Adjustable Frequency Drives Basics

why do you have these can anybody tell me why we're looking to use V of these the main reason basically is energy savings why if we're using an example for a standard application where we are controlling a flow by example okay the classic one will be with what so we have a motor that is powering a pump the pump is pushing the flow okay how are we controlling the flow in that case in a classic way without a VFD with dampers inlets all kind of mechanical gear okay is that efficient not for the motor why because the motor is gonna work always at the full spin and it's gonna take the full amount of power required okay now if we're using it on VFD instead of that we can actually control the flow by controlling the speed of the motor so we don't have to run the motor at full speed all the time so it's all about saving dollars okay now when we're trying to look at V of these we have to look at multiple things okay so what we came up we came up with an acronym you know that is illustrating a little bit what we have to look at when we're trying to pick up a VFD for a certain application okay and we're calling that a backpack it's easy to remember be basic AC motor and drive theory design so this will be your and think about this backpack is exactly like when you're packing an actual backpack you will have to put everything in there so it's as balanced as possible okay so you're stripe starting from the base of the backpack where you're putting whatever it's more heavier you know the basic stuff and you're going up and adding all kind of other things like accessories and so on and you will see so in this case for our base it's gonna be this basic AC motor and drive theory okay a application evaluation to the Terr in drive size and performance see for control scheme scheme for analog digital BIOS and communication and so on P is for protection process motor drive and line a ambient environment pollutants temperature moisture and see it's gonna be commissioning an integration okay and we're gonna touch on each of these ones today now for the basics everybody knows how the electric motor is built okay so we have our motor frame somewhere in covers stator rotor and belts okay it's pretty standard now how the motor is working basically when we're passing the three-phase through through the windings basically we will have a magnetic field that basically will push the rotor so it's gonna rotate and will create that motion okay three-phase applied to the stator winding or taking magnetic field is created okay the speed of the rotating field it's gonna be cold your synchronous speed okay and we have a formula to calculate that it's pretty straightforward it's 120 multiplied by your frequency and divided by the number of poles now the frequency here in North America we're using sixty Hertz okay so it's pretty straightforward number of poles what can we have for poles for a motor there's a number of poles so the speed in this case it's gonna be influenced mostly by the number of poles that we have okay higher number of poles how it's gonna be the speed higher or lower okay more poles less speed okay now if we're changing the frequency we can also change the speed because in the formula if we're looking at 120 multiplied by freak in polls now the motor is already built we have two polls four polls or six polls so we cannot play with that anymore so what we can play with is the frequency in this case okay ten Hertz is gonna be slow thirty it's gonna be faster sixty is gonna be the synchronous speed okay now for the same formula two polls four polls six volts okay we are going to touch this but what do you think on a real motor is this what are we gonna see on the nominal plate of the motor are we gonna see 3600 rpm for two-pole motor what do you think in a real life real motor less any idea why we're gonna touch a little bit on that but okay so we have the slip okay so in an ideal situation your rotor will rotate at the same speed with the magnetic field created okay but we have some losses okay who do you think it's causing those losses in a motor why it's not rotating at the same speed with the with the Baraat or with that okay we have some losses in the windage right come on don't be shy bearings so we have some frictions in there so there are all kind of things that may influence that so we will never have that speed so all the time on your nominal plate you will see something a little bit lower than the calculated the speed okay and who's gonna calculate that basically the manufacturer of the motor so they will measure that and depending on how well it the motor build you will see something close to that number or something but far from that number okay the higher the load on the rotor shaft the more the rotor will sleep okay so okay we touch so the actual speed will be your synchronous speed - that sleep okay now let's introduce a little bit bit work so measurement of course of a rotational force I don't think that they have to explain too much to the engineering side on that you're pretty familiar with this it's the ability basically of that rotating element you know gear shaft or whatever you have to overcome that turning resistance and the torque requirements will be considered when you're selecting your drive horsepower unit of measurement okay so these are just some basic formulas and we just put them there so you know what we are talking about in order to rotate that motor shaft basically under specific load that motor will need to convert that electrical energy to mechanical mechanical energy okay and the horsepower rating of the motor will be actually the amount of the power that you need to provide the torque required by the by the application in order to maintain your motors nominal motors nameplate speed value at full load okay this is a little bit more developed so if we're replacing the wattage in the horsepower formula with what actual what it means you'll get something a little bit more evolved and based on this formulas you see what we can play in order with in order to adjust certain elements like the speed of the motor so if we're looking at this formula what do you think we can touch in here and then winds mostly voltage and the amperage basically okay the rest are pretty standard okay okay this is a the torque speed curve for a standard NEMA B motor winema B because maybe it's probably the most used motor on North America okay so if we're looking at this we have our locked rotor or stall torque so this is where our motor is starting at zero speed okay so in order for us to start motor we have to break that okay once we're starting you have pull up torque while the motor is speeding toward the nominal speed and you're getting in the pull out or breakdown torque when the motor is getting close to what the nominal speed is okay and you will have your full load torque at pretty much the nominal speed as you can see it's not exactly 1,800 why because of your sleep and remember we already said that okay between here in here you have your sleep okay and for us really that's what Doug was saying earlier this area where your slip is is the area where we can play the higher the efficiency of the motor the shorter interval will have word that sleeps so less room to play with with the view of the end the motor okay so this is pretty much the standard curve now if we're looking this is what I was saying we have couple NEMA designs for the motor there is an mi which is not here but this is mainly what you have NEMA a B and the you see all the difference between this if you have let's say an application where you're demanding a lot of torque in the initial when you want to start the motor you know like a big flying wheel or something like this which motor do you think will handle better that situation without having the VFD d that's correct okay but they it's really expensive it's I think about two or three times more expensive than a NEMA B okay so it's not very good solution in most of the applications now with the VFDs we don't really need this because the VFD can control differently how the motor is starting so it's gonna help with that so that's another good reason to use the VFD okay now how did they build the D algorithm and theory behind the VFD drives they took the motor and that they translate that into an electrical circuit okay so if we're looking at our motor it's gonna look pretty much like this in a electrical diagram okay so we're gonna have your stator you're gonna have your rotor okay your applied voltage at the motor terminals you have a current flowing through the Statler you have the resistance in there you have the magnetizing voltage okay so they looked at I'm not gonna insist too much but there they looked at all the elements involved in two days and they translated that into an equation that can be used in the algorithm so for the drive okay for us the ability to produce that torque basically it's gonna be proportional with the wattage okay this is the formula remember for the wattage volts multiplied times 1.73 efficiency and power factor okay yeah and the applied voltage will be determined of course by your source okay the amps it's gonna be applied by your total voltage and the total the impedance of the motor circuits what kind what do you think it's forming the impedance in a motor do we have capacitance in there okay what kind of capacitance between different elements of the windings basically because we have an air gap okay so you have some content in capacitance resistance of course your windings and stuff like that so these are the elements basically that will will contribute to that so the whole idea of this is just to know that in order to build that algorithm that is embedded in the drive we need it to look a little bit more deep into the theory of the electrical circuits okay and we translate it basically the motor into that they actually not we but let's move on to our dry so we have our motor we basically so what we have to do with our motors now the drive dry it's mainly are built from couple main elements okay you have your operator controls which can be I don't know your keypad can be an HMI through a communication or something like that you have your control unit you have your power conversion unit and at the output of course you have your motor okay most of the times with the drives you have three phase in three phase out okay is that all the time can you have one phase input yes you can can you have one phase output actually yes we can now we have the DA one drive which basically at the DC one sorry which can do single line input goal-line output okay so we can control actually some of the single line single-phase motors that are out there okay with the previous drives and most of the drives that are available we can do single-phase at the input but always three-phase at the output and most of the time when you have a three-phase driving you're using a single phasing on the input of the drive you'll have to do rate that drive and sometimes you have to add some extra capacitance okay on that but now this is the important thing to know we have a single phase two single phase drive okay now if we're looking a little bit further into our drive basically we will have our input line voltage we have our rectifier we have our DC bus and we have our inverter at the output so what we're doing basically we will we are taking the three-phase okay input we have our sine wave we transform that into DC and at the output we have the inverter that will produce what is called pulse width modulation why we're doing this why are we converted where we're not taking that AC and do something with it we can't really change the frequency in needham on that so by converting that to DC and after that through the inverting into a peer pulse width modulation signal we can play with frequency and open with the amplitude and stuff like that if we're looking at our converter or rectifier section and you will see you'll find in literature this called both converter and rectifier through the bit fold the output inverter basically it's also a converter okay because if you're thinking converter designed something you're converting from AC to DC or from DC to AC okay so sometimes in literature you'll find the input rectifier called all Converter okay so it's nothing wrong with with that rectifier basically is just most of the time it's a six diodes bridge which will take your base E and converting to DC everybody knows how a diode is working so it's either allowing or not allowing the current to pass through okay so depending on where your input phase will be it will let or not the current to pass and that's gonna create your DC voltage on some drives you will see we can have more than six diodes on the rectifier or on some of the drives we can even have AG be T's that commonly you find them on the output but if you want to put back in the network when you are in a region mode we're using igb T's to let the current flow in the other direction - so basically it's becoming an inverter okay at that time okay the pre-charge circuit this is standard with most of the drives why do we have a preacher circuit on the DC bus we have capacitors okay which are here and we will touch a little bit later about them capacitors usually when you're starting the drive they want to do what to suck a lot of current okay they're very very hungry if you let that you'll have a big inrush current okay which can damage your rectifier so in order to temper it a little bit what the capacitors are sucking in you are using this pre-charge circuit which basically it's a resistor connected in series from one of the branches of the DC bus okay once the capacitors reach a certain level of charging then we're closing the contact that is gonna bypass your resistor okay usually with the drives you will hear whenever you power the drive you're powering up the drive you will see you'll hear a click okay that click it's actually that contactor that basically it's closing and it bypassing the resistor and that's when you will basically get your ready signal for the drive okay kill that is not closed the drive will not be able to operate okay very important some people I know electricians in the field out there you know they like to connect and disconnect the drive if you're doing that too fast and too often you're governed that register most of the time that register is basically directly on your PCB board okay it's not easy to replace so sometimes you have to replace the entire board just because of that okay we have our DC bus capacitors they are basically there to provide a little bit of extra energy when it's needed okay so if your application is demanding at a certain point a little bit more you know in order to avoid any new Russian because the rectifier will not be able to provide that as fast as possible you have those capacitors that will basically help a little bit in the inverter the inverter basically is built with igbts there again six of them the nice things were nice thing with diabetes is that you can basically control both when it's open and it's closed you know and you have a higher switching frequency on them okay also on some of the drives on some of the smaller frame sizes we are offering this by example as a standard option on some of them it's just an option the motor can be a motor or can be a generator in some situation when it's a generator it's gonna push all that energy that generates back on to our inverter if we're not burning that energy that's gonna basically hit your DC bus if you're hitting your disability m''d that your IGBTs will not handle in time so you want to burn off that energy how you have basically on seven IGBT in this case that basically when it feels that you have some energy put back it's gonna burn that through a resistor so it's gonna just this heat on some of the trail drives like I said instead of having a braking you can regen back into the grid by using scr's or igbts on this side okay this is how your post with modulation is looking and this is how we kind of try to recreate that sine wave at the output of the drive it's not an ideal sine wave because it's a pretty chart and we can see in here so we have different durations for the input when we open or we're closing the circuit and based on that you're creating something you're trying to simulate basically that that sine wave now hired the switching frequency a little bit more nicer sine wave you will have but higher switching frequency not very good for your motor because think about this you're just opening and closing opening and closing so it's not that ideal perfect sine wave so you basically just open shoot into your motor windings closed open again so the faster you're doing that the worse is for the motor and it's gonna start to heat up okay and this is how your typical PWM the voltage and current waveforms are looking like so you see it's not a very very nice sine wave but it's working okay how efficient is your V FD they're pretty good actually these days you know and the amount of losses that you have on a VFD basically they're very small compared with the savings that you're doing in energy by using a VFD okay why you have some losses in the VFD because we have all kind of electronic involved okay we have some resistance in there that is going to just burn off some of the energy so there are some fixed losses you know which are a very very small amount of the total power transfer usually we're about at 98.5 percent like efficiency you know that can vary but that's pretty much so it's really really low okay the losses of course you have your control logic you have your cooling fan this is one of the biggest loss probably but even for that we have options we can have the fan to run only when the temperature reaches a certain value and on bigger frame size drives basically we have fans that have their own little small VFD okay so they're not going all the time at the maximum speed but they're also speed control okay conduction loss is proportional to your current flow and you have your switching losses into the eye gbts okay

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Id: IT23yjCYjDc

Length: 25min 21sec (1521 seconds)

Published: Mon Aug 25 2014