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A critical element of motor servicing technique is being able to determine
whether or not a replacement motor that is not an exact duplicate of the
original is suitable for the application. As a technician, you must consider
a number of factors, but for this article, I'd like to focus on one of
the more important issues: nameplate amps.
One typical way that technicians determine whether the replacement motor
has sufficient power output is to compare the nameplate amps of the original
motor with the replacement model. If the replacement motor's amp rating
is at least as high as the original, you can consider the replacement
suitable. In many cases, this comparison simply confirms what other factors,
such as nameplate horsepower and rated voltage, tell us. This practice
is most satisfactory when there is little or no variation in the efficiency
from the original motor to the replacement. This method works well, for
example, with most three-phase motors.
In other cases, however, comparing amps may be misleading. The comparison
process tends to break down when the motors in question are single-phase
models where there is a wide range of efficiencies common for a single
design. This category includes permanent-split-capacitor motors, shaded-pole,
and some types of split-phase and capacitor-start motors.
Since nameplate amps reflect the total current consumption of the motor
(which includes both the current converted to output power and the current
lost to heat due to design inefficiency), higher nameplate amps can just
as likely mean poor efficiency as higher power output.
As motor manufacturers become increasingly sensitive to the energy efficiency
issue, they work hard to develop motors that deliver higher power while
consuming the same or fewer watts. That efficiency may or may not be reflected
in the amp rating of the motor. For the service technician, this generally
means placing more importance on comparing the horsepower of certain motors
rather than comparing amps of the replacement to the original.
Since there are no efficiency standards for most single-phase motors,
there is one good way for the technician to verify that a replacement
motor of the same horsepower but higher or lower amps is a satisfactory
replacement. The method is to measure the actual amps delivered to the
replacement motor in its normal operating state (under normal load) and
compare that measurement only to the nameplate amps for that particular
replacement motor.
In summary, using just an amp comparison is not sufficient with certain
types of motors. You need to be aware of variations in efficiency and
take this factor into account when determining a successful replacement.
by Neil Simon, aka the Motor Doctor - regional sales
manager for A. O. Smith Electrical Products Company.
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