Introduction

Efficient use of energy continues to be a topic of concern. Therefore, it is important that motor users and specifiers understand the selection, application, and maintenance of electric motors to improve energy savings. In this context, energy savings considers the overall application to identify factors to reduce energy consumption. One of these factors is the motor itself. An electric motor converts electrical energy to mechanical energy. For this reason, an electric motor should be considered as always being connected to a driven machine or apparatus having specific operating characteristics such as starting, speed, and load. Selection of the motor for a given application involves weighing a host of characteristics, many of which conflict with one another to some degree. Small (fractional horsepower) motors in the 1/20 through 1 horsepower size are generally connected to single-phase power systems that are found in homes or small businesses and are most frequently used to drive household or commercial appliances.

System efficiency is affected by the efficiencies of all the components in the system. These components include belts, pulleys, fans, pumps, gears, and, in the case of refrigeration, such items as the compressor. Other components that are not a part of the system also affect the overall system efficiency. Selection of a motor to provide for the most efficient system is based on factors such as speed, load versus horsepower, duty cycle, type, and initial motor cost, as well as the efficiency of the motor itself. Successful energy management is when the motor-driven product performs its intended function while maximizing energy savings.

General Concepts

The design of an electric motor involves balancing design features such as starting and running characteristics, thermal performance, and material utilization. Operating efficiency involves a careful consideration of these features and relating them to the requirements of the specific application and the efficiency of the system.

In general, for a given type, motors with larger horsepower ratings are more efficient than those with smaller horsepower ratings when operated at their rated output. However, motor efficiency is also affected when operating at a load less than the designed rating of the motor. Motors operating at less than rated load may operate at a substantially reduced efficiency. Therefore, oversizing—that is, the use of a motor having an output rating greater than the load—should be avoided.

Motor efficiency can be improved by matching the voltage and frequency of the motor with those of the power supply.

In addition, motors with higher synchronous speeds are generally more efficient than those with lower synchronous speeds. This does not imply, however, that all apparatus should be driven by high-speed motors. When the end-use application requires a speed different than the motor speed, consideration must be given to speed changing mechanisms since each component added to the system will impact system efficiency.

Many motors are used for short periods of time and/or a low total number of hours per year. Examples of such applications include can openers, food waste disposers, electric lawn mowers, power tools, etc. In these instances, a change in motor efficiency would not substantially change the total energy consumed since very little total energy is involved.

Yet, there are many applications where motors are used for long periods of time and for a high total number of hours per year. Examples include air moving equipment, circulator pumps, refrigeration compressors, etc. Such equipment, when designed to use a high-efficiency motor, can substantially reduce the total amount of energy consumed.

Types of Motors

The most commonly used single-phase motors are those of the induction type because of their simplicity, dependability, and relatively constant speed. Induction motors include the following sub-types: shaded- pole, split-phase, capacitor-start, and permanent-split capacitor. Universal motors are also commonly used on single-phase power systems in homes on specific applications.

Capacitor-Start, Induction-Run Motors

Capacitor-start, induction-run motors are most widely used in ratings of 1/8 (93 W) horsepower and larger for applications where higher starting characteristics are required. They are characterized by high starting torque, low starting current, and medium efficiency.

Capacitor-Start, Capacitor-Run Motors

Capacitor-start, capacitor-run motors are most widely used in ratings of 1/3 horsepower (248 W) and larger for applications where high starting torque, low starting current, low operating current, and high efficiency are required.

Conclusion

Proper selection, application, and maintenance of electric motors is essential to an effective energy management program. With today's increasing costs of energy, and potential shortages in the future, energy management is crucial from the standpoint of conservation of natural resources, energy independence, and energy availability. As part of a system, electric motors play a significant role in determining total energy consumption; however, they cannot be considered alone and are only one factor in the energy management analysis of an entire system.