Advantages of using adjustable speed drives, part 1: Energy efficiency in motors

Tim Albers and John Malinowski discuss energy efficiency in motors and synchronous motors and the benefits engineers can realize.

By Plant Engineering Staff August 25, 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).

The combination of a motor and adjustable speed drive (ASD) is called a power drive system (PDS). Specifying adjustable speed drives can help a manufacturing process become more energy efficient and productive.

On variable torque applications (pumps and fans), one can expect energy savings of 20 to 40% with a PDS when compared to an application using a valve or damper for flow control.
Selection of the proper motor for a PDS will be discussed. Induction and synchronous motors will also be introduced. The new synchronous motor types are more efficient and offer a higher power density.
Electronically commutated motors are a type of synchronous motor technology and typically programmed for a single purpose at the factory because they are designed for specific use by the original equipment manufacturer (OEM). General-purpose drives can be programmed at any time to do whatever job is required.

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

John Malinowski: We’re primarily going to talk about energy efficiency with regard to induction motors and synchronous motors. Energy efficiency is different from efficiency, so system efficiency. We’re going to talk about ASD savings basics and variable load versus constant load, which is different from duty cycle. Then we’ll discuss some research that NEMA has done and we’re also going to look at some DOE research. Tim is going to cover power index, how to use it and then we’ll look at some typical savings values when we use drives.
What is a power drive system (PDS)? It could be a one-piece device like an ECM that has an integral motor and drive together, or it could be two pieces, a motor and a separate drive module. In our definition, it does not include the driven load. That’s a separate device. So why should you add a PDS?

If your equipment cycles on and off, or the load varies, or it’s too loud, or you’re throttling it with a valve or a damper, a pump or an HVAC system, or it doesn’t operate at full capacity. We all oversize our equipment to operate at worst-case capacity, but it rarely operates that. It’s usually 60 to 70%, so you can throttle it down with a drive or if you always have a lot of mechanical maintenance with breakage and things, the drive can help you out by soft starting.
Why is benchmarking for energy savings important? It lowers the cost of energy use. If you know what you use, you know how much you can possibly save. Many people say, “We do co-generation here. We generate our own power.”

Well, in many cases, you’re actually selling the surplus of your power back to the utility. If you save electricity in your plant, you have more to sell. It also allows for expansion before you have to increase the generators you have in your plant for generation.

There are many good reasons to save. There are some non-energy benefits, like we said such as decreased maintenance. The motor can be set up to soft start, reducing wear and tear on mechanical components. If you have a lot of things on your conveyor, soft starting prevents bottles from tipping over and things from clunking into each other. You can improve process control by using the drives to optimize feed rates from all your basic feeder systems.

You have system connectivity and visibility. You can remotely monitor what’s going on and see if something is out of spec, overloaded, or even underloaded. If something is not feeding into the system as it should, you can set up an alert for that as well. A drive essentially corrects for power factor. With the right drive, it can be a regenerative drive that takes surplus energy from an overriding load and puts it back on the line. It also compensates somewhat for voltage regulation. 
Here we’re going to get into types of loads, and this is a little bit different from what you might be used to when we talk about duty cycle. This is not duty cycle; this is variable torque.
Variable torque means when you change the speed downwards, like in a centrifugal load pump, fan, or compressor, it follows the affinity laws. As the load decreases by speed, by the cube, half speed is like a quarter load. So as the speed goes down, the load on the motor goes down exponentially.

Energy savings directly decrease with reduced load. In this case, with a variable torque load, one can expect 30 to 40% energy savings when used with this. A constant load is like a conveyor or a hoist or a constant displacement pump. You change the speed, and the torque requirements are the same regardless of the speed change. The energy savings that are easy to see with a variable torque load are more difficult here. You’d have to benchmark it by how many widgets you produced per kilowatt-hour. That’s much more difficult to do, but it’s possible.

Then there’s constant horsepower, like a spindle drive. This is a case where you’re running the product above the base speed or the synchronous speed of the motor. As the speed goes up, the horsepower tapers off directly above the base speed. Those are very rare applications. So we’re looking at energy savings here with the power drive system. At 70% flow, we’re getting a 49% power reduction. So there’s a significant energy savings and if you drop below 70%, the savings are even bigger. This is for centrifugal loads like fans and pumps and compressors; this is not a constant load like a conveyor.

Tim Albers: John, I just wanted to add one comment. If you go back to that slide, the comparison in energy efficiency is actually not in comparison to a fixed speed running all the time unconstrained, but that top level that it’s comparing to is actually the throttled value. So that would assume that there’s a valve or a damper in place. If it’s a completely unconstrained flow, the savings are even greater. I’m just pointing out that the significant savings John mentioned, which are substantial, are actually in comparison to a throttled value. It could be even more significant if you’re not throttling.

John Malinowski: So, now we’re talking about variable versus constant load systems. This is not constant torque or variable torque; this is constant load. A constant load would be if I’m running the motor with the drive at a fixed speed. Again, we said a lot of times the motor is sized for worst case and maybe the optimized feed rate is 70 or 80%. I always run at that 80% and running it other than that is not usual. Variable load is more like a demand load situation. You might run it at 25 or 50% or whatever flow for shorter periods of time as needed. And that’s more like when you’re in a hotel and have a water system, everybody gets up in the morning and takes a shower.

The demand on the pumps is high in the morning, but during the day when nobody’s in the rooms, the water supply is low. So that’s what we’re talking about, variable load versus constant load. All different types of applications can have energy savings, some more than others, and we’re looking at the difference between variable load and constant load. So you can see the difference. There are savings everywhere here.

Tim Albers: What this shows is in different applications, one of the unique things here, and I think a lot of people have heard about variable load energy savings. John pointed out it’s 30 to 40%. You can see here it can be as high as 50 or even more than 50%. But one thing that’s more unique is this concept that even something that runs on a constant load can actually have energy savings. And we’ll talk a little bit more about that later, but one of the things that makes this so efficient is the fact that very often an induction motor running at a fixed speed on the sine wave just running on utility power has a very specific speed that it’s going to run at.

And very often the pump or the fan or the compressor’s best efficiency point is not at that exact point.
Even though the load is constant and you may even cycle on and off, having the power drive system with a VFD and motor in place allows you to run at that best efficiency point. And running at that best efficiency point is where you get these energy savings of 20, 10, 15 and even up to 38% over just running on a sine wave. That’s a unique concept, but we have a significant amount of research now, both independent research from some energy advocate organizations as well as the Department of Energy that show that this is the case. So it’s kind of a unique concept, and I think that’s one of the biggest things that we want to get across here is that for variable torque type applications, particularly even constant load systems can show significant energy savings.

John Malinowski: And to Tim’s point, this data is from NEEA, the Northwest Energy Efficiency Association. They’re an advocate that works with all the utilities in the northwest and I think some in Canada, in the Vancouver area. Just to kind of mess with your mind a little bit and to basically state that adding a drive with a motor doesn’t make something more efficient. It adds about 3% losses to the system for the drive losses, and then it adds a little bit of losses to the motor. So it’s how it’s used together with the motor and run at a different speed or load or such that is saving the energy, and that’s the difference between variable torque applications and constant torque applications. This is another thing that came up in the NEEA work.

We looked at the variable load and the constant load systems and how much savings there was by horsepower. And once you get up to the 20, 25 horsepower range, the amount of savings percentagewise is pretty even. Everybody talks about going from a premium motor, an IE3 level motor, to an IE4 level motor to save energy. While the bottom graph there is showing what happens when you simply switch motors out without a drive, by going from a NEMA premium motor to a NEMA super premium motor. You’ll get some efficiency improvement, but it’s nothing like putting a drive on the existing motor that you have in your plant.

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