Advantages of using adjustable speed drives, part 3: Questions and answers

Tim Albers and John Malinowski answer questions from the audience on variable frequency drives (VFDs), determining the optimal speed for efficiency and more.


By Plant Engineering September 8, 2023
Courtesy: CFE Media and Technology

Motor and drive insights

  • Engineers can save energy while keeping control with adjustable speed drives (ASDs), but they need to know the applications for power drive motors.
  • It’s also important to know when a synchronous motor is advantageous over an induction motor.
  • Engineers also need to know how to install and add a variable frequency drive (VFD).

John Malinowski, motor industry consultant for JMAL Consulting and Tim Albers, an IEEE senior member and NEMA MG1 technical committee chairman, answer audience questions in the February 16, 2023, webcast: “Motors and drives: Advantages of using adjustable speed drives.” This has been edited for clarity.

Plant Engineering: Can you talk briefly about VFDs and stray voltage causing issues with motor shafts or bearings?

John Malinowski: There were quite a few questions about circulating currents in a motor and shaft grounding brushes and such. We normally see those in systems that are not well grounded where a shaft grounding brush needs to be added. A shaft grounding brush is sort of a Band-Aid that doesn’t address the shaft or the main grounding problem. But when you’re in a commercial building, a lot of times they don’t have the same robust grounding practices that you might have in the industry. So we can use a shaft grounding brush that’s usually on one end of the motor.

Alternatively, you could use a ceramic bearing insert that would insulate the bearing from the frame or use a bearing with ceramic balls. Two downsides of the ceramic aspects, the ceramic bearing insert sometimes tends to crack. And then in those cracks, you kind of get a carbon dust buildup that’s conductive, so that kind of goes away. The ceramic ball bearing is a good solution. However, they’re expensive and in larger sizes, they’re quite difficult to source.

Tim Albers: There are a lot of solutions now that are potentially for circulating currents and there are some standards that people do apply now with insulation. However, you insulate at least one end of the motor and then provide a shaft grounding system of some kind. And there are lots of options for that now that didn’t used to exist. So the other thing that does exist now is there’s also a number of manufacturers. I’d say most of the larger manufacturers are also capable of putting a shaft grounding device inside a hazardous motor. So you even have a potential solution here for your hazardous product which didn’t used to exist.

John Malinowski: And to that point, a lot of manufacturers have motors with shaft grounding brushes available from inventory. It’s not something that’s special, it has to be added later. So a lot, particularly in the HVAC portfolios and a lot of manufacturers on their inverter-only motors have added a shaft grounding brush.

Plant Engineering: Tim, how do I determine the speed for optimum efficiency?

Tim Albers: You’re going to have to work with the pump or fan manufacturer. There is going to be a pump or a fan curve, and if you’re familiar with reading those, you’ll be able to point out the best efficiency point on that curve, which will then give you the speed versus the flow that you need. A quick discussion with your supplier or even your distributor will give you that answer. So that’s how you need to do it. It gets a lot more complicated if you’re dealing with an older piece of equipment and you don’t have those curves. Can’t answer that now. There are ways to do it, but if you have that and other stuff, then it makes it a lot easier.

Plant Engineering: John: How does the improved motor efficiency affect the inrush current of the motor?

John Malinowski: This is something we’ve been pulling our hair out and you can tell by both of us that we’ve done a lot of that over the last few years. As you change efficiency, we either have to have inrush current go up or starting torque go down, and nobody wants starting torque to go down. So inrush has gone up. Our charts in NEMA MG1 we adjusted a few years ago and raised them by up to 15% on design B motors. And as DOE is pushing us to the super-premium IE4 levels, you’ll see a new category of inrush currents come out that we’re going to model after the European IEC standard, and they’ll be called a design BE motor.

They’ll also be a design CE, but it will still be at a premium efficiency, but it’ll allow for the higher inrush currents. Presently on premium-efficient design C motors, they exceed the allowed design B inrush, so most manufacturers call them design A motors with design C torques. We’re trying to get away from design A motors which are undefined. We think propagating the industry with more design A motors is an electrical safety hazard waiting to happen. So we want to quantify what inrush currents are so the proper starters can be selected.

Plant Engineering: Tim: Why would you combine a synchronous motor with a drive?

Tim Albers: I think that’s something that we talked about a little bit earlier and most synchronous motors, there are some synchronous motors that can run on utility power, but the vast majority would be optimized by running on a drive. And the point is that putting the synchronous motor and a drive together, there are a number of manufacturers, both motor and pump and even fan manufacturers that are starting to do the combination of the motor and the drive, synchronous motor and drive, and even the final application, pump or fan or compressor, and they’re selling them as systems. And the reason why is because they’re able to then as a system efficiency, as total power use to get out, to get things out, they’re actually optimizing with the synchronous motor and drive. So that’s the primary reason people are actually able to hit the lowest power usage for the output of whatever they’re trying to do by implementing those together.

Plant Engineering: John: When you are retrofitting a drive, does the motor need to be rated for inverter duty?

John Malinowski: Depends on the application. This gets into what Tim talked about, where it is inverter-capable or inverter duty or inverter only. DOE calls it an inverter-only. Is it going to be a motor that you’re going to need extreme high torque throughout the speed range, maybe operation above base speed with constant horsepower range, something akin to almost a servo motor type of an application, more torque-oriented than horsepower-oriented? Most motors, even TEFC, general-purpose motors, we call them inverter capable, CSA makes us put inverter duty on the nameplate. Those motors that have gone through the CSA for Canada will have both variable torque and constant torque speed ranges on them.

So you know what you’re getting even on a general-purpose motor. And I would say that the vast majority of applications, a general-purpose motor, or even like an IEE 841 motor or whatever people call their motors for severe duty, outside duty like chemical process duty, those motors are all capable of running any kind of variable torque load. And on constant torque load, it will depend. Smaller motors have a little bit more headroom as far as temperature rise. And as you get into larger motors, a larger motor might only, several hundred horsepower might only be able to do constant torque two to one or four to one. You need to really talk to the motor manufacturer and figure out how you’re really going to use the motor.