How Motors Work for Beginners (Episode 3); Three Phase Induction Motors: 034

the three-phase motor is by far the most widely used in industrial applications especially where you need good consistent speed even under varying loads the induction motor is very rugged it's only got a few parts as I'm going to show you here in a moment and it's actually simpler than the single phase induction motor and that's why I want to show you this one first let's begin by breaking this guy open and then we'll talk about the components that are there now that we have it apart let's take a look at the anatomy here and decided I have a single-phase induction motor just for comparison with the three-phase motor you can see we have this part which is essentially the rotor it's got these aluminum rings on the end sometimes it's made out of copper copper is a better conductor but aluminum is a little bit cheaper so sometimes they'll made out of aluminum like this now if you look right along the edge you can see these little raised areas around the ring those are actually aluminum bars that go all the way down to this other side and what you end up with there is a little circuit they're connected at both ends and the aluminum bar is essentially the same as a wire it can conduct electricity to the other side but the thing I want you to notice about these rotors here's another one as you can see the single-phase motor is made the same way what was interesting about this though is that there is no electrical connection between the rotor and the three-phase motor or the single-phase motor if you don't have a commutator you don't have any way of putting electricity directly on the rotor well what in the world makes this then that's the concept we want to come back to you in just a moment here we have the stator which is the concept you should be familiar with now and as you can see similar to what I've shown you before a large number of windings the last part I want to point out here is these caps on the end as you can see these serve as places for the electrical connections but again there's no connection to where the rotor Syd's that's just where the bearing is supported by the in bill and it also happens to be a convenient place to plug your wires in to the stator but no electrical connection to the rotor and then the other end bill in this case is attached to the rotor I could take that off but I want to be able to put this one back together so I'm going to leave it on there but that's it you've got your stator housing with the stator field the two in Bill's the bottom one usually serves to support the bearing as well as give you a place to mount all your electronics and then you've got the rotor itself which doesn't connect electrically to anything in the motor now let's have a look at how induction works all right let me explain the details of my model here and then we'll demonstrate this principle for you so here you see I just have a simple disc that's got two bearings on it and that's just to allow it to spin on the other side of the Plexiglas which is only here just to show you that there is no connection they're not touching each other I've got an aluminum disc which is a very good conductor and as we saw earlier many motors are made out of aluminum all right listen this guy up and see what happens as you can see the aluminum disc is spinning and it's spinning just a little bit slower than the nine that it field is spinning behind it if you've been paying attention and following this you ought to know why this is occurring from the lettuce okay and for justice again all right so the concept you're observing here is induction this is what makes the induction motor different from say the DC motor that we saw where electricity was directly conducted to the rotor through the commutator in this case there is no electrical connection between our rotor and our spinning magnetic field and when the magnetic field spins the aluminum disc becomes an electromagnet current begins to flow because all of the conditions for electromagnetism have been met we've got a relative motion between the magnetic field and the conductor and those are your three requirements once you have that relative motion your magnetic field and your conductor electricity or current will begin to flow in the conductor it has a north and a South Pole now just like any other magnet so once it has a north and a South Pole it wants to follow the North and the South Pole of the spinning magnetic field it's just like a compass in fact if I hold the compass here and spin it you can see the very same action the compass spins to align itself with the magnetic field the same way the compass will align itself with the magnetic field of the earth and bring you in just a little bit closer and let you see that one more time and the closer I hold it this better the induction is and the better spins and that's basically what induction is of course if you have to spin the magnets manually that would be a terrible motor so we want to create our magnetic field with electromagnets that we can move electrically well I showed you in the very first episode that when you run current through a wire it produces a north and a South Pole and that's dependent upon the direction of current flow if you switch the current flow to the opposite side then the polarity of the magnet will also switch instead of having the North and the South Pole oriented like this now this is your North Pole and this is your South Pole taking that one step further if we run alternating current through this wire you're going to have an oscillating North and South Pole north south and then south the north south and so on it'll continue to switch back and forth okay let's make a diagram here we have our rotor and we're going to draw in our coils and for the sake of simplicity we're going to make this a six pole motor now generally the way the motor is wound and it's part of the coil will be on top there'll be one wire going to the bottom and then the rest of the coil will continue in the same direction so if it's clockwise on the top the bottom will be cold the same way when this is energized okay just looking at one coal for now with AC current there's our AC sine wave and we know that we're going to call this positive up here and negative and here the current flow is zero the direction of that cycle is going to vary depending upon where you live but it's usually 50 to 60 Hertz 60 Hertz would be one full cycle 60 times per second Hertz the cycles per second okay now with that in mind remember when the voltage is positive your polarity is going to be one way and then when it goes negative it's the other way so we're going to say for now initially this one is energized north here and South and I said this is a 6-pole motor so let's draw in a couple more we've got our six poles if we wire all these up to single-phase power when the current flows through it three of these will be North and three of these will be south and they'll continue to flip back and forth does that make sense so let's just say yeah north like this and then sound stuff and then half a cycle later these wall flip and all of these will be south and all of these will be north and that happens very rapidly right 60 times per second that gives you an oscillating magnetic field but it doesn't actually rotate the rotor and what happens when you wire a motor like this even single-phase motors which are designed to operate with single-phase power just hum they don't actually rotate you need something additional to get it to rotate and we're going to talk about that a little bit later the way the three-phase motor salsas problem is they use this 3 AC supplies shift it out of phase with each other so that they all don't have the same polarity at the same time okay and I'll just put a picture on the screen to help this time let's draw another motor if you've got three different supplies of AC current and there are 120 degrees out of phase with each other that's 120 electrical degrees if you think of one full cycle as 360 degrees then if you shift to be Veatch 120 degrees the three phases they'll be evenly spaced over the full cycle okay so looking at my diagram on the screen when we initially energized this guy you can look at where the phases are and one will be like this and one is just starting to become north this one's just starting to get south and this one is getting ready to flip right now it's just barely north over here and south over here and it's getting ready to flip to the other side okay so just one third of a cycle later this guy is fully north this guy is just starting to flip this one south and this one is now starting to flip now if you notice a pattern going on here as this cycle keeps going what you'll have is a north and then a north and then the north and then a north and then in north and you see what you end up with is a spinning magnetic field and therefore any rotor that's immersed in this rotating a magnetic field will have would take its own poles and try to align it with the magnetic field that's rotating there's just one problem with this three-phase induction motor and that is three-phase power is usually only found at industrial sites most residents don't have three-phase power we have single-phase power if you take single-phase power and hook it up to this three-phase motor you won't get anything it's not designed to function on that well that's where single-phase motors come in like this but just like I told you earlier the single-phase motor if you supply a three-phase motor or a single-phase motor with just straight single-phase line it doesn't spend it just homes current is still being induced in the rotor but it's not actually producing a torque because the magnetic field doesn't spend it's just oscillating top and bottom well we've obviously found a way around that and that is going to be a topic in the very next episode so when you come back we're going to talk about the single phase induction motor I'm going to break out my trainer here we'll inject this rotor end and you'll actually see this guy get fired up and I'll teach you how the motor gets started I forgot to mention this in the last episode but if I've made any technical errors whenever I do a teaching video like this it's always possible that I've omitted something important edited it out forgot to say something it was just wrong and if that happens I will add notes to the description so please be sure to have a look at that if you have any questions about motors please go ahead and scroll down and leave them in the comment section at the end of the series I'm going to make a video where I just go through your questions and answer all your questions about motors so be sure let me know what your questions are I'm going to put a couple videos on the screen for you the one on this side is going to be a link to the playlist so that you can watch this whole series in order if you are not a subscriber you can join my neighborhood by clicking on the subscribe button right here and I'll welcome you into the family all right I'll see you guys soon

Channel: Jeremy Fielding

Views: 481,385

Rating: 4.9502578 out of 5

Keywords: three phase motors, induction motors, single phase motors, motors for beginners, electromagnetism

Id: TumJjfcVVuo

Length: 12min 50sec (770 seconds)

Published: Wed Jul 12 2017