155. Idea – VFD Integration: Things to Consider Transcript
The application needs to be much more specific and much more understood than will one harm the other. If you don’t have a lot of experience my advice is to partner with a good VFD specialist that can walk you through these considerations prior to specking out and trying to purchase the equipment that would be associated with when these conversions. Knowledge is everything and doing your due diligence on front end and being prepared for what the aspects are going to be I think is probably your biggest benefit.
Welcome to EECO Asks Why. Today we’re going be talking about how to convert a standard AC induction motor from across the line to a variable frequency drive application. So with us today, we have our subject matter expert, Mr. Mike Rathbun. Mike, welcome today.
Good day. Thanks, Chris.
Hope you’re doing well, man. Looking forward to going through this topic with you and I’m sure you’re going to bring a lot of value to our listeners and maybe just get us started. Just talk about why would end users even want to change their starting method of a motor.
There’s a number of reasons why somebody would consider this. And it really brings me back to some conversations that I’ve had recently with some customers and changes in equipment could be one, whereas as the process may be influenced by some variable speed operation could be one. Sometimes it’s as simple as the customers are like looking for energy efficiency.
And there are efficiencies associated to putting motors on VFDs as compared to across the line star, but I think what’s important in this conversation is that to have a basic understanding of the impact of what’s going to happen to that traditional AC motor, or that may not necessarily be designed for VFD operation.
Okay. If you think about applications, Mike, are there any applications that are, typical in industry that will be like a front runner from a benefit of converting from across the line to a VFD?
I see a lot of them associated with fan operations could be involved in a boiler. Some are FD and ID fans. Traditionally a lot of fan operations going back through time, the air and air flow was managed typically through some type of dampener operation. There are greater efficiencies that can be gained by managing that air flow and air pressures without a dampener but using the variable speed capability that a VFD could bring into that operation.
Okay. So just by changing that speed on that particular fan application, they could get the required output for instance by using a VFD
Exactly as opposed to more of a mechanical arrangement such as dampeners within that air flow.
So when you’re thinking about the cost factor for converting a cross the line starter to a variable frequency drive, you have upfront costs, but then you have cost savings because you mentioned earlier care about, the efficiency and picking up efficiency. So when you think about cost, how would you put up a fence around all the costs and explain them to our listeners?
You touched on w with your state with your question there is the obvious cost of the components that would be used or replaced in this case, the VFD, potentially some of the interconnecting devices that may be utilized in the process, but there is the component of potential cost savings due to power efficiency that VFD brings into that. So I think when you’re looking at this type opportunity for improvement in your process it is important. There are pretty standardize application tools that could provide calculations based upon horsepower and torque and those values associated with the application, plugging them into a VFT type of operation that would give you an understanding of energy and potential costs savings that could be associated with that.
The other thing to consider that may not be or have as much differentiation is what is the cost over time. There really are no moving parts in a VFD is compared to, and across the line starter where you have contacts continually opening and closing. So there could be a trade-off in the long-term cost of ownership due to the limited or minimized maintenance and replacement parts that would be associated with that.
Now for typical applications outside of the VFD itself is it, from a call standpoint I’m talking about here, what other components would be involved or are there any, can you use the same cabling? The same enclosure? Things like that to physically make that conversion or are we looking at it more, associated unit costs from a hardware standpoint just to get this done?
So that’s a great question that really leads us into the heart of this conversation. And that’s some of the things you need to think about or consider if you’re going to make this conversion, and there are aspects of operating a motor on a VFD that are much more impactful and it may change or add to the componentry and some of the hardware associated with VFD operation.
I think you touched on one idea and that is to mitigate some of the electrical harmonic effects that are induced in VFD operation it’s quite often recommended that special cabling be used between the VFD and the motor, so that could be one aspect. Another aspect of the hardware to be considered for costing could be an input and an output filter associated with that VFD to help also mitigate that type of anomaly associated with harmonics in voltage, distortion, or current distortion.
Another aspect that you may have to consider is. One of the biggest things to manage or handle in making this conversion is there’s going to be a lot more heat, generally created within the motor and once it’s placed on VFD operation. And that can vary widely depending on what speed you want to run. So depending on the specifics of the application, you may have to consider adding a blower type cooling motor to a traditional AC induction motor to maintain the cooling levels that will be required to meet the installation and operating requirements of that motor.
So just physically you need potentially would have to move more air, just to keep it cool.
A traditional AC motor, it generates its own cooling through a fan that is mounted on its shaft. The movement or the rotation of that fan drives the air down across internally through the motor structure or externally to remove the heat component. Now that fans have delivered a exact amount of cooling at a fixed speed for a fixed speed AC induction motor. We take that motor and drive it down to 50% of its normal operating speed obviously we can conclude we’ve reduced the cooling effectiveness of that motor by 50%. That’s the key at that point that’s probably an application that we would truly have to consider adding an additional blower motor to provide or make up for the inadequate cooling.
Absolutely. Mike, one thing we see a lot of times when we’re converting, motors to across the line starters. From across the line starters to a drive is, we saw this in the motor service division at EECO for years is the phenomenon of fluting and what the impact that can have on the bearings. So maybe give our listeners a little walkthrough of what floating is and things to potentially look for from those harmonics that could potentially damage their equipment that not everyone typically thinks of when they’re just, doing this conversion, lots of times this doesn’t sneak up on you until everything’s done. And maybe you haven’t, put proper grounding or whatever into the system. So can you just kind of walk that through for our listeners?
Sure. And hopefully to keep it pretty basic, this phenomenon has existed forever since VFDs started to becoming popular and applied, and that is the effect of the coupling of the high switching currents that are being delivered from the VFD through the motor, which basically builds up a charge of electricity on the rotor while that motor’s in operation. And over time, that charge will find a way to discharge. Typically the way that would happen would be through the bearing to the frame of the motor and then to ground, obviously. And that’s the ultimate source for that type of fluting failure that we commonly see in bearings and motors that are being rebuilt.
So some of the mitigation tasks that we talked about or touched on associated with our harmonics, it comes into play really here to a great degree. And really what it boils down to is we can look at two primary aspects of managing this condition. And one is if we can clean up the voltage that is being generated by the VFD to the greatest degree that we can. That will minimize this effect from building up in a current and the other aspect that is if we can somehow prevent the discharge of this voltage from the rotor or through the motor housing to ground. That would also eliminate the effect of the failure to the bearing.
So there’s a number of techniques and a lot of variables to this equation that come into play when you’re trying to manage or diagnose this condition, some of the readily available processes that could be used on this is when we touched upon VFD cable lane. Another could be an output filter. There are also other devices such as grounding brushes that could be applied to that motor that basically gives a parallel circuit for that discharge to occur to ground without having to go through a bearing other technology that we’ve used, going back to our shop days, could be a insulated bearing and insulated bearing housing. Those types of approaches, break that circuit and prevent that discharge as well.
All of these types of approaches to that condition are relevant. I think what we’ve seen in our history with a lot of motors is sometimes things work better for one customer and sometimes another approach works better. And the reality of that is the specific installation and the details of some of the variables associated with the installation and electrical circuit and the plan has a great impact on this all the way from, how good of a ground do you really have? How good is the bonding on your grounding from your motor back to the bus of a VFD? All of these factors can come and play.
My advice when a customer comes to me with these situations now is, you really source a good motor repair. And have them walk through the different levels of options and there’s all levels and pricing and complexity. They could go into this from the rebuild process to being able to be applied to a motor in the field.
So Mike, thank you for walking us through the fluting phenomenon. There’s so much that we can unpack there, but I’m also thinking about when end users want to do that conversion. And a lot of times when you specify a drive, you have these options out there, you have filters, you have reactors, you have breaking resisters. We don’t have to unpack each one of those, but maybe a high level overview of different components they should consider before they begin this conversion process that may help.
So real quickly, I think with any VFD application, I like to look at two primary aspects that you touch on there. One is a line filter or line isolation and that’s, commonly done through an isolation transformer. What that does on the line side of that VFD is actually removes that physical connection from the drive and what happens in its circuitry from the rest of the distribution system, that gives us the ability to isolate some of the negative aspects that occur in power within that VFD from the rest of the distribution system. That’s one aspect of it.
The second touches on some of these factors that we’ve already discussed is having an output reactor or a filter on the specs along with that VFD of the appropriate current rating to accommodate the drive can cure a lot of the ills we’ve talked about from cleaning up the voltage to minimizing or potentially minimizing bearing fluting condition should absolutely be considered. The length of the run for the motor. How far is it going to be from the VFD out to where that motor is? And typically anything over a hundred feet, you really need to start looking at some of these options between output filters and specialized VFD cabling to manage some of that.
You touched on a braking resistor, and that really would apply to the specific application that we’re doing. And how do we need to manage the slow down or the stopping of the motor? Something that it has a high inertia type load, such as a flywheel operation. You’re probably gonna want to consider a breaking resister to help manage that power as the VFD is pushing that power off during that down process, but those were a couple of examples.
Absolutely that really helped Mike thank you so much. I mean, and may be to kind of wrap us up here, any parting advice or wisdom? I know you’ve been involved with a lot of these conversions with your experience in the past, anything you would offer up as, “Hey, don’t forget to do this,” or, “Definitely do not do this,” things like that to, to help our listeners here?
I would say it’s probably the biggest thing is there’s no right or wrong. I’ve had customers come to me recently and are looking at doing this type of a conversion and has asked me is a particular VFD guaranteed not to damage the motor. And unfortunately, that’s not really the perspective to approach this. The application needs to be much more specific and much more understood than will one harm the other. If you don’t have a lot of experience understanding some of these anomalies and conditions that we’ve talked about here today, my advice is to partner with a good VFD specialist and there’s plenty of them out there all the way from your OEMs to your distributors that can walk you through these considerations prior to specking out and trying to purchase the equipment that would be associated with when these conversions. That would be my big thing is just knowledge is everything and doing your due diligence on front end and being prepared for what the aspects are going to be I think is probably your biggest benefit.
Absolutely. And then I love the way you made the tie to partnership. You find the right partners and industry, no matter where you’re at there are subject matter experts in all these different fields that can help you through some of these because let’s face it some of these topics are complex. They’re difficult. Mike, thank you so much. You really helped us here with this topic of doing the conversion from a cross the line starter to drive, really appreciate the insight that you brought. Hopefully our listeners, you picked up some things, some pointers here that they’ll take in the future on these projects. It’s always a pleasure, Mike. Thank you.
Yeah, thanks Chris. I appreciate it. Enjoyed it as always.