Efficient Plant April 2022 - 24

motor maintenance
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Reject These
Motor Myths
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Have some myths replaced facts in your motor setup and
maintenance practices?
THE TONGUE-IN-CHEEK saying, " If it's in black
and white, it must be right, " is a helpful reminder that
not everything we read (or hear) is accurate or complete.
It's always best to check sources and verify facts before
accepting consequential statements as true. A similar
adage underscores the importance of this advice in the
digital age: " If it's on the internet, it must be true. " With
these things in mind, here's a selection of common
misconceptions about three-phase squirrel-cage
motors and the facts that deny them.
SOFT-STARTING MOTORS REDUCE
UTILITY DEMAND CHARGES
Soft starters typically ramp up the voltage applied to a motor over a few seconds
at start-up, reducing winding heating and starting current. This may extend the life
of the winding for motors that start frequently, but it doesn't affect utility demand
charges. That's because the electric meter averages the kilowatts consumed over each
15-to-30-min. period, not just for the few seconds that the soft starter reduces input
power to the motor.
POWER FACTOR CORRECTION CAPACITORS
CAN REDUCE MOTOR ENERGY CONSUMPTION
Applying power factor capacitors at the motor terminals increases the power factor
on the supply cables but does not change the motor's power factor. Increasing the
power factor on the supply lines reduces current in them, causing a corresponding
but typically insignificant reduction in I2
A MOTOR CAN BE LOADED UP TO ITS SERVICE
FACTOR CURRENT
An example of this would be loading a 1.15 service factor motor up to its service
factor current (typically ~1.15 x rated current). That would be a problem, according to
clause 14.37.1 of NEMA Stds. MG 1-2016: Motors and Generators (MG 1): " A motor
operating continuously at any service factor greater than 1 will have a reduced life
expectancy compared to operating at its rated nameplate horsepower. Insulation life
and bearing life are reduced by the service factor load. "
Further, the service factor only applies to Usual Service Conditions (MG 1, 14.2).
These include operation at an ambient temperature of 5 F to 104 F (-15 C to 40 C)
and at an altitude of less than 3,300 ft. (1,000 m.) when rigidly mounted in areas
or supplementary enclosures that do not seriously interfere with the machine's
ventilation.
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This article is an excerpt from the original article by Thomas
Bishop, Bust Nine Common Motor Myths. That original
article and several other motor-maintenance publications can be
downloaded at easa.com/erc.
24 | EFFICIENTPLANTMAG.COM
OVERSIZED MOTORS, ESPECIALLY MOTORS OPERATING
BELOW 60% OF RATED LOAD, ARE NOT EFFICIENT AND
SHOULD BE REPLACED WITH APPROPRIATELY SIZED
PREMIUM EFFICIENCY (IE3) MOTORS
On the contrary, matching motor horsepower (kW) rating to the load will usually mean
a slightly lower efficiency at that load than using the next larger-sized motor. The
reason is that motors tend to peak in efficiency between 75% and 80% of rated load.
Motors that drive supply or return air fans in HVAC systems generally operate at 70%
to 75% of rated load, making them candidates for use with oversized motors. Further,
even at 60% of rated load (which more than one industrial motor study found to be
the average load level), the next higher-power-rating motor could be more efficient
at that load than the appropriately sized power rating. Some high-inertia loads also
require more HP/kW to start than to run the load. Reducing the HP/kW to match the
running load could result in the motor being unable to start the load. -Thomas H.
Bishop, P.E, Senior Technical Support Specialist, EASA Inc., St. Louis, easa.com. EP
APRIL 2022
R losses (energy) in the supply wiring. The
primary reason for reducing supply circuit current is to add electrical loads without
rewiring a facility.
http://www.easa.com/erc http://www.easa.com http://www.EFFICIENTPLANTMAG.COM

Efficient Plant April 2022

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