re-positioned baffles or filters have re-directed
air flow or decreased the amount of air flow.
Another condition that may cause unexpected thermal tripping is excessive
ambient temperature. Almost all motors are designed to produce their nameplate-rated
output up to a specific temperature. If the environmental temperature
is higher than the nameplate rating, the motor is at risk, even if everything
else about the load and power supply fall within normal ranges. If a motor
begins to nuisance trip, consider the environmental temperature issue.
With motor-driven devices located in mechanical rooms, check to see if
additional heat-generating equipment has been installed in the room. Or
perhaps room ventilation has changed, either due to construction or ventilating
equipment failure.
Consider the effects of foreign material or contaminants. Material can
build up on the surfaces of motors, even enclosed designs that prevent
foreign materials from entering the motor. Oily vapors caused by cooking
oil or other chemicals can condense on the motor. Dust and lint adhere
easily to these oily films, creating a very effective insulation. This
may affect both the air movement over the motor as well as the motor’s
ability to dissipate heat. The results are often higher internal temperatures
and thermal tripping.
One last item to check is the power supply leading into the motor. Overvoltage
and undervoltage can cause the motor to overheat, resulting in nuisance
tripping. To check for this condition, use a volt meter to measure the
power supply while the motor and the other equipment on the same circuit
are running.
It’s very easy to blame nuisance tripping on the thermal device.
Don’t be fooled! Listen to what the thermo is trying to tell you
and always take the time to search for the root cause of the problem.
You’ll avoid bigger problems down the road.
by Neil Simon, aka the Motor Doctor - regional sales
manager for A. O. Smith Electrical Products Company.
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