It has been over 100 years since the invention of the AC motor by Nicola Tesla. You would think that after all this time, that getting the motor and drive size right would not be a problem. Yet it remains one of the most common difficulties in modern manufacturing. This is due, in part, to the advent of power semiconductors for controlling electric motors, which requires special consideration. Electric motors and power semiconductors conduct electricity in different ways and it is crucial to understand those difference in order to get it right.
The fundamentals, however, have not changed. It’s all about the load.
Electric motors in industry consume 40-50% of all electricity used in the United States. This makes motor sizing much more significant and a greater priority to make sure that we get it right. As manufacturers seek to reduce operating expense, large energy users can benefit directly from better motor sizing practices. According to the Department of Energy there are a lot of motors that are incorrectly sized. A small change in efficiency across a large number of applications can result in a big change in power requirements. Not only in total energy required but also in power factor, starting loads and peak loads.
Specifically motors and drives are different in one primary way. Motors are mostly copper and steel and drives are mostly silicon. As conductors the two materials behave in profoundly different ways.
We can consider the stator by itself for the purpose of understanding the electrical behavior in a circuit. An AC motor is a giant inductor, copper wire wound around electrical steel. In spite of the fact that electrons move pretty quickly, they are very small, and it takes some time to fill up all the copper and magnetize the steel. So in one respect it behaves like a capacitor, it has a time constant in which it becomes charged. The copper exerts an electrostatic field on the steel and exerts work on the molecules of iron causing them to align and sustain a magnetic field, which also takes time.
All of this work takes time and produces heat. The bigger the motor, the longer the time constant, the more the heat. AC motors can take between 3 and 7 seconds to come up to speed depending on the load. If they are highly loaded they can take 20-30 seconds or more depending on how well they can dissipate the heat. Motors will pull in current until the load moves or the winding fails, which can reach current levels of 600-700%
The semiconductor on the other hand responds in microseconds and is much more sensitive to the rate at which current is applied and is limited generally to 200% of its rating. Therein lies the problem.
More next week.