In this episode, host Mike Murphy is joined by Rick Hoadley, Principle consulting Applications Engineer for ABB LV and MV Drives, to discuss active front-end technology in the mining industry. Discover what AFE is and why a facility would be interested in this type of technology.
Learn more about solutions for the mining industry.
In this episode, host Mike Murphy is joined by Rick Hoadley, Principle consulting Applications Engineer for ABB LV and MV Drives, to discuss active front-end technology in the mining industry. Discover what AFE is and why a facility would be interested in this type of technology.
Learn more about solutions for the mining industry.
Welcome to the ABB Solutions Podcast where we discuss some of the challenges faced in our industry. I'm your host, Mike Murphy speaking you from Greenville, South Carolina. Today we have Rick Hoadley, Principal Consulting Applications Engineer for ABB Low Voltage and Medium Voltage Drives. Rick is here to speak with us on Active Front End Technology in the Mining Industry. Welcome, Rick.
Mike Hoadley:Thank you for having me today.
Mike Murphy:Yeah, it's great having you. Okay, Active Front End Technology, AFE. I've seen that acronym before. So can you again, go over what is active front end? And why would a facility be interested in this type of technology?
Mike Hoadley:Well, it all has to do with really power quality, principally, the reduction of harmonics, and also improving power factor. And yeah, really everyone is concerned with, with harmonics, including mining facilities, oil and gas, even ice cream makers, which I really want to make sure they make a lot of ice cream. So how does an active front end really help? Well, there's a number of different names that are used for this type of technology. Sometimes it's called active front end. Sometimes it's called a synchronous rectifier, or an active rectifier, or a ULH, which means an ultra low harmonic type of a rectifier. And actually, that's what we use in our low voltage ACS800 and 880 drives. But when you think about the way a drive is designed, there's really three key pieces to it. There's the front end, which is a rectifier, and that's why this is called an active front end. That's a rectifier portion. And that takes the three-phase AC and converts that to DC. Then we have a DC filter that's in the middle, and that smooths out that voltage. So we have a smooth DC voltage. So it's has a little bit of ripple on it. But it's sort of like a big battery, almost like a battery in your car. And then we have the inverter section which takes that DC and it changes to a variable frequency, variable voltage, three-phase power, which goes to the motor. And that's how we actually control the speed and the torque of that motor and the direction of that. Now, a typical diode bridge type of rectifier, which is not an active front end would be made up of six diods for three-phase. And within six diodes you actually get six pulses of current every cycle. And so these pulses of current is actually what supplied power to the drive to maintain that DC bus voltage. If we didn't keep providing these pulses, the DC bus voltage would just drop off fairly rapidly. So we got pulses going into the drive, pulses of current, you can think of more like like power pulses. A 12 pulse type of a drive would have twelve diodes, and you get 12 pulses per cycle. An 18 Pulse we'd have 18 pulses per cycle and so on. This is very similar to, actually a car engine. If you have a like a four cylinder engine in your car, you actually get two power pulses per revolution of the crankshaft. Alright, that's not too bad, car engines always have a flywheel on them to help smooth out those torque pulsations. Otherwise your engine would would be kind of jerky feeling all the time. An eight pulse or eight cylinder engine actually has then four power pulses per revolution. And so it's a lot smoother running that way. And maybe that Duesenberg V 16 has 16 cylinders in there. And so you get eight power pulses per revolution, which is a nice real smooth running car. Well, this is the same type of thing with these rectifiers. We have these diodes, which provide these power pulses going into the drive to be able to, you know, have a fairly smooth running type of a drive. And the more pulses you have, the smoother the DC is. And along with that, the less harmonics you would have on the AC line side. Now, diodes don't have any on-off control. And so the pulses, you can't really tell when the pulse will start or stop. It happens simply based upon the voltage across that diode, and the inductance in the circuit and so on. But with an active front end, we don't just have diodes, we actually have IGBTs, which are transistors, which we can actually turn on and turn off. We can control when they turn on and control when they turn off. And so an actual front end really is more like, I would say closer to a 24 pulse rectifier, which has well, 24 per cycles. 60 cycles a second is like you know, a little over 1400 pulses per second. And so we have a lot of pulses coming in because we're turning these IGBTs on and off at a fairly high rate. We also have these turned on and off in such a way that we don't create low harmonics. We don't create fifth harmonics, we don't create third harmonics, we don't create seventh harmonics. And so those are not in the current waveform, then when we use an active front end. A six pulse bridge creates those harmonics, it creates fifth and seventh, 11th and 13th, and so on. But an actual front end doesn't create those harmonics, because what we're doing is we're actually creating a sinusoidal voltage on the front end, just like the sinusoidal voltage, which we are creating on the back end going out to that motor. So we have a nice smooth current going to that motor, a nice sinusoidal looking current. And that's what we're doing on the front end as well. And so, because of that, we can have very low harmonics, and in fact we are able to meet like IEEE 519 standards of being less than 5% current distortion, right at the input terminals to the drive. Now, if part of the drive, the active front end is also what's called an LCL filter, which means there's an inductor, a small capacitor, another inductor, and that helps us smooth out some things. But, but with that, we have very low harmonics. If we were to actually have a like a 12, or 18, or 24, pulse type of rectifier, we would need to have a transformer to do that to be able to shift, phase shift the the different phases there. And so with an active front end, you don't need a transformer. So that means a footprint can be a lot smaller. And there's a couple other things that an active front end also does. One thing is with that we can actually control and maintain the power factor over the entire load and speed range. And we set it up so it operates at unity power factor. So it's never leading, and we don't have it lagging to to pull down the power factor of your plant. But we have it at unity power factor. And operating at unity power factor means a few different things. One, means that we're going to be drawing less current than even the motor would if that motor were running across the line. Because at unity power factor, we've reduced the KVARs the KW is really the same. And so if the KVAR is reduced to practically zero, that means the KVA is reduced, which means the current is reduced, and the power factor is improved. So it's basically at unity power factor. What this also does, the transformer that's feeding this power frees up, frees up the capacity for that transformer, so that can support additional loads. And then, a lot of places in the US, the utility charges for low power factor. If your power factor is maybe less than .85, then you get a penalty charge because of that. But with an active front end drive, you're not going to see any penalties like that by adding this drive to your facility; even if it's a big drive. And then a couple other points too, is that if you're running on a genset and a lot of facilities have gensets, like in mining you need gensets. Suppose using a drive for say ventilation in the mine, you need the air moving, even if power goes out elsewhere. So, might have gensets just for things like that. And for gensets you really want to have very low current distortion, less than 5% and very little, if any leading power factor. Now with a diode bridge type of rectifier for a drive, you have to actually oversize a genset about six times in order to accomplish that. The power factor is fine on the on the diode bridge rectifier, but the current distortion is not. And so that's the reason why you'd have to oversize the genset which gets expensive. But with an active front end, typically, all you need is maybe a genset that has two times the KW rating of the motor that you're you're using there. So, it's a smaller genset, that's less expensive, got lower current, got unity power factor and the leading power factor. And so, so an active front end drive is actually really nicely configured for use with gensets. Some other things that sometimes come along that they help out with is unbalanced line conditions. I mean, nobody has perfectly balanced lines. I can usually figure there might be about maybe a 2% imbalance, but an active front end can operate really better with unbalanced line conditions then, a multi-pulse, like even a six pulse or 12 pulse can. What happens with a diode bridge? When when you have a diode bridge rectifier and you have unbalanced lines, you'll start to see some third harmonics. And they're going to be there, you're going to see that when you take a look at the pulses, I mean, some pulses will be higher and other pulses will be lower than some of the others. And because of that unevenness that actually shows up as a third harmonic. And with the active front end, you're not going to see that, even if the lines are unbalanced. It will basically be drawing, about the same amount of current from each line, even if there is an unbalance. Another thing that you sometimes run into are low line conditions. And with the active front end, we're not based upon operating with a certain voltage, because we can actually boost the voltage that's coming in to the the drive. So if the, if the voltage were to sag to, let's say, 80%, say a 20% sag. Now, if you have a 20% sag, and you have a six pulse drive, you're not going to have the same DC bus voltage, and that means you're not going to have the same voltage available to send out to your motor. So, the motor is going to be starved for voltage, which means it's going to draw more current, which means it's going to run hotter. And so it's just not a good thing for for your motor under those conditions. But with an active front end, we're able to maintain that DC bus voltage, even when the line voltage drops down to even 75% of nominal. Now we're going to draw more current from the line, because we still need the same amount of power coming in. But we're able to maintain that DC bus so that the motor doesn't see any change whatsoever in the voltage going to that motor. So it can still provide the same torque, at the same speed, with the same current going to that motor. And so its sort of sort of as isolated from what's going on the AC line that way. Then, one other thing that also sometimes comes into play is up is a power loss. What happens if you actually get an interruption? Well, this really has a little bit more to do with what the inverter section of the drive can can can do then then the active front end, but but they work in unison with with one another. If there is a power interruption, what'll happen is we'll basically shut off the IGBTs, which are taking care of the amount of current coming into the drive and maintain that DC bus. And instead, what we're going to do is we're going to use the kinetic energy, which is in that rotating mechanical load that we have in that motor, which is run, which is connected to maybe a large fan, a large inertia fan or something like that. We can use that inertia along with that motor, which can actually make act like a generator, and we can maintain the DC bus voltage. Now the motor is going to slow down during this period of time because we're actually taking energy out of that rotating mass and using that to maintain the DC bus voltage. But we can keep that going as long as we have some inertia there we can draw from. So then, when the line comes back on again, all we have to do is say, "hey, great line is back", we start modulating the those IGBTs, we bring the DC bus back to its normal voltage level, and then we can then accelerate that motor back to where it was before. And so those are really the key things that's nice about an active front end is how we can take care of the harmonics, how we can work well with gensets. And how we can even with unbalanced or low line conditions, we can still provide the the voltage and the power that's needed for that motor.
Mike Murphy:Okay, great. Yeah, that was a great description of what active front end is, and I definitely understand why we use the term active. Also, I'm sure our customers like the less current draw, as well. You know, that means less power consumption. So, Rick, let me ask you this, in a mining facility, let's say they've got a downhill conveyor. Would a product like this still work in that application?
Mike Hoadley:Oh, well, yes, it certainly can. In fact, it's, it works extremely well for those applications. When you have a downhill conveyor. You have these rocks or coal or whatever it might be on this conveyor it's heading downhill. And so the conveyor is going to want to speed up. And the only way you can control the speed is you have to apply some some braking torque with those motors or on the shaft. Sometimes they have like disc brakes, just like like in your car, which they might have to use to sort of slow things down. If you don't slow things down, the conveyor is simply going to go faster and faster. And the guys down at the bottom of that hill, they're going to they're going to scatter because all these rocks are coming out faster than expected. So what can we do? How can we sort of control that maybe electronically as opposed to mechanically with breaks? Well, if we had a diode bridge rectifier, the the motors can can act like a generator, and we're going to try to draw power from that to slow down the how fast the motor is spinning and slow down the conveyor. And as we draw that power from that rotating load, that power is going back into the into the DC bus. And in the bus, we have these capacitors which smooth out that voltage. But if we have power coming in from the from the motor, the voltage on the capacitor is going to start to rise. And if it rises too high, and the capacitors will blow up; we certainly don't want that. So, typically what we do is we connect what's called a braking chopper with a resistor across the DC bus. So that when the voltage across that DC bus reaches a certain level, we start turning on that, that resistor to bleed off that energy as coming from that, that downhill conveyor. And that way the motors act like a brake, it actually can slow down and we can maintain control of the speed of those motors, even with a downhill conveyor. But with a braking chopper and resistor, all of that energy is just going out as heat. So So that's really a waste that way, and it just leads to to warming the environment there. Now, if you're up in Canada, and it's really cold, you might like that little extra heat, you know, put those resistors in the your cabin or something like that. But what can we do with an active front end? The neat thing about an active front end is it can control power flow, not just from the utility lines, or from the genset to the motor. But we can also control power flow from the motor when it's acting like a generator, back to the utility lines; which is ideal. So this way, we can actually control the amount of braking torque which that motor's providing on that downhill conveyor, by taking that energy, putting back onto the utility power. And if you were to take a look at your, your kilowatt hour meter right at your drive, instead of you know it spinning in one direction, saying "I'm using energy", it'll actually spin backwards. That means you're putting energy back on the power lines, and you're reducing your total energy usage that way. So if you've got a conveyor, that's always doing a downhill type of a thing, you can actually recover energy that way. So that's great, it will reduce your electric bill. And again, it'll be operating at basically unity power factor, whether it's regenerating, or whether it's motoring. So it's it's a fantastic type of an application for this. Now, the drive is the same if it's an active front end, which does not have this re-gen capability, or if it has the re-gen capability. And it's really just a parameter setting. And so so it's an optional feature. And so that can be set up even in the field if necessary. If you find out, if you've put things together, and you need some additional braking, coasting to a rest isn't enough for maybe a hoist or something else, maybe a pump. And maybe you want to stop things faster. And and using this re-gen capability, you can stop things faster than it would if you simply let it coast to a stop. And so if you see that you need that. Well like like on an ID fan. Sometimes it takes you know, 30 minutes for an ID fan to stop. Well, maybe you don't want to wait around 30 minutes before you can do something. Well, if you have this re-gen feature enabled, you can stop that sooner, and then get to work on it.
Mike Murphy:Okay, great. In some of our pre-recording questions, you mentioned that this is a voltage source inverter or VSI type of drive. Rick, I've also heard of current source inverters. So can you quickly go over like what are the some of the differences between the two? And would there be an advantage of using the VSI design?
Mike Hoadley:Well, that's a good question. When we talk about a VSI for a voltage source inverter, like you said, what that means is we have current pulses coming in to the front end through the rectifier. The filter in the middle is a capacitor, and that filter really smooths out the voltage which those current pulses are putting into the capacitor to maintain that voltage there. And then on the output, what we're doing is we're switching that voltage of the capacitor onto that motor, and it looks like a voltage source just like our you know, if you plug the motor into the wall outlet. We have a voltage source there and the amount of current that has drawn is based upon the load on that motor, but you have that voltage there all the time. Any type of filtering, and like even for an active front end we do put a small amount of filtering on the line side - it's called an LCL filter. That means an inductor small capacitor and an inductor and that helps smooth out the current that's coming in. So that again, we don't have any low frequency harmonics. It takes care of that the higher switching frequency stuff, but but that's much higher frequency. And CSI drive, the current source overdrive is built differently, in the sense that the filtering in the middle part of the drive instead of having capacitors there for filtering out the voltage is actually a large inductor. And it smooths out the current that's flowing. So the currents can be flowing in a loop from the utility through the rectifier through the inductor, through the inverter section out to the motor, and back, then through all of that to the utility again. And so what we have going on there is they're controlling the amount of current that's going to the motor all the time. Now, that smooths out the current in the filter. And to help smooth out the voltage on the front end, they have some large capacitors on the front and filter, on the line side. So they're like mirror images of each other. But we have capacitors and a voltage source drive, we have inductors and a current source drive. And where we have inductors and a voltage source drive, they have capacitors on a current source drive. Now, current source drives have good harmonics. So there really is isn't an issue with that. But one thing which does sometimes come into play, and often does require additional engineering, when you're going to be putting a system together, is the the input power factor. That can be an issue because of these larger capacitors they have on the line filter. There can be times when operating perhaps at low load, where you'll have leading power factor coming in. Now, maybe that's not going to be a problem with respect to utility. But if you are operating on a genset, that could be an issue for that genset. Because, the alternators really don't like any leading power factor loading. They can handle a small amount, but a very small load. The other thing that can occur is with a significant amount of capacitance on the line side there, they can cause resonance with other filters that might be on the line, or the filters, or loads, or other systems, or maybe power factor correction caps, which are maybe at a substation. And so for a system using a current source drive, like I said, an engineer really needs to sit down to know okay, what am I going to be connected to, and I may have to do some modifications, to be able to make sure that we don't run into any of these issues with that. Now, with the active front end type of a drive, we don't have those issues. We don't have to do additional engineering. It's sort of like, it's an ideal type of a load for utility; it's ideal type of load for a genset and really don't have to worry about possible resonance issue on on those things. That's really the key difference between those two.
Mike Murphy:Okay, we're gonna pause right here, but make sure you tune in to Part 2, where Rick and I continue our discussion on the ACS 2000 medium voltage drive, and how the drive can save space, simplify installation, and also some of the embedded safety features. If you'd like more information on the ACS 2000 active front end technology, reach out to your local sales representative. You can always go to www.abb.com. If you have any questions or possible topics about our Podcast Solutions Series, shoot us an email at us-solutions@abb.com. Thanks and have a great rest of your day.