Challenges of Motor Selection and Sizing

The range of sizes, types, and configurations of electric motors can seem endless. Here are a few ideas for navigating the choices.


CFE Media - machine and motion control - motor-drive iconParadoxically, electric motors are simple, yet complex. Their simplicity comes from having a single purpose: to convert electrical energy into mechanical energy. Their complexity comes from myriad applications where motors are used. A motor’s usefulness is in how it is applied. A spinning motor with nothing connected to its shaft is a waste of time, money, and energy.

However, the value of a motor is how efficiently and effectively its mechanical energy operates conveyors, fans, pumps, and other types of industrial equipment. To specify and apply electric motors, engineers must thoroughly understand the electrical and physical characteristics of the motors and the applications in which they are used.

Terms such as torque, horsepower, inertia, friction, acceleration, and load come to mind when designing motorized equipment. And there are formulas that apply to every parameter. For example, the relationship between horsepower, torque, and speed is fairly straightforward and is calculated using simple mathematics:

Horsepower = (torque in pound-feet x motor speed in RPM)/5,250

Torque and speed can be found by changing the formula algebraically. However, nothing happens unless the motor actually starts spinning, which requires it to overcome inertia of both the motor and its load. This is why pre-EPAct (Energy Policy Act of 1992) motors require five or six times their full-load amps (FLA) to come up to speed, and NEMA premium efficiency motors can require eight to 10 times FLA to reach operating speed.

Inertia and friction work together to resist starting a still motor. Although coefficient of friction is another frequently used motor application term, it can’t be found through direct calculations; it must be measured experimentally. The ratio of friction force to normal force is a simplified definition of the coefficient of friction.

While the coefficient of friction depends on the properties of two materials that come into contact as with motor shaft and bearings, for example, there are other factors that come into play. Temperature, velocity, atmosphere, shape, and lubrication affect the coefficient of friction as well. Obviously, lowering friction increases motor efficiency.

Motors are used in a plethora of applications. While many books about motor design and applications have been written, they barely scratch the surface of possibilities. One of the sidebars with this article gives you an idea of how complex the calculations can be if you want to consider all the relevant variables connected to an application. If you read this article online, there is additional detail and a second example.

Jack Smith is an industry consultant and writer, and served as an editor for Plant Engineering. Reach him at

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