Defining Brushless DC Motors #1 of 4: Six(6) Step Proven Procedure

10 Oct Defining Brushless DC Motors #1 of 4: Six(6) Step Proven Procedure


Defining Brushless DC Motors #1 of 4:

Six(6) Step Proven Procedure

By John Knott, President of JD Technologies, LLC


In a previous blog, Composite Motors, Inc., a high quality motor manufacturer that JD Technologies LLC represents spoke about the importance of defining a brushless DC Motor.

They stated and rightfully so that when designing a system, you want to guarantee the motor delivers the intended performance. Conditions and specifications ultimately dictate what type of motor best fits your needs when it comes to DC motors. Brushless DC Motors (BDCM) offer advantages that include but are not limited to higher efficiency, lower maintenance and more torque.

In this series of blogs I will present a procedure for defining a permanent magnet BDCM. We are writing for the designer who has no prior knowledge of motors as well as the expert who needs a refresher. I will help you use the laws of physics to define a motor for your application.

First what does a proven procedure mean and where did it come from. When I was a young designer of hydromechanical systems I was able to use my mechanical engineering degrees (BSME from University of Connecticut & MSME from Northeastern University) to design mechanical and hydraulic sub-systems and components. With these degrees and my experience I was a confident designer. Until one day the Vice President of Engineering called me into his office and informed me that our company was getting into the electromechanical systems market and that I now needed to work with motors.

I dusted off my electrical engineering text books, started researching motors and quickly realized I needed a BDCM mostly because of the small envelope I had to work with and the harsh environment (high temperature and pressure). After some research I started working with numerous motor manufacturers who were very helpful during this critical development process. Initially I was using no-load speed and stall torque to define a motor. Using my customer’s requirements for power and just using these parameters the BDCM had a hook on it! The motor running steady state would be so big (over 8 inches in diameter) and heavy it required a hook to transport it. That was not going to work for my 3 inch diameter envelope! What saved the day was the duty cycle which for my application was not steady operation but rather a very short duty cycle of a few minutes. Because of this I was able to use a much smaller BDCM.

The procedure I developed is not patentable since it simply draws from the laws of physics to arrive at the equations to use and to derive the associated constants. The procedure involves these steps:

  1. Note the excitation voltage, torque rate (stall, rated and no-load speed).
  2. Using the efficiency (of motor and gearbox) determine the torque and speed requirement at the motor.
  3. From this you can determine the slope of the torque – speed curve.
  4. Now one can determine the back emf constant and the motor sensitivity.
  5. Knowing the motor sensitivity one can get the theoretical resistance value.
  6. Now one is ready to approach a motor manufacturer and pick out a motor. Since R is physically limited the value will probably change somewhat.

This simple six step process does work so it is proven but one cannot minimize the importance of working with a competent motor manufacturer like Composite Motors, Inc. to help with the process.

More to come

I hope you enjoyed this overview of our proven procedure. In the upcoming blogs we will get a little more technical by defining the equations to be used and then give an example.

Stay tuned!

Let’s Talk

I am confident that Composite Motors, Inc. with their Team and resources would be an essential foundation for supporting your BDCM development. All of Composite Motors, Inc.’s products are made on American soil. Let’s talk! Email me at or call me at 781-864-2220 or visit Composite Motors, Inc.’s website at

About JD Technologies, LLC

JD Technologies LLC provides premium sales and marketing services to a group of high quality, complementary manufacturers of engineered services and products who serve the aerospace, military & defense, industrial, and medical industries. They develop long term relationships with key customers who value the consultative style of selling and who wish to interface with sales professionals of high integrity. Their approach is to use proven consultative selling techniques. Consultative selling is a collaborative process that leads customers/prospects through an analysis of their current situation to a resulting improvement. For more information on JD Technologies, LLC, their products, services and their consultative selling methodology, visit



by E.S. Smith. Pub. by Wiley, Inc


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