Getting the Best Performance From High Efficiency Motors

At Motors@Work, we have talked to customers who upgrade several motors to more efficient models only to find that the new premium efficient motors were actually consuming more than the standard motors. How can that be? There are two potential issues at play. First, many electricians don’t measure power directly (as kW), but infer it from amperage readings (as compared to full load amps). Remember, kW = V x Amps x PF x a constant. Power factor might be different between the old motor and the new motor so amp readings are not a true measure of potential kW increases or reductions. Secondly, and more likely, Premium Efficiency motors do operate with a higher full-load speed. If you turn a centrifugal fan or pump at a greater speed (in RPM), your flow and head increase and so does your power draw---in accordance with the affinity laws that state that input power increases with the third power of the speed ratio i.e. ((speed final)/(speed initial)) is raised to the third power. So, if the original motor had a speed of 1760 RPM and the replacement motor drove the same rotating equipment at 1775 RPM, flow provided would increase by (1775/1760) = 1.0142 or 1.4% while input power would increase by the ratio cubed or 4.3%. Driving the equipment faster does indeed increase the load imposed on the motor by the rotating equipment and the motor has to draw additional power to meet that increased load. That increase in power is likely to be more than enough to negate the energy savings due to the efficiency improvement.

So what to do? If the equipment has a belt drive, then the pulley sizes can be adjusted to once again drive the rotating equipment at its original operating speed. Then the operator will receive the full benefits of the increased efficiency of the Premium Efficiency motor. They could also look at adjusting the speed or actually downsizing the motor.

If you are using Motors@Work, feed motor readings into the system (manually or automatically) to immediately identify the problem. Motors@Work will identify the increased speed (load) and pointing the engineer to either modify the design (as an example resizing the pulley), adjust the speed, or downsize the motor.