The potential energy savings realized with premium-efficient motors makes a replacement analysis worthwhile. Any motor that operates continuously is a viable candidate.
You should not focus strictly on high-horsepower motors, as they might already be the most efficient in the plant. By replacing many smaller motors (100 hp and below) you could see the best payback.
Right-sizing the motor
System designers sometimes oversize a motor to provide a safety factor in an application. For example, if 200 hp is required, 250 hp will provide a safety factor and run cooler. This requires a more expensive initial motor purchase, as well as a larger motor starter.
Operation of the motor at a slightly lighter load does not compromise efficiency, but the power factor is lower (Fig. 2 and 3). If the motor operates at 50 percent of full load, both power factor and efficiency drop off. For best performance and efficiency, motors should be sized for the load. Avoid continuous operation in the motor's service factor load range, which is meant to accomodate short-term overloads ," not continuous operation.
If motors currently are using or might be candidates for adjustable-speed drives (ASDs), use of IEEE 841 or NEMA Premium efficient motors should be specified. Significant energy savings is realized from an adjustable-speed drive when the motor is driving a variable torque load and the motor is operated below its base speed. The adjustable-speed drive is throttled back to provide the exact amount of cooling air requirements.
As the speed on a variable torque load is changed, the hp/kW requirements change by the cube of the speed change. At 80 percent of base speed, the HP/kW requirements are reduced to about 50 percent (see Fig. 4). In a typical fan application using a damper or a pump using a valve for flow control, hp/kW requirements change very little as flow is reduced. Significant energy reductions now are possible because of the lower power requirements. Based on the operating profile, the added energy savings can be calculated.
Figure 4. The Advantages of Adjustable-Speed Drives
Using an adjustable-speed drive to control flow results in a greater reduction in power consumption than using a fixed-speed motor with a throttling device.
When a motor runs alone at less than full load, the lightly loaded motor is less efficient and has a lower power factor. When this motor is operated from an adjustable-speed drive, the power factor is held at near unity because of the drive. The lower current draw from unloading the motor is so significant, it outweighs the loss of efficiency.
Most motor manufacturers and DOE offer software to assist with a motor survey. The software performs the math and calculates the payback. Ideas on how to reduce plant downtime by practicing sound motor management are available from "Motor Decisions Matter" (www.motorsmatter.org.) By working with a motor manufacturer and specifying IEEE 841 motors with NEMA Premium efficiency levels, you can ensure the motors will provide good service with reduced downtime. Energy costs also will be reduced.
Malinowski is marketing product specialist for Baldor Electric Co., Fort Smith, Ark.