3 Phase: Star Connected Loads, How to Calculate Line and Phase Voltage & Line and Phase Current

[Music] hello and welcome to this electrical principals training video in this video we're going to continue considering the subject of three-phase electricity now in this video and in the subsequent video we're going to look at loads that are connected in both star and Delta so you may have heard those expressions before in connection with three-phase and they describe the two ways that we can connect up loads into a three-phase system more specifically in this video we're going to define and describe the different voltages and the different currents that you find in three-phase systems and we're going to have a look at the relationships between those voltages and between those currents so in this video we're going to specifically look at a star connected load and if we just have a look at the rig that I've created behind us here you can see that we've got three loads connected now we're getting familiar with the idea that we can use resistors to represent loads and in this case this could be viewed as three heating loads we could also start to consider this being the three windings of a three-phase motor or perhaps the three windings of a three-phase transformer these are all different ways that we can consider that gets a little bit more complicated let me start considering those but the point is we've got three loads and they're connected to each other in star now how do we know this is a star connection well each load has two connections to it electrically speaking so you can see that there's two connections there and there and two connections there and there one of those connections is connected to a supply line so you can see here this one is connected to the supply line coming along the bottom and this one is connected to this supply line here and the other end of each one of these loads is connected to each other and then to the neutral conductor and that's what makes it a star connection as you'll see in the next video the Delta Connection looks very different now it's important to bear in mind that when we look at a star connected load we lay it out like this graphically and I've laid this out like this as a demonstration piece but actually in reality the three loads don't necessarily have to sit 120 degrees apart physically like they are here if you look inside the windings of a transformer you won't see the windings arrayed like this when we had three heaters or three lamps plugged in through our three-phase system in a previous video they weren't laid out like this but it does help us just to see the relationships between the voltages and the currents so let's start thinking about the voltages and the currents inside these in the relationship between them now in a three-phase system there are two kinds of voltage that we're interested in there's what we call the line voltage and there is what we call the phase voltage now what we're going to do is we're going to give these definitions that make them work for a star connected load and for a delta connected load so let's think about that first of all what is the line voltage well line voltage in a three-phase system is the voltage that can be measured between any two supply lines so we might measure it between l1 and l2 as we've got here we might measure it between l1 and l3 or we might measure it between l2 and l3 those are the three different ways that we can measure line voltage in a three-phase system now some of you may be watching this video and thinking ah now I know how he's going to define phase voltage the way I define phase voltage it might seem a little bit odd to start with but actually it helps us to understand how the same definition applies to star and Delta because the way I define the phase voltage is it is the voltage measured across the load so it's the voltage across the load now in a star connected system that's basically the same as measuring between any one of the lines and the neutral conductor you can see that measuring between here and here will give us exactly the same value as measuring between here and here so we'll think of phase voltage as being the voltage across the load now let's have a think about our line and phase current so though we've got our two different kinds of current we've got line current and we've got phase current now at first glance the definitions at my to give here might seem pretty absurd for a star connected load you can see that they're going to be really useful when we get to the delta connected load so the line current is the current flowing through the supply line to the load so is the current flowing through the supply line the phase current the other kind of current in a three-phase system is the current that's actually flowing through the load now what I've done on this rig is I've built in these little loops in the cable so that we can measure what they are and actually this really helps to demonstrate very easily what the relationships going to be between the line current and the phase current in a star connected system so here we've got we'll be able to measure our line current and here we'll be able to measure the current that's flowing through the load the phase current so what we're going to do now is we're going to power up our star connected load we're going to measure the voltages we're going to measure the currents and then we're going to show the mathematical relationships between those voltages and currents so what we're going to do now is we're going to connect up our star connected load to a three-phase supply which is just off screen here now we're working under controlled conditions but obviously a three-phase system is even more dangerous than a single-phase system which is already pretty lethal so please do not attempt to repeat this experiment in your own time so what we're going to do will power up so we've turned that on now there's now electricity flowing into our three phase load and what we're going to do is we're going to first of all measure the line voltage so that is the voltage between any two supply lines I'm going to pick l1 and l2 and I'm going to measure the voltage between those so using my beautiful mega AVO eight three five I'm going to set that to measure voltage make sure that it's on AC and then we're going to measure across here now I'm using these rather lovely Vargo connectors to make sure that I am absolutely safe there I'm going to put these on at roughly the same time so I don't live and waggling around and then you can see that the voltage that we're measuring here is 426 426 point three volts got there so 420 6.3 volts so that is our line voltage for this system that we've got connected up so what we're going to do now is we're going to measure our phase voltage and we're gonna see what that is so remember phase voltage is the voltage across the load so in this case we're just going to connect up across here so if I can just get a connection onto here and a connection onto here in a safe fashion you can see that we've got two hundred and forty six point eight volts two hundred and forty six point eight volts so hopefully those values are vaguely familiar to you because of course on nominal voltages in the UK we say that three-phase is 400 volts and we say that single-phase is 230 volts so we had those values pretty close what we're going to do now is we're going to nip across to the white board and we're going to have a look at how those two voltages are related to each other and what the kind of the number is that connects them so you can see on the screen here we've got our three-phase connected load we've got our three resistors that are connected in star with each other and we've got various voltages and currents that we're going to be measuring now the first value that we measured was the line voltage which was between l1 and l2 and measured 426 point 3 volts so we measured that at 426 point 3 volts and then when we measured our phase voltage it came out at two hundred and forty six point eight volts so how are these two numbers related to each other well when we look at these two numbers there is a relationship between them but it's not immediately obvious we need a number that connects these two together and that number is really important in three-phase systems and it is the square root of three the square root of three so if we want to figure out what the relationship is between these we look at it in this way we say that the line voltage VL is equal to the phase voltage VP multiplied by the square root of three now this number the square root of three will often be written down as one point seven three one point seven three two which is an approximation of this value root 3 is are much more accurate value to use and if you have a scientific calculator it's a very very easy one to use so let's put our numbers in and see if this holds true so we'll put in our values so we've got first of all two hundred and forty six point eight for our phase voltage and we're going to times that by the square root of three so let's bring up our Casio FX 85 GT plus and we'll have a look at what that calculation comes out at so let's do our calculation now we've got two hundred and forty six point eight and we're gonna times that by the square root of three so here's our square root button and then we put a three underneath there it's not only best to do it in this order if you put the root three in first sometimes you end up putting this multiply under the square root which throws us off completely so try remember that as a tip times root 3 is the second value hit the equals button and we come out with four hundred and twenty seven point four seven which is very very close to the four hundred and twenty six point three volts that we were hoping to get so we can see that the relationship between those two numbers holds true that is a really really important thing to bear in mind this formula here is the one that we want to take into our exams in our minds V L is equal to V P times root three for a star connected load so now we've seen the relationship between line voltage and phase voltage in a three-phase system we're going to have a look at the relationship between line current and phase current in a star connected system so now we've looked at the relationship between line voltage and phase voltage in a star connected three-phase system we're going to have a look at the line current and the phase current in a star connected three-phase system and see what that relationship is so for this I'm going to use my rather lovely DCM 305 a leakage clamp now this device is absolutely fan tastic for measuring earthly kids in an installation it measures very very low current very accurately and that low current measurement is going to come in super helpful now because we're going to be able to measure the small amounts of current that flowing into my three-phase rig here so I'm going to first of all measure my line current so that's the current flowing through the supply line so I'm going to clip my DCM 305 II on here and measure that and you can see that we've got there 74 point 1 milliamp soar 74 milliamps that settle down that so that's quite nice now it might seem a little bit mad to check what the phase current is going to be here because again if you think about this logically any current flowing through this supply line here must just flow down here through our star connected load here and out of the system again so therefore what value do you expect we'll get here well let's have a look so we've got amazingly 74 milliamps again so this is reading 74 point 2 if we let that settle down it'll probably come out at 74 again so you can see there that the phase current and the line current are actually exactly the same value inside a star connected load that might seem like an obvious thing to say but understanding that now will help us to gain a deeper understanding of how our delta connected load looks like in the next video so just for the sake of completeness we'll go over to the white board and we'll add that relationship on so that we can see our relationships very clearly so we took our measurements for the line current and the phase current so the line current was 74 milliamps and we found that the phase current was also 74 milliamps just about so we can see there very simply that these two values are the same and that makes sense because any current flowing down this line conductor into the load must also flow through the load so it seems pretty obvious to state it at this point but we'll write it down for the fullness of the situation we've got al is equal to I P in a star connected system the line current and the phase current are exactly the same so there's our second key formula to take away from this video il is equal to IP in a star connected load so for your exam just some tips here that will come in really helpful you might be shown a drawing of a star connected load that looks something like this with various a meters and voltmeters connected to it you need to be able to identify whether those a meters are measuring line current phase current line voltage or phase voltage so if you remember the definitions that we mentioned earlier on in the video that will really help you with that and you also need to remember the relationships you may be given a value of line voltage and asked to calculate the phase voltage and that all-important number of the square root of 3 is the number that relates them together the way I always think about this is if I can remember my line voltage which is 400 volts we state that and if I can remember my single-phase voltage which is 230 volts that's my phase voltage I can kind of get the relationship between those two by doing 400 divided by 230 that will give you about one point seven three so then you've just got to think well if I'm given a line voltage do I want the phase voltage to be bigger or smaller obviously we want it to be smaller therefore you would divide by that one point seven three or the square root of three if you want to be a little bit more accurate and of course in the star connected system because we've got two different voltages that kind of means that the two currents must be the same so if you're given a value of line current for a star connected load you can very very easily figure out that the phase current will be exactly the same thank you very much for watching [Music] [Music]

Channel: Joe Robinson Training

Views: 72,647

Keywords: Electrical, training, electricity, voltage, current, resistance, ohm, ohms, electrical training, electrical training video, matrix, EAL, City and Guilds, City, Guilds, C&G, Science, Principles, Science and Principles, Joe Robinson Training, level 1, level 2, level 3, level 4, level, maths, calculation, formula, formulae, HNC, BTEC, Engineering, 2365, 2357, 5357, three, phase, 3 phase, star, delta, line, line current, phase current, line voltage, phase voltage, a-level, a level

Id: rB1NafJ6XPk

Length: 15min 28sec (928 seconds)

Published: Wed Jan 08 2020