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Electric Motors

Purchasing Tips

The best time to purchase a premium efficiency motor is when you are planning new plant capacity or when a motor has failed and must be replaced or repaired. Failed motors need to be replaced quickly. This is why it is always best to have a motor management plan in place to ensure that a suitable NEMA Premium™ replacement model is available. A motor management plan can be as simple as having a set of criteria for all repair/replace decisions. But ideally it is better to make those decisions in advance, starting with the motors that have the most critical applications.

Now Available! CanMOST &##8211; the Canadian Motor Selection Tool
ELECTRIC MOTORS Energy Efficiency Reference Guide

In some cases it is worthwhile to replace motors before
they fail. If the motor is old, then chances are its efficiency is below even the regulated minimum efficiency standards, so switching to a NEMA Premium™ motor would offer even greater operational savings.

Motor Decisions Matter offers more information and advice on motor management plans.

Selecting an efficient motor

To select the most appropriate motor for an application, you must know the motor type (size, speed and enclosure) and motor load (constant or variable).

CanMOST – the Canadian Motor Selection Tool – is a useful software tool for selecting energy-efficient motors. Modelled on the successful U.S. industrial motor energy management software program MotorMaster+, CanMOST was developed for Natural Resources Canada by the Washington State University Extension Energy Program in partnership with the U.S. Department of Energy and an international consortium. With a database of more than 43 000 motors (25 000 60-Hz North American and 18 000 50-Hz European) from 1 to 2000 hp, this software program allows you to compare the dollar and energy savings associated with any motor purchase, rewind or replace scenario.

CanMOST – The Canadian Motor Selection Tool – will help you compare motors and choose the best one for your application.

Motor size: Motors should be sized to operate with a load factor between 75 and 100 percent. As shown in Figure 1, the energy efficiency of a motor drops off at load factors below about 50 percent. Using a motor that is oversized for its application will result in a loss of efficiency.

Figure 1. Motor Load Efficiencies

Figure 1. Motor Load Efficiencies

Source: Premium-Efficiency Motors, Natural Resources Canada technical fact sheet M144-20-2003.

Watch the motor speed. Some energy-efficient motors will run slightly faster than the motor being replaced, and this speed increase can negate the energy efficiency of the new motor. For centrifugal fans and pumps, the power is proportional to the speed cubed. For example, a 1 percent increase in speed will result in 3 percent more electrical consumption. If the new motor is 2 percent more efficient than the old one, the energy-efficient motor will actually use 1 percent more energy than the old one. Always select a new motor that has a full-load operating speed that is less than or equal to that of the motor being replaced. The Canadian Motor Selection Tool (CanMOST) provides full-load rpm data, so that speed changes can be taken into account and motors can be properly matched to your equipment needs.

Minor speed variations can be sometimes be adjusted in belt-driven equipment by changing the pulley size; in direct-drive pumps, speed can be adjusted by trimming impellers.

Use a variable speed drive (also called a variable frequency drive). For fluctuating loads with long duty cycles, a variable speed drive can help minimize energy consumption for an induction motor. A variable speed drive works as a frequency inverter and can regulate, over a wide range, the speed of the motor to fit the load demand. Energy efficiency is increased, in some applications by as much as 50 percent.

Watch the power factor. Power factor is the ratio of the real power (kW, or the power the motor is actually using) to the apparent power (kVA, or the power you are paying for). When a motor operates near its rated load, the power factor is high, which is good. A motor that is oversized and lightly loaded has a lower power factor. In addition to increased electrical costs, a lower power factor may reduce the building's voltage, increase electrical distribution system losses and reduce the system's capacity to deliver electrical energy. A facility with many induction motors and a low power factor can address this issue by replacing these motors with properly sized premium-efficiency ones. Power factor can also be improved by adding power-factor correction capacitors to the in-plant distribution system. Although choosing the most energy-efficient motor is most important, try to ensure that the power factor of the new motor at its operating point is higher than the power factor of the motor being replaced.

List of motor manufacturers

Consult our list of manufacturers that make energy-efficient 60-Hz electric motors for the North American market.

Next: Operation and Maintenance