EV Motor Mania: The Strengths & Weaknesses of Induction vs Permanent Magnet

hi I'm Paul turble and this is Monroe live welcome uh we're with Monroe and Associates an engineering consulting firm and we do lots of tear Downs of uh Motors vehicles and and we like to highlight great examples of amazing engineering and this is one of those things um we've been asked a lot of questions from our previous videos about the difference between induction Motors and permanent magnet motors they're both electric motors but they have different uh strengths and weaknesses and so we find that they work very well together in electric vehicles so I'm going to go through uh and talk about the physical differences of the two kinds of Motors um a little bit about how they work um and we're going to discuss those strengths and weaknesses advantages and disadvantages of the two different kinds of motors and then we'll look at the performance differences and come to the uh the conclusion that it's a good idea to have both kinds of Motors in the car so that we can take advantage of the strengths of each so that's the the plan and uh it's a lot to to it so let's dive in this video is brought to you by an ER and their new Mago wireless charging station the Mago wireless charging station is the ultimate companion for your Apple ecosystem allowing you to charge your iPhone airpods and Apple watch all at the same time all of the modules are magsafe compatible for a hassle-free wireless charging experience and when you're not charging they can be folded up for Easy Storage the compact size of the Mago wireless charging station is true to our values of lean design but it doesn't compromise on capability thanks to its 15 wat Ultra fast charging you get a lot of power in not a lot of time its size also makes it the perfect travel companion so you can stay charged no matter where in the world you are best of all the Mago wireless charging station is designed to keep your devices safe equipped with Advanced temperature control over voltage production and many more unique safety features you don't have to worry about heat or damage while you charge use the link and discount code in the description to get your own Mago wireless charging station from anchor Today first of all let me talk about the physical differences between these two Motors um in the case of the permanent magnet motor the rotor is has permanent magnets in this case um this is an interior permanent magnet motor where the perent magnets are down inside the rotor you can't see them from the surface um and there's lots of different ways to put these permanent magnets in the rotor this is a different rotor uh where you see this kind of a vshape and that's a whole another topic for another video uh let me know in the comments if you'd want to dive into why they arrange the magnets in the permanent magnet motors this type of motor it's called an interior permanent magnet synchronous machine we're going to talk a little bit about that later induction Motors again these are both electric motors um and they both use the same kind of a stator where doesn't matter how you wind the stator you can use stranded wound or bar wound um so both perent magnet motors and induction Motors use the same kind of stator but the difference is the rotor and induction Motors don't have any magnets so instead the the rotor is made up of a whole stack of laminations like this and then typically we take molten aluminum and pour it down the slots uh the gaps in the in the lamination and that forms These Bars uh called rotor bars and then at the other end all the bars are shorted together with a ring to form a circuit um and in some cases when I when I remove all the steel you get something that looks like this and we've been calling this in the motor industry the squirrel cage and so induction Motors are sometimes called squirrel cage Motors so you have a bunch of bars of conductors that uh are shorted at both ends by big rings of either copper or aluminum uh any conductor that's convenient um the aluminum is usually a little bit cheaper and if you want higher efficiency you can go to um copper um these are both um used today in production um in electric vehicles so that's the physical differences uh between permanent magnet motors and induction Motors uh but we need to talk a little bit about how they work so both of these are examples of AC Motors but in reality all Motors are AC Motors even the so-called brush DC motors that people may be familiar with um the brushes and commutator turn the DC from the battery into alternating current inside the motor in this case we use something called an inverter to turn the DC current that comes from the battery into AC current that the motor needs to work so we use a computer controlled inverter which is a set of six switches um and how inverters work is amazing uh maybe another topic for a video let me know in the comments if You' like to hear more about how inverters work but we use the inverter to turn the DC into AC uh we feed the AC current or three phase AC current into the windings of the motor so when you put electricity into a windings these are coils of copper wire surrounded by Steel you get an electromagnet so we turn this thing into an electromagnet with this AC current but the special thing about it and the great gift to humanity that Nicola Tesla gave us is that by putting AC current into a stator like this you get a moving magnetic field it's a little bit like at a stadium where there's the crowd does the the cheer like the wave so each individual person is just standing up and sitting down in their seat at the right time but everyone can see the wave of the chair going around the stadium so the wave moves even though the people don't and we do the same trick in the stator of this we just turn on and off the electricity in one place but by doing that we can create a wave of magnetic field that travels round and round the stator so when we do that with the stator and we put a permanent magnet rotor into into the stator then the magnets want to try to follow the magnetic field around so the if I an observer sitting on the rotor so if I was like a person sitting on a merry-go round and I was looking at the that wave in the crowd in the stands um as I'm going around I would always see just people standing up as they as I go around it's it's as if the wave was standing still from my perspective as a person on a merrygoround so from the perspective of the rotor the this moving wave going around the stator is stationary compared to the rotor it's in sync so as the rotor spins the wave of the uh magnetic field of the stator is synchronized with the position of the magnets and it's in a position such that um the magnets of the rotor are always between the magnets of the stator so this is a North Pole it always wants to pull towards the South Pole that of the stator and always being pushed by the North Pole which is here so uh we always have a both a push and a pull that's pulling the stator rotor around with that uh magnetic field so as the magnetic field rotates around it pulls and pushes the magnetic fields of the permanent magnets um in the rotor so that's how permanent magnet motors develop torque by using this moving wave of magnetic field from the stator induction Motors on the other hand have no magnets and so what we do we do the exactly the same trick with the stator we make a rotating magnetic field but we make the magnetic field rotate just a little bit faster than the rotor in the induction motor and so the field is moving as compared to the um the rotor as the mo the field the magnetic field moves you have this changing magnetic field which uh generates a voltage and when that changing magnetic field that's Faraday's law is changing magnetic field creates a a voltage and that voltage drives a current in the in the bars of the rotor and the current just goes round and round in a circle an Edie current round and round so as the magnetic field passes by it generates current inside these ma these rotor bars and the current flows around a circle as soon as you have current flowing around in a circle with a steel around it you get an electromagnet so we use the moving magnetic field from the stator to induce a magnetic field in on in the rotor and the magnetic field in the rotor and the magnetic field in the stator interact in exactly the same way as it did on the permanent magnet motor so once we have a uh induced magnetic field in the rotor then this motor ends up operating just the same way as the permanent magnet motor so that's the 5 minute physics explanation for how Motors make torque with electricity um so let's talk about the advantages and disadvantages of these two different ways of making torque with electricity induction Motors and permanent magnet motors uh so we got of listed here and induction Motors on the left and permit magnet motors on the right um from the advantages perspective and Engineering uh and certainly Monon Associates we help people save money uh on all kinds of uh uh manufacturing products if you're doing that with uh electric motors one thing you need to look at is cost on the rotor and these permanent magnets that we put in the permanent magnet motors are expensive and the molten aluminum that we use or even the copper um are both less expensive than the permanent magnets for the permanent magnet motor so the induction motors have the advantage of lower cost um it's not as big an an advantage as one might think because in order to make the same Torque from A induction motor you need to make it a little bigger uh because the permit magnet motors have produce more torque for the same package size and so you're making it a little bigger that means you're paying a little more for the copper and steel in the stator and rotor in order to get the same effect but after all said and done the induction motor has a cost advantage and that's beneficial but it's not the the primary reason it's not the make or break for why one might choose an induction motor the second thing this lower spin loss is uh the key advantage that induction motors have and I need to talk a little bit about what spin loss is so when you turn the motor off and the motor is just spinning along so you're uh just gliding in the car so the car uh is moving but you've turned all you've taken your foot off the accelerator there's still some loss because it has bearings and the bearings are not are they're almost frictionless but not perfectly frictionless um and you're spinning and you're stirring up the air inside the motor and that produces a little bit of loss but induction Motors the the spin loss which is the loss when you when the motor is off is very low um permanent magnet motors on the other hand have this as a disadvantage um because of the permanent magnets when it's spinning even if you have the electricity off on the stator the the permanent magnets are creating this moving magnetic field which wants to induce electricity the same way we induce it uh purposefully in the in the induction motor the permanent magnet motor is inducing current in the steel of the stator it wants to induce it producing a voltage and it wants to produce electricity in the windings but we we uh open up the switches on the windings when it's turned off and so no current can flow um but even so the the little bit of eddy current losses that you end up generating with a permanent magnet motor um leads to higher spin loss so there there's this issue with permanent magnet you can't turn off a permanent magnet and so because of that It suffers this extra spin loss when the motor is turned off fortunately in electric cars the motor isn't turned off very often except when you want to brake uh come to a stop and in which case uh you kind of want the extra drag so uh per magnet motors this is not a huge deal except for uh certain situations in the car where this causes us trouble um the permanent magnet motor by contrast it has higher torque density um and higher efficiency than the induction motor in general um they uh and this leads to a smaller package a lighter weight uh um and uh generally the higher efficiency is the key here because every watt that you save by with the motor is a watt that the battery didn't have to deliver and so if you want to go 300 miles in your electric vehicle you want to do that with the smallest battery possible and if you can do it efficiently you don't need as big a battery to go the the miles so higher efficiency is a real premium for um EV for electric vehicle applications um and so that's why the permanent magnet motor this alone is why permanent magnet motors especially this interior permanent magnet synchronous machine um is usually the choice to go with um so th those are the advantages and disadvantages of permanent magnet and induction motor um I put up here the synchronous versus asynchronous um the way we make torque as you call it is with these um we put the electricity into the stator to make uh this traveling wave and we'd make that traveling wave move in sync with the permanent magnets in a permanent magnet motor but in order to produce uh power with the induction motor we have to have make sure that the traveling wave of electricity and magnetism in the stator is always out of sync a little bit faster than the um than the rotor so we call this an asynchronous motor um so those are the advantages and disadvantages and a little bit about how they work um so let's look at some examples of performance um the performance differences between these two kinds of motors which will I think drive home the point of why it's a good idea in many cases to have both of these kinds of Motors in the car to take advantage of the strengths and weaknesses of each well take advantage of the strengths of each and ignore the weaknesses um so for the induction motor um what I've plotted here is a torque here and speed of the motor so torque and this is called a torque speed curve and so at uh at zero RPM the each of these motors can produce uh large torque um and then as the RPM increases the these these motors can hold what's called a constant torque out to some speed usually the RPM here is roughly you know 35 to translates to 35 to 45 miles an hour so you have this constant torque which gives you a great feel off the line so you get this constant um acceleration for the car um right at the limit of what the tires are able to deliver so uh this is a wiconsin torque is an ideal feel off the line up to some point and at some point we reach the limit of what the motor is able to do this would be the the maximum power um coming out of the motor and then the torque drops off along what's called a constant power line so as the speed goes up the torque drops off but we're delivering the same Peak power and then with induction Motors something happens at about uh at this this would be near highway speed or 60 miles an hour or so um that uh magnetic field from the rotor uh starts to work against you it starts to fight back against the current of the stator so as you're putting the current into the stator uh the impedance of the circuit the AC circuit increases with frequency we want to get as much torque for every amp that we put in as possible so that's great when you're down here at low speed but that same high inductance that generates the large magnetic field for every amp that we get that later that large changing magnetic field starts to work against us and we can no longer put in the current that we'd like to put in and the torque drops off so induction motors have this issue of not being able to produce high power at high speed permanent magnet motors on the other hand um do the same trick with a constant torque um up to 35 or 45 miles an hour and then they produce nearly constant power up to essentially infinite speed the speed limit here is only determined by the speed at which you the rotor holds together um so we try to make the rotor uh mechanically robust so it can spin really fast and that allows us to use a uh High a large gear ratio to give us great torque at the wheels so it can hold that really high um constant power out to extremely high speeds so that's one of the advantages of per magnet motor over the induction motor um they also have a little bit different uh regions of high efficiency um one thing I wanted to point out with efficiency is that the definition of efficiency is power out over power in and since the power out is zero along this line so if you're putting in zero if you got zero torque you're getting zero power out and it's also zero along this line if you're if you're not spinning then you get zero power out you can produce torque but no power and so the efficiency is zero along this line and it's zero along this line but it reaches some high some Maximum efficiency out at moderate torqus and moderate moderate speeds um the induction motor is efficient out to high speed but not so efficient at low speed because at low speed I have to put energy in in order to get the magnetic field and so that energy that I'm putting in to get the magnetic field I don't ever recover and so at low speeds where the powers are low uh the efficiency is low for induction Motors permanent magnet motors on the other hand since we get the magnetic field for free we don't put well we pay a lot of money for the magnets but the uh in terms of energy we don't put energy in to get those uh get the magnetic field so at very low speed the the permanent magnet motor reaches a very high efficiency so it's really great for City Driving uh the the asterisk here I've put is the point where a typical City Driving operating POS point so when you're and you're driving the city you have this you know torque that's in this neighborhood and speed like his so it's like 35 miles an hour and you know uh one third pedal position for your torque typical City driving this other asterisk is highway driving typical highway driving so this is 70 M an hour you're on flat level Road um you're only fighting against wind resistance and uh rolling resistance of the tires and so so the torque needed to keep the car going 70 M hour is small you don't need to be flooring it to keep the car going so um It's relatively low torque um so if you're delivering the torque so one option if you have both these motors in your car one option would be to split the difference and have uh half of the torque delivered from the induction motor and half the torque delivered from the permanent magnet motor but that means that your each torque is cut in half and you're down near the uh both of these are down near the x axis which is where efficiency is low and so it's much better on the highway if everything is if you're flat and level and you're not passing anyone to turn one motor off and let then deliver all the torque with one motor motor um and so that it's much closer to the highest efficiency operating point now you might think oh let's turn the permanent magnet motor off because the induction motor is very efficient at high speed and that would be great except that the permanent magnet motor when you turn it off you can't turn off the permanent magnets and so the perent magnets are still spinning they're still generating this back EMF this volt Vol AG in the stator which is driving inducing currents in the steel of the stator and therefore creating a drag torque and so the permanent magnet motor produces this drag torque and so the net effect your Your Efficient induction motor um is has to carry the permanent magnet motor and its extra drag torque it actually works out better to turn the induction motor off and use the permanent magnet motor um and this one is the induction motor is not producing any very low spin loss only the bearings and the air and this one is operating near its most efficient point but not right on it um but still the net losses between with this off and this running is optimal it's the best way to go and the so what we'd have here is and by knowing that the companies are putting induction Motors and permanent magnet motors in their cars we know that they're running this live efficiency optimization algorithm and so every instant as you're driving down the road listening to Tunes kicking back there's a computer that's deciding what is the best way to deliver what the customer wants right now and it's so it's selecting which motor to use sometimes it is the uh induction motor sometimes it's the per magnet motor sometimes it's a combination of both it selects the best motor to use live every uh 100th of a second uh and so that's why and when we see this kind of brilliant engineering at uh as we um tear apart vehicles and analyze what the industry is doing you know we at Monroe just just love to see that and I think the engineers um deserve a lot of credit for coming up with this type of thing and we wanted to highlight this um and I personally on behalf of a driver here as an EV driver I just want to say thank you to all those Engineers that came up with uh brilliant stuff like this and it just is very exciting to me to uh uh get a chance to see it in action thanks a [Music] [Music] lot

Channel: Munro Live

Views: 94,009

Keywords: EV, BEV, Sandy Munro, Munro, Electric Vehicle, Benchmarking, Electric, Insight, Lean Design, Design, MunroLive, ElectricCars, Review, Car Review, Automotive, Automotive Review, Teardown Titan, Tesla, video review, Elon Musk, Munro Live, Ask Munro, Technology, Luxury, Electric Car, Automotive Engineering, Automotive Technology, Innovation, Cybertruck, Motor, Electric Motor, Induction Motor, Permanent Magnet Motor

Id: FHufjrP0xDI

Length: 29min 17sec (1757 seconds)

Published: Mon Mar 25 2024