There is a subtle premise that often escapes us as we talk about motors and the controls that run them. It is that the motor and controller operate as a package. In most situations, a customer specification is for input voltage and output torque and speed. That’s all that is important. How you get there doesn’t matter a great deal.
But ironically, most motor manufacturers are predominately mechanical engineering centered. And most drive electronics companies are electronics centered. And they have very little in common with each other. Except that their products must work together. And oftentimes, that’s where the trouble starts.
The drive manufacturer warrants that his drive will produce current and voltage. But the the motor can have very complex constraints to deal with in response to the excitation of the electronics. How accurately a 6 step approximation of the sine wave performs, for example, can result in overheating in the motor depending on the loading of the system. And as the motor winding heats up, the resistance in the motor can change dramatically, especially in the low inductance windings that are common in many specialty motors available today.
Then there are the cabling issues for connecting the motor and drive electronics. The ac drive industry found out quickly that long wire runs can result in stored energy in the wires themselves. Standing wave phenomena could cause higher voltages than expected and blow holes in the winding insulation in the motor.
Power semiconductor prices have fallen considerably in the last few years creating situations where it is sometimes cheaper and more reliable to put in parallel devices than to attached single power devices to large heat sinks. This leads to some serious new options for packaging the electronics. How about drive circuits in the end bell or junction box attached to the motor? Actually, some models of the GE ECM motor (now owned by Beloit) are ac fan motors with variable frequency drives and intelligent controls built directly into the motor end bell. You may have one in your main air handler in the air conditioning system of your home. I was surprised to find out that I did.
I used to think that thermodynamics of these systems would be impossible to manage. But the fact is that the drive efficiencies are getting really good. One team I worked with was producing a 500 Watt brush drive that only shed about 20 Watts of loss at full load. That’s some incredible efficiency. So the notion of integrating motors and drive electronics is much more reasonable than it used to be. And there are stepping motor packages that have been doing it for years.
So where is this all heading?
The fact is that the motor and drive electronics must work together as a package. There is an increasing need, and an opportunity to create further performance enhancements, by the two technologies working more closely together. More innovation will lead to better energy efficiency and new design opportunities and a chance to recharge (pun intended) an industry that has been losing share to offshore competition in the last few years.