Advantages of using adjustable speed drives, part 2: Installing and adding a VFD

Tim Albers and John Malinowski outlines installing and adding a variable frequency drive (VFD) into a system.

By Plant Engineering September 1, 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).

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

Installating, retrofitting a VFD into a system

Tim Albers: What I’m going to talk about is actually a retrofit or the installation or addition of a variable frequency drive (VFD) into a system. So we’ve already talked about the different types of loads and one of the things that we made very clear is that we’re focusing on variable torque loads. In this case, a fan or a pump load is perfect; it could be a number of different compressors as well. So one of the things, there are some kind of best practices I’d say that we would recommend. One is to start with a NEMA premium motor or better if it’s synchronous.

Obviously, we’re putting the drive in before the motor. And then the second thing is taking out the valve or at least opening the valve or the damper, making sure that those are not being used anymore. And then adjusting flow, using the VFD and taking whatever feedback was going to the valve or going to the damper, and instead of taking it there, now you take it back to the drive.
So that means that whatever that set point was based on flow, pressure, temperature, tank level, it could be any of those measurement devices that would now come back to the drive. Now, that’s assuming that you’ve got a variable flow system. If you have a constant flow or a constant load system, you wouldn’t need a feedback device at all because you’re just setting it at the best efficiency point and running. So one of the other things that should be considered for the installation of a VFD, which is unique then, it is when you’re running a motor on a sine wave, is the cabling between the drive and the motor. So it is an absolute consideration. Keeping that distance as short as possible is always recommended.

There’s very specific cabling designed for use between motors and drives. There are, the drive manufacturers, in many cases, are going to have specific recommendations on what that cabling should be. One of the best things that I’m going to tell you to pay attention to as part of that is also going to be the grounding system and how you actually ground from the motor back to the drive and end to the earth ground from both the motor and the drive. So I just tell you there are a number of best practices. We’re not going to spend a lot of time on that today, maybe in a future discussion, but just a little bit of input there.

A few other things to take a look at when you’re putting a VFD into the system is there are these things called common mode voltages. I’m not going to get into a lot of detail about why a VFD creates a common mode voltage, just trust me it does and there are some things that we can do in the system to make sure that the common mode voltage does not affect the motor or the system very much. So one is, one of the primary ways, again, is the grounding from the motor back to the drive using a high-frequency ground that is specifically designed for this purpose. So again, many of the drive manufacturers will tell you what that is and even have some recommendations on what type of cabling that is, but it does take care of all of these issues, the line imbalance, the oscillating carrier frequency, and even other poor grounding potential. So what I’m going to say is make sure you pay attention to grounding and follow the recommendations of the manufacturer, particularly as it pertains to the grounding from the motor back to the drive.

So I’m going to go over just some terms and I think it’s important to know when you look at these terms so that they shouldn’t be scary to you or whatever, just so people understand what they are. These are some types of adjustable speed drives. One of them is volts per hertz inverter. So these are the easiest drives. They don’t have in many cases all the complicated instructions and things like that. Basically, when it says volts per hertz, it’s designed to basically, if it’s 460 volts at 60 hertz, then it’s half 230 volts at 30 hertz. It just does that. It doesn’t have a lot of capabilities necessarily for doing some high-end machine controls, but for a variable torque load like a pump or a fan, it’s perfectly suited for that and will do a great job. There is something called an open loop flux vector and a closed loop flux vector drive.

In many cases, those drives can be the same and it just comes down to whether they’re connected with a closed loop speed reference or not. The thing is that a flux vector drive can do a variable torque load. It has the capability to run your fan or your pump. It just may have a lot more capabilities than that. So it may be a little more expensive potentially, and it might take a little bit more to set up because it’s going to have a lot more capabilities, so therefore it has a little bit more that has to be set up. So some of them, the hardware is the same between a volts per hertz drive and a vector drive, and in some cases, it’s not. My point is all of these will work for a power drive system, and particularly if you’re doing a variable torque load, all of these will work. They just may vary in expense and maybe a little bit in complexity for setup.

Naming electric motors and their role

So let’s talk about naming electric motors and what they’re used for on a drive. Let me start by saying first of all, most NEMA premium motors are going to be capable of being run on an inverter. If it’s a NEMA design A, B, or C or an IEC design NRH, they’re going to be capable and have an insulation system that, at least in a general-purpose mode, will have the capability to run on a drive. And in many cases, you’re going to see that the motor manufacturer is going to say that yes, this is capable and suitable for running on a drive.

The question is going to be what is the term? There are many, many terms in the industry that refer to inverter-capable or inverter duty or inverter-rated type motors. There is an industry term that the Department of Energy has started to use, which is called inverter only, and inverter only does mean something specific. And what that means is that it’s not even capable of starting on a sine wave. So if you see an inverter-only motor, that’s a motor, you’re going to have to run on a VFD, not really capable of running on sine wave power.

There are a few things about it. Because of that, the DOE doesn’t regulate it, but it’s also very specific, usually used for a very high-speed range, constant torque loads, very specific, definitely an overkill motor for a variable torque application such as a pump or a fan. So what I’m telling you is most motors are going to be capable of running on an inverter. Be careful of saying, I need an inverter-only motor or the best inverter duty motor. If you’re running it on a pump or a fan, you may not need that and you might be spending a little bit too much versus what you really need.

IEC standards to know for motors

The next thing is I just want to address from the IEC perspective, if there are some folks that are mostly in the IEC world, there are a couple of IEC standards that do address inverter-duty motors or what they call IEC converter capable motors. So they’re calling the VFD or ASD a converter, converting I guess to the speed of the frequency. So one thing is that there are a couple of IEC standards there, you can see them listed. You can actually, when you get a copy of this, you can click on those and go to those standards. But those are standards for testing of the IEC motors. And again, just to recognize that there’s some different terminology within the IEC world and a converter-duty motor will also run very well on an adjustable speed drive, but you may not necessarily even need to have that to be able to run again, a variable torque load for your power drive system.

The equivalent NEMA standard as we talk about it here, a converter-capable motor, the NEMA standard is this part 30 within NEMA MG1, and that is kind of a general-purpose motor as I talked about before, like a NEMA premium motor running on a drive and then this converter-duty motor is the more dedicated motor. And that’s similar to the NEMA MG1, part 31, and I know I can’t talk about companies, but NEMA is generic, and so I will make the comment that if you go to the NEMA website, you can actually get the NEMA standard, NEMA MG1 for free. So you can download that and have fun reading through that document. It’s a big one, but there’s a lot of great information in there.

So I’m going to go back to, if you remember from the very beginning of this presentation when John was talking, he defined the power drive system and the power drive system you see right in there is circled in red, PDS, power drive system, and it includes the drive modules that make up a full VFD and it also includes the electric motor. So that is the power drive system. Now, a motor system then includes some other things and starters and contactors and other things that we’re not going to talk about here today. And then when they bring it out to this extended product, that includes things that aren’t included in our discussion, but that would be then the pump and the fan and the compressor and those kinds of things. A lot of this is defined in an IEC standard.

The IEC standard is called 61800-9-2 and also 9-1. NEMA helped write those standards and we actually utilize those as well. But there is a significant amount of information in those standards about calculating your power drive system energy use and comparing it to other power drive systems and also comparing it to running something that’s actually across the line. So if you have an interest that goes into a significant amount of detail, but I would tell you that you don’t necessarily have to get into that level of detail to understand the energy savings that you’ve got, and I’m going to talk a little bit about that in the future. But NEMA has a power drive system standard that helps calculate savings, which is actually simpler than the 61800-9-2, but it also doesn’t account for anything except for variable torque. So the NEMA standard is just variable torque. This standard from IEC can do pretty much all different types of applications.

Synchronous motors’ role

The next thing I wanted to go over is the fact that we’re not just talking about induction motors. So I wanted to go over a little bit about synchronous motors and kind of take you through what I’m talking about when it comes to synchronous motors.

So two things, first of all, if you look at that little chart over here on the right, this is a 60 hertz chart and you see the number of poles, the synchronous speed of a motor and then the asynchronous speed. So when we talk about what an asynchronous motor is, an asynchronous motor, in most cases, we’re referring to this cage motor right in the middle of this chart here. And that cage motor follows across electric motor, can go drop down to AC motor, then go to AC asynchronous induction and then go to a cage rotor that is your induction motor.

That is 90+% of all the motors that you’re going to deal with are going to be that cage asynchronous induction motor. When we talk about synchronous motors, primarily what we’re referring to is a follow, the AC motor drop down to the synchronous induction motor, go over to the right and there’s a few different motors down here at the bottom. There’s switch reluctance, permanent magnet, and synchronous reluctance. And I’m going to tell you that there’s a very strong derivative of synchronous reluctance motors called synchronous reluctance with permanent magnet assist or something like that. So the reason why we wanted to talk about synchronous motors is that power drive systems are absolutely capable of using induction motors as well as using these synchronous motors. And the synchronous motors actually provide even higher savings, particularly as it pertains to part-load. So I will get to a little bit of that in a minute.

One thing that we talked about briefly, John talked about briefly at the beginning is that this point of ECM and PM, there’s a lot of terms for synchronous motors and an ECM motor is not anything special. An ECM motor is just a permanent magnet motor or a synchronous reluctance motor of some type that is then just integrated with a drive. So when you’ve got an ECM motor, don’t think that ECM is a motor technology in and into itself, it’s not. It just means that they’ve taken a synchronous motor of some type, most likely a PMDC or a synchronous reluctance or a synchronous reluctance with PM assist and they combined it with a drive. So that’s what an ECM motor is and really nothing else.

One thing I do want to comment on, and that is that the DOE has taken notice of synchronous motors and has also as part of the latest test rule that actually becomes effective in April of this year, 2023, there is now a defined DOE test standard. There’s no efficiency standard yet, but there’s a test standard for synchronous motors. So that’s brand-new from the DOE, and there are a number of DOE rules for end equipment and some smaller ones like furnace fans for houses and a number of other items where the DOE has effectively ended up requiring a synchronous motor to be able to get to the system efficiencies. So just recognize that synchronous is definitely a thing and it’s going to continue to be a significant portion of power drive systems that are actually being implemented as we move into the future.

As I said, I kind of talked about the pool pumps, the circulator pumps, and I’m going to tell you that in at least commercial applications, synchronous motors are taking a significant share of commercial applications. And when I say that, I’m talking about hydronic pumps and other types of blowers and things like that within commercial and even agricultural applications. So synchronous is definitely growing. It started in the smaller horsepowers and it has definitely grown to significantly larger horsepowers, and I think most of us in the industry would all agree that this is going to continue to grow. And so getting comfortable with synchronous motors and the application of a power drive system with a synchronous motor is definitely going to be much more common.

So I just wanted to show some pictures and applications. I start with some of the pumps here. These are a couple of pumps where we’ve got some synchronous motors that are integrated in. I’m going to show you a few more pictures here of some synchronous motors. The one thing is that, again, basic can be integrated as an ECM. The nice thing about an integrated motor and drive is obviously you’re getting it from one manufacturer and there’s no field wiring. It’s a good compatibility, should be easier to retrofit. So that’s definitely something when it comes to integrated. So that is whether you call it an integrated motor and drive or you call it an ECM, it’s definitely something to consider with synchronous motors.

So here are just some pictures, no names of companies, but giving you an idea. This is a synchronous motor. This is actually a synchronous reluctance motor with PM assist and it’s got an integrated drive with it. You’re actually seeing it without the in shield on it, so don’t be concerned. That’s just to show the inside. And here’s a couple of other versions. Here’s one with the drive actually integrated on the end of the motor. Here’s one integrated on the top of the motor. So again, a pump and a fan.

Benefits and drawbacks of synchronous motors

I was just going to talk a little bit about some benefits and some drawbacks of synchronous motors. So one thing is that you have to recognize is that a synchronous motor does run at the synchronous speed, which is different from the induction motor. So that does take some planning and recognition that the synchronous motor will run faster. Another thing to recognize is that if there are fluctuations in your system, and when I say not just smooth fluctuations, but quick ones that you might call like cavitation or instability, a synchronous motor might need some other things in the system, such as the addition of a resistor on the DC bridge because otherwise, it becomes a generator pretty quickly.

So there are some things like that to consider, particularly as it pertains to deceleration. That’s what I said, you don’t want to turn your synchronous motor into a generator, your drive’s not going to like that. So there are some things to consider. I would say work with your motor and drive manufacturers, make sure you do those considerations. They do increase efficiency, they are higher efficiency, but they are in some cases have some considerations that you have to consider when they’re applied.

John Malinowski: Tim, just as we’re getting into this next one, let’s just, if you can go into the details that these are not synchronous motors that people may be used to in large sizes. These are generally permanent magnet rotor-type motors.

What I mean by that is I’m not talking about your 5 and 10,000 horsepower synchronous motors that actually have a wound field on the rotor that allows for power factor correction. I’m talking about mostly 500 horsepower and smaller with a whole bunch of it being 20 horsepower and smaller synchronous motors that usually in many cases use magnets or they’re using reluctance torque.

Tim Albers: So I’m going to reemphasize that here and we could spend a lot of time on this, but again, I wanted to just familiarize people again that here’s an induction motor.

This is the standard over here on the left. It has some rotor slip to it, so it’s considered an asynchronous motor. The ones over here to the right, all four of them are all synchronous products. So this is a like interior permanent magnet. So this would be a PM motor. This is a pure synchronous reluctance motor with no magnetism. Here’s a synchronous reluctance with magnets, and here’s a switch reluctance motor. So just to give you an idea that when someone says a synchronous motor or an ECM motor, it could really be any of these technologies that are actually being integrated. There’s not just one and they can actually all be pretty significantly different. And really when it comes to the different types of technology for synchronous motors, there are some advantages and disadvantages to different products. For example, an induction motor, it’s very robust, easy maintenance.

You can run it on the line, you can run it on a drive, but it’s got lower efficiency, it’s got higher rotor losses. These synchronous motors have no rotor losses, so that’s their advantage. They have zero rotor losses. Permanent magnets got some high efficiencies, but you know what, you have to have a drive and there’s a little bit higher systems cost and you can have some maintenance issues and it’s going to be a little more complex to set up, very similar to some of the synchronous reluctance. The other thing is the synchronous reluctance also has a lower power factor, a little less complex because there’s no magnets in it. So there’s always trade-offs between efficiency, maintenance cost, robustness. There’s not a single technology that is a solution for everything, but there are going to be some advantages and disadvantages to each.

One thing I wanted to cover here, and this just got a couple of charts that show the same thing but in a different way. So if you look at the next chart and say, I think that tells me the same thing, it does tell you the same thing, but I just wanted to show it in a couple of ways. And this has a pump or a fan curve on it. And what it shows here is if you look out here at full load, you’re going to say maybe 5 or 6% in efficiency might even only be a few percent inefficiency if it’s a larger horsepower at full load of a synchronous versus an induction motor.

If you come down here to a part load, if you’re down here at 25% load, you can have as much as a 12% gain of efficiency on the synchronous motor versus the induction motor because the synchronous motor maintains its efficiency as you slow it down on a drive better than you do on an induction motor.

So when you start talking about 12 points of efficiency gain or even more, it is extremely significant the difference of applying a synchronous motor versus an induction motor.

So the next chart basically shows the same thing. This is a much larger horsepower, the one I showed previously was a 5 horsepower I believe, and this one is a 100 horsepower. So you can see again at full load here, this is a 100 horsepower, it’s only half a point of efficiency savings like, well, that’s not a big deal, but down here at the part load, it’s gone from a half percent up to a percent and a half, so it tripled in your savings. And here is the induction motor. So these are a couple of different types of induction motors, and this is your standard NEMA premium.

The last thing I wanted to cover, and just one last advantage of a synchronous motor is the fact that it does have a little bit more maintenance and reliability capable. And one of the reasons why is because the rotor doesn’t get hot. So this actually shows you an induction motor on top and a synchronous motor on the bottom. They’re actually the exact same electrical design. One is running with an induction rotor and one with a synchronous rotor. And you can see the heating difference here between the two. And one of the big deals here is what gets to your bearing. So your bearing is actually going to get some of the heat from this rotor and on your synchronous motor it will not. So it’s just one of the incremental advantages of synchronous motors that a lot of people don’t take into consideration. And I think it’s important because it will give you an extended bearing life and that is still the number one failure mode for electric motors.