# 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.

06/12/2012

Paradoxically, 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 jacksmith.writes@gmail.com.

The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
2017 Lubrication Guide; Software tools; Microgrids and energy strategies; Use robots effectively
Prescriptive maintenance; Hannover Messe 2017 recap; Reduce welding errors
Safety standards and electrical test instruments; Product of the Year winners; Easy and safe electrical design
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Diagnostic functions for system safety; Specifying industrial enclosures; Effective decision support for a crisis
Transformers; Electrical system design; Selecting and sizing transformers; Grounded and ungrounded system design, Paralleling generator systems
Natural gas for tomorrow's fleets; Colleges and universities moving to CHP; Power and steam and frozen foods

### Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.