Motion control and mechatronics continue to embody the extreme opposites of engineering, phenomena that are incredibly simple and incredibly complex at the same time. This apparent contradiction is experienced in many areas. The operation of a fan is one of the most common and basic motor applications, yet to understand the starting torque and acceleration requirements, you have to do some exponential equations because the force of air pressure acting on the fan varies with with cube of the speed. The relationship between motor speed and the number of cubic feet of air moved is only the first tradeoff constraint. The amount of time allowed for the acceleration of the fan is a second order constraint which is coupled to the amount of power needed by the motor over time. The current utilization of the motor combined with the voltage and frequency will define the efficiency of the overall system. Assuming there are no belts and pulleys to transfer mechanical power to the fan load, which, of course, there usually are, we are dealing in units of volts, hertz, amperes, revolutions, cubic feet of air, torque and time. If you are keeping track that’s 7, count ’em, a 7 variable system to turn a fan. 8 if you consider efficiency, but efficiency can be derived from the others. So much for simplicity.
What I find really interesting about this situation is that the motor industry and machinery builders continue to apply electric motors using simple time (speed) and torque tradeoff analysis. Heck, that’s only 2 (or 3) variables depending on how you look at time. Yes, it has some sophistication. Motor sizing programs take into account acceleration and thermal limits. And it should be said that all systems of energy conversion must ultimately be considered as thermodynamic systems. That is, how much energy can I convert, and how quickly, before the waste heat becomes an uneconomical burden to the process. But it’s a bit of an oversimplification to apply a motor to a load using only two variable.
To make matters worse, all motors that are for sale are solutions that are based on manufacturing and cost constraints. They are an average of many designs and requirements blended into an off-the-shelf package that is readily available. The motor’s design has very little to do with the particular requirement that you and I might be faced with in a given application. The information exchange between the motor vendor and the engineer applying the motor is to understand the peak and continuous torque that the motor can produce and fitting the curve to the load requirement. Making this a predominantly mechanical exercise, motor output (mechanical) to load requirement (also mechanical).
But in the age of the Personal Computer, with ever more powerful design tools available, it appears to me that the next frontier will be applying the new tools to the old motor and load problem. I know some of the people working on it, and the progress is very good. A new and very powerful age of design tools is dawning.