3 Phase Motor. Stator creating a rotating magnetic field

[Music] welcome to a short video explaining what we mean by a rotating magnetic field that's being created by the stator so if we start off here by looking at our original sort of image of a very simplified three-phase motor here we can see we've got our three coils or our six poles and as we've described earlier in in the notes we know that the the top and the sort of middle top and bottom that forms one set that's one one set of poles uh going across the diagonal we've got another two poles forming the other set uh we think of that's like the blue phase and then the other diagonal sort of going top top left to bottom right we have the the yellow phase now um if we translate that into a sort of schematical diagram or a drawing on the right here we can see our individual sort of stator phase windings uh representing orange and what i've done is i've represented the magnet magnetic field that they create as an arrow here and arrow is pointing in the direction of the north to south which is the direction of the magnetic field so what i've done here is you know i've replaced the diagram the real life diagram with our representation that you'll remember from looking at star and delta you can see here the representation of the three phases well that those those three phases here are demonstrated by the drawing on the right where we've got the red blue and yellow phases and as you can see those those phases are displaced by 120 degrees from each other so what i want to examine here is each phase individually and what happens when we apply an ac sine wave to each of the phases well as you can see in the top left here i've got i've got a single phase ac sine wave and it's for the red phase so let's imagine that we've hooked the red phase up to a single phase ac and we apply current to it and what's going to happen is as the current initially begins to flow up to the peak it's going to flow through the coil in one direction so in this case i'm suggesting it's flowing in the top coil and through the coil and then down and out through the bottom coil as indicated by the direction of current and that is going to generate a magnetic field in the middle here and in this case the magnetic field has risen to its maximum point because where we've stopped or at the maximum current and is going from north to south top to bottom if we carry on though as we move along as the as the current now begins to drop off in the sine wave we can see when we reach zero that that the magnitude of the magnetic field in the middle is now is gone basically of course at this point we cross the x-axis and now the the current becomes negative or switches direction and we're into the negative half cycle of the sine wave of course what this means is that now current is going to change direction and so now the current is actually going in the bottom coil through the coil and then up to the top and then actually out of the top coil as opposed to what was doing in the first half and then you can also see the direction of the magnetic field has actually reversed and now the north pole is at the bottom and the south pole is at the top so we've had a switch in the direction of the magnetic field as well and then you can see again when the current drops back off to zero that magnetic field disappears once again now this is exactly the same thing happens in the other two phases and we're going to have a look at that right now so next we're going to examine the blue phase and as you can see i've drawn the blue phase here in the top left notice that his phase shifted by 120 degrees away from the red one and that's because the the motor here is receiving three phase ac and each of the phases are 120 degrees apart from each other so again you know when when it's zero then the magnetic field will be zero that current is going to gradually increase flowing through the coils in one direction it will get to its maximum producing a magnetic field in one direction so in this case north to south from right to left and then it'll drop off again reducing the magnetic field and then again it'll also go through its own negative half cycle reversing the current flow relative to what was in the positive half cycle and as a result the magnetic field also switches again and then once again that will drop off to zero and magnetic field will disappear and the same is true for the yellow phase here we see it increasing up to its maximum decreasing to zero flipping direction so now the magnetic magnetic field goes in the opposite direction and then dropping off to zero now the key thing to take away from this is that actually all of these three phases are doing this at the same time except there's a slight delay between the red blue and yellow phase so typically think of the red phase going first therefore its magnetic field rises at the same time as the red field is beginning to collapse the blue fields begin to rise and as the blue fields begin to collapse the yellow field is beginning to rise and then begin to collapse and of course then they they all begin to switch directions so then at that point the red field is beginning to increase in the opposite direction then begin to collapse and the blue fields begin to increase in the opposite direction and begin to collapse and then finally the yellow field is beginning to increase in the opposite direction and collapse so it looks it should look a bit like this so first the red field followed by the blue followed by the yellow then the red blue and yellow in the opposite direction so what we've seen there where each of the fields rise and collapse in in sequence basically and the reason that they were spaced in time is because of that 120 degrees shift in the three phase that has been supplied to each of the phases now of course that that didn't look really that didn't really look like what we call a rotating field what that looked like was three fields switching on and off in sequence and that's exactly what it is if i now begin to speed that sequence up it's going to begin to look more and more like a continuous field that is actually rotating so here is a bit faster and once again a bit faster still and now you can see that that is actually beginning to look like a rotating magnetic field in fact if we went fast enough i.e if we were doing this as 50 hertz or a 60 hertz system then that field would begin to rotate even faster faster it would rotate so fast that it would actually appear to be a rotating field even though in reality all that's happening is each of the individual fields are just sort of switching off and switching on again and then reversing direction one after the other in sequence with each other and the net result is what appears to be a rotating magnetic field

Channel: Gordon Brown

Views: 24,373

Keywords: Wondershare Filmora

Id: vmgLfl-Xcb8

Length: 7min 37sec (457 seconds)

Published: Fri Jan 08 2021