If the decision has been made to replace an existing standard efficiency motor with a high efficiency version, care must be taken to ensure that the result will in fact yield actual savings. Done incorrectly, the new motor could, in some instances, consume more energy than the old motor. That is not to say the new motor is less efficient, but rather the new motor load combination performs differently than the old combination. This can result in more work being done when the increase is not needed. Therefore when changing a motor, ensure the load is adjusted to match the new motor.
a. Motor Resizing
A common practice was (and still is) to oversize a motor to be sure it is capable of driving the load. An oversized motor is less efficient and contributes to a lower power factor. For variable loads, ensure the motor never exceeds its full load or service factor rating.
b. Speed Adjustments
Energy efficient motors typically operate at lower slip than standard efficiency motors. This means for a variable torque load, an energy efficient motor will operate at a higher RPM at the balance point and will consume more energy. To correct this problem, the pulley ratio between the motor and load need to be adjusted for belt drives.
c. Load Adjustments
For direct coupled variable torque loads, an energy efficient motor will tend to run faster as noted in the previous subsection. For these situations the load will need to be adjusted. Examples include changing fan pitch or trimming pump impellers. This requires special skills to accomplish.
d. Power Factor
Energy efficient motors typically have higher power factors than standard efficiency motors. Existing power factor correction capacitors will need to be re-evaluated to ensure overcorrection is not occurring.
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