149. Idea – The Difference Between a Soft Starter and a VFD Transcript
99% of the time, the question is, do I need to continuously vary the speed or not? If you have an application that’s always going to run the exact same speed, you could get a soft starter, you could use a contactor. It doesn’t really matter. If you ever want to vary the speed or maybe you want to slow something down, maintain a certain building pressure or something like that then that’s when you’re going to need to move to variable frequency drives.
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All right. Welcome to EECO Asks Why. Today we’re going to be digging into an interesting topic about, how do I select between a soft start and a VFD? Different applications out there in the industry. And sometimes it can be difficult just to make the right decision. So we’re going to hopefully help you make the right decision today with us today, we have Mr. Chase Boehlke from Siemens.
Very excited to have Chase. He’s been with us on a couple of episodes now and looking forward to his insight on how to make this decision and make the right one. So Chase, maybe just for the listeners out there. Can you just give us a quick recap of a soft starter and a VFD and how they apply to the industry, please?
Sure. So there’s I think we talked about it a little bit in another episode, but there are three main types of ways to run a motor in the industrial plan. You’ve got a starter and it’s got its own line and load protection. And that’s just on and off, right? It’s just like a light switch and you’ve got a soft starter. And the primary goal of the soft starter is to ramp the load up slowly. You’re slowly building torque with a soft starter. So you don’t rip belts off and damage things mechanically by providing too much torque. And then there’s a VFD where you can provide a continuously variable frequency and provide much more efficiency in a certain application. Those are the main differences.
Very good. Well, thank you for walking us through that. So when you’re looking at a high level from an application standpoint, are there any clear-cut areas to make a decision on when to choose that soft starter versus a VFD?
Yeah, sure. The primary thing. 99% of the time, the question is, do I need to continuously vary the speed or not? If you have an application that’s always going to run the exact same speed, you could get a soft starter, you could use a contactor. It doesn’t really matter. If you want to run the same speed, you just want to make sure that your motor and that your gearbox are sized appropriately. And then you start them with a starter if you don’t need to worry about starting torque issues or if you need to build that torque over a slow amount of time, then you use a soft starter.
That’s fine and well and good if you just want to run at the motors rated speed all the time, but again, if you ever want to vary the speed or maybe you want to slow something down, maintain a certain building pressure or something like that then that’s when you’re going to need to move to variable frequency drives
Absolutely. Without having to make those mechanical changes to the load itself or to the driven equipment.
Right. So with a VFD, we would set up the VFD and the motor rated speed to be typically in typical applications, the top speed that you would want to go at. Some people like to over-speed things, but if we size it to go the standard regular rate of speed of the motor is your, the speed. Then we can vary at almost any speed in between those, with a VFD and the advantages, because the VFD does provide a variable frequency and we also know that the motor only wants to spin at the frequency that it’s given.
So if we slow down those sine waves, the motor inherently wants to slow down. And that’s what the VFD does for us. So the motor can run at a much lower speed and not overheat, not have overcurrent pull. Whereas the only way to slow down a motor running off of a contactor is to just overload the thing until it can’t go any faster or until it can’t go on here.
That’s right. Until you have reached that locked rotor point potentially. So when you talk about VFDs versus self-starters physically, how much larger are they as a VFD than a soft starter, typically?
That’s a great question. So in many cases, the VFD and a soft start for very small applications, you know, 1, 2, 3, 4, 5 horsepower. They’re not that much different in size. A lot of times they’re very close. Sometimes if using an advanced soft start versus a small, very simplistic drive, they can actually be a little bit smaller, but that changes very much as the size builds. As soon as the size and horsepower starts going above 5, 10, 15, 20 horsepower, you will see a disparagingly large difference in size between a VFD and a soft start as drive sizes have come down due to advances in transistor technology and things like that it’s still nothing to compare. If you took a 500 horsepower drive, and a 500 horsepower soft start, the drive will definitely need far more room and typically it’s gonna need a lot more cooling as well.
Okay. Cooling is definitely important on the drives. No doubt. So another area that really comes into play when you’re trying to make these decisions at the plant. You know, we have to talk about it here is budget right? Cost. And is there a typical rule of thumb when you start thinking about a percentage increase of costs and selecting a VFD over a soft starter? Know that it can vary a lot, you know, but just didn’t know if there was a rule of thumb out there and you could give us an idea about.
Yeah. So it’s roughly the same principle of size for the smaller drives versus smaller soft starts. They’re usually somewhat close. The drive is almost always going to be more expensive, even if you used a more advanced soft starter versus very simplistic soft starter. The drive is still going to be more expensive, but the difference is not too bad, especially to use what we would call like a more simplistic product.
We have a little v20 drive. Very simplistic in nature, still has some cool benefits and features, many of them in fact, but it’s going to cost more than a very simplistic, soft starter at the same horsepower level. Now, the size builds, once again, you’re going to see that cost go much more on a drive than with a soft starter, but you get a lot more with the VFD. So it’s a trade-off, right?
I guess what that trade-off is, what are some of those main functions that the VFD would offer you that the soft starter, you know, doesn’t inherently?
So VFDs can do many things, but one of the main things is we talked about the very first question, right? The VFD, they variable frequency drive. It can spin the motor at a different speed and it can do it very efficiently because of that, the software has to ramp it up. You can’t vary the frequency. So if you’re starting with a soft start, it’s going to pull a truckload of current starting that motor and getting it up to speed.
A VFD, doing the same application is much more efficient. We had a customer recently that we went out to go see. And in the application, they’re using a soft starter. And we actually an oscilloscope out to the soft starter and we measured the amount of current going to the motor and it was pulling 300 amps for four seconds or something of that nature. We ended up putting a drive-in that same application, the exact same mechanics only pulled 85 amps for three seconds to start up the same load.
So because that drive varies the frequency, all the physics change and all the electronics change of how that motor ramps up to speed how much torque it has, because we can provide the appropriate amount of voltage and we’re giving a motor the proper frequency and that slip is always just behind not to go too deep, but the drive is a much more efficient tool. It’s starting high inertial loads as well. That’s one big thing I would say.
The other things are obviously like we said the changing of speed, you get a lot more diagnostics. You get all sorts of information out of your motor. I can tell you how much torque my motor has on it. I can tell you how much current it’s putting out. Soft starters can do that too, but VFDs can get typically much more detailed. I can tell things like how much flux current is going to my motor. There are a lot more capabilities in general.
The soft start can do some of these things, but like I said, the VFD can run in what’s called a torque mode. I can supply a constant torque to a load, all day long and I don’t have to care. The drive won’t overheat, the motor won’t overheat. I can just tell the drive, sit there and hold this torque all day. You can come back tomorrow and the thing will be fine. Obviously, soft starters can’t do that. They’re just made to start that load and get it going.
And now did I hear you say 300 amps down to 85 on that application?
Yeah, I was very surprised. It was a weird scenario. It was pretty decent inertia start not extravagant, but it was pretty high. And they wanted to put VFDs in there so they could vary the speed. And we came back, I did a bunch of calculations. I actually put it into our soft starter tool because we have soft starter app. And, you know, we have tons of apps and all this stuff, which are no charge to customers, but I threw it into the app and I back-fed the amount of load and inertia that the load had and made it equal 300 amps.
And then I backed, fed it in size, the drive, according to what the soft started tool spit out in reverse. And then I also ran it through our sizer software, which is our software for sizing loads and drives. And then I came out with, I think I came out with 95 amps or something, and I thought it was wrong to be honest, but after talking through it with a lot of other people and, going back to my good old motor and drive physics books, I found that because of the drive actually supplies the appropriate frequency to the motor.
NEMA designed B motors, which are one of the most common industrial motors. They give their best torque a little bit shy of what their slip frequency is, but if you’re changing the frequency, you’re also changing that maximum torque point in the motor. So there’s just a lot of behind-the-scenes stuff that the drive does that a soft starter can’t do.
That’s killer man. That’s great. And you said it was a NEMA designed B motor?
Yeah. And NEMA designed B motors are pretty standard kinds of motors. And they have a different speed torque curve than like a NEMA design D motor and NEMA designed D motor provides its highest torque at the very low RPM range and that, so it doesn’t stall. Whereas any even design B provides its highest torque higher than it’s RPM band. Okay.
You know, you’re talking to a motor guy but yeah let’s not go down and Chase those rabbits. We’ll do that on another one, you know, we’ll definitely do it though.
So I got a question for you. So a lot of plants, sometimes they work with limited inventory, right? And they may only have drives on the shelf, so can not take a drive and put that in a soft start application every time? If all I had was drives and my soft starter went bad, I do that?
I really can’t think of a situation where you can’t. Me being a drives guy, I always want to put a drive in there anyway, but there are a couple of things you need to consider things that we’ve already talked about. One is panel space that drives going to be. It needs typically it’s going to require more cooling power, right?
Remember that in a very inexpensive, soft start, it’s gonna switch over to a little internal contactor a lot of the times, not all manufacturers, not all types, but sometimes it wants to do that. The drive is always running off of its transistors. So it’s always making heat in that heat needs to be gotten rid of. So you need adequate cooling in the panel and it’s also physically.
So a lot of times, what I see is people will put a VFD in a panel and they’ll put the cable tray down right on top of the drives. And you’ve really done nothing, but make a convection oven. So now we’re just spinning hot air around the heat sink, but it’s not getting out so you can make bread in there, but that’s about all it’s going to be good for.
Right? Yeah. I got you, man. I got you. What about from a maintenance standpoint, any advantages of one versus the other?
You know, typically in the, standard size range soft starts are going to be a rip and replace, drives off for a couple more replaceable components than soft starts in general, but drives also have more pieces and parts sometimes.
So it depends on a lot of factors. If you go back to that standard kind of soft start, where it uses the transistor. Well, if it uses it switches to bring the load up to speed and then it switches over to its internal contactor. And it does that very often. You can still wear out that internal contactor, whereas the drive is properly maintained and kept cool the electronics will last quite a long time. And you may have to replace a fan every now, but there are not many moving parts.
So the longevity of both different kinds of products here is typically very high at this point. VFDs, as long as we’re on the topic, one thing that you want to watch out for is if you shelf a drive for more than a couple of years at that point you have to bring the capacitors back up to full voltage again, very slowly and there’s a process to do that. So you have to be careful of that kind of thing. If you bring a drive out of maintenance, you have to make sure that you reformed those capacitors properly.
Right. Or that could be a bad day.
The drive might work for five minutes, it might work for zero minutes, it might work for two days. I’ve been told it sounds like crackling and that’s a bad sign. If your drives ever talking to you other than the standard 4kHz carrier frequency whine, you probably want to shut it off,
Right. Absolutely. How about overload protection from one application to the other? What varies there?
They both offer pretty similar overload protection speaking on generalities, right? If you have a 4.3 amp motor, you’re going to set the overload in both the soft starter and the drive to 4.3 amps, but advantages with the drive, you know, you probably get to do a more advanced application with the drive.
But advantages of the drive are going to be things like I can monitor the torque. So we had a customer one time they were making a product and the motor was too powerful for the application. And so occasionally when stuff would get bound up the wheels would break apart because the motor was too strong.
And so in that situation, the motor wasn’t overloading, the motor was fine. It was ripping apart their very nice expensive application. We went into the drive and we put a torque limiter on the drive. And so as soon as the drive found out that it was going above three, four or five Newton meters or whatever it would limit that torque. And so now the customer had saved their application by using a drive. That’s something that you can’t do with a soft start.
Absolutely. Absolutely. Speaking of torque, you know, does constant torque and variable torque play in consideration with making these decisions for drives versus soft starts?
Yeah. So soft starts have their own set of guidelines for sizing. You should always check with the manufacturer. Some soft starts have to be started only a certain amount of times per hour, or, you know, so many times per given amount of time. Drives are very different. They’re always running off of the electronics, so it doesn’t matter.
You can start and stop a drive 85,000 times it doesn’t care. As long as you’re not going over the drives rated current, right? So that’s what speaks toward the variable torque and the constant torque. If you do need to go over the drive’s rated current for any reason, then you have to make sure that it’s sized appropriately for it.
Motors are really good at holding thermal energy, right? Or taking a long time to overheat. You can put 200% of current through a motor for a pretty long time. And the bigger it gets, the more time you can do it for, but a drive because of its transistors they overheat very quick. And so using a constant torque application, we would use what we call a high overload drive. And that basically means it can supply more current to the motor for a longer amount of time.
Okay. Very good. Well, thank you for walking us through that, Chase. Just to wrap us up here, really, thank you. You’ve done a great job of breaking this down, but just from your experience. And I think that story you gave was just outstanding, but is there any other advice you’d give our listeners when they’re trying to make the decision to go between a VFD and a soft starter?
Absolutely. I sell drives so you can call me directly and no, I’m just kidding. It really boils down to that very basic principle. Do I need to vary the speed a lot, or even sometimes? If I have a conveyor that one day it’s running a certain product at 900 RPMs and the next day it’s running at 1800 RPMs, could I do that with a soft starter? Well, no, not unless I changed gearing. And not many people want to sell multi-speed gearboxes anymore. So really looking at whether I need to vary the speed or not, or whether I want to do advanced things like maintaining a limited amount of torque, maybe it runs at the same speed all the time, but I never want it to go over a certain amount of torque and it’s too expensive to replace those kinds of situations.
Right, right. Well man, you really helped us here. You brought a lot of knowledge to the table. Thank you. You helped our listeners understand how to make these decisions because ultimately we want to make the best decisions out there in our plants, you know, to make things the most reliable, the safest and to run the longest. So Chase, thank you again for so much knowledge that you bring us. Really enjoyed having you on this episode.
Thank you. It’s great to be on the show.
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