Work, power, and energy defined

Electric motors, in essence, are conversion devices. They convert one form of energy (electrical energy) into another form (mechanical energy). In the process, they consume power, and they do work.

It is easy to be imprecise about these terms as well as the units of measurement we use in connection with the terms, such as horsepower, watts, and amps. Obviously, it’s beneficial to have a precise understanding of the relationship between these units of measurement, especially since they can help you make sense of the related issues of efficiency and power system sizing. So, here are some precise definitions of terms.

“Work” is performed when something is moved. It may be the work involved when several stagehands move a piano or when a gas engine moves an automobile. Appropriate examples for this article include water being moved through a pump by an electric motor or a garage door being lifted by a motor-driven opener.

“Power” is the measurement of how much work is accomplished in a specific amount of time. A bulldozer is capable of moving a hill of earth much faster than a garden tractor, therefore we say the bulldozer is more powerful than the tractor. Before bulldozers and garden tractors, horses performed much of the heavy work needed by humans. Thus, when the scientific concept of power was developed hundreds of years ago, they described it in terms of “horsepower,” a term still in use today.

“Energy” is the ability to do work. Energy is stored in such things as coal, gasoline, and the food we eat. For energy to be released, some chemical or mechanical action must be performed on whatever stores that energy. Coal is burned, gasoline is compressed and heated to make it explode in an internal combustion engine, and our bodies oxidize the food we eat.

Electrical energy is produced mechanically by a generator or chemically by a battery. Electrical energy performs work once it’s applied to an electro/mechanical conversion device, such as a motor.

One of the measures of work is a unit called “foot/pounds.” A foot/pound is simply the work done when a one-pound weight is lifted vertically the distance of one foot. So, if a 55-pound weight is lifted vertically 10 feet, 550 foot/pounds of work has been accomplished.

As I said before, power includes a time factor. It takes more power to move the 55-pound weight in our example 10 feet in one second than it would to move the same weight the same distance in two seconds. The power it takes to move that 55-pound weight 10 feet in one second would be measured as 550 foot/pounds per second. This power value is equivalent to one horsepower. Therefore, a four-horsepower electric motor would be able to move a 2,200-pound load (4 x 550) a vertical distance of one foot in one second, or an 1,100-pound load two feet in one second.

Just as horsepower has an equivalence of foot/pounds per second, a “watt,” the unit of electrical power, has a relationship to horsepower. One horsepower equals 746 watts. Therefore, a 10-horsepower motor also can be said to produce 7,460 watts of power. The input watts to this motor, however, will be higher because not all the electric power can be converted to mechanical power. Some of that input power is wasted in the form of heat. The relationship of the input power and output power represents the motor’s efficiency.

An electric motor does its work by turning a shaft. Whenever work is accomplished by rotating something, it is referred to as rotational power or “torque.” A common measurement of torque is the pound/foot. When a force of one pound acts on a radius of one foot, the result is one pound/foot of torque. For example, if a motor drives a pulley with a two-foot radius, and the belt on the pulley has a force on it of eight pounds, the torque supplied by the motor is 8 x 2 or 16 pound/feet of torque.

Having the precise, scientific definitions of terms not only enhances your understanding, it also helps you to better see the relationship of concepts such as efficiency and torque when you look at electric motors and their applications in the field.

by Neil Simon, aka the Motor Doctor - regional sales manager for A. O. Smith Electrical Products Company.