Pratique du Shadowing: How Electric Motors Work - 3 phase AC induction motors ac motor - Apprendre l'anglais à l'oral avec la vidéo
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161 phrases
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This is an electrical motor is one of the most important
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devices ever to be invented these motors are used everywhere from pumping the water we drink to powering elevators
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and cranes even cooling nuclear power stations
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so we are going to look inside one
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and learn in detail exactly how they work in this video
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which is sponsored by the Great Courses Plus visit the link in the video description to start a free trial
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and get access to some truly great online courses.
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The induction motor will look something like this.
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They turn electrical energy into mechanical energy which we can use to drive pumps,
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fans, compressors, gears, pulleys etc. Almost all the parts are held inside the main housing.
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At the front we find the shaft.
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This is the part that rotates and we can connect things like pumps, gears and pulleys to this to do work for us.
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At the back we find the fan and a protective cover.
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The fan is connected to the shaft and so it rotates whenever the motor operates.
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That's because the induction motor can produce a lot of heat
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when in operation so the fan blows ambient air over the casing to cool it down.
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If the induction motor became too hot
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The insulation of the internal electrical coils will melt.
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This will cause a short circuit and the motor will destroy itself.
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The fins on the side of the enclosure help to increase the surface area and that lets us remove more unwanted heat.
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The shaft is supported by some bearings, which sit inside the front as well as the rear shields.
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The bearings help the shaft rotate smoothly and also hold it in position.
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Inside the housing we find the stator.
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The stator is stationary and does not rotate.
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This consists of a number of copper wires which are wrapped into coils between the slots positioned around the inner perimeter.
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The copper wires are coated with a special enamel which electrically insulates the wires from each other.
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This means electricity has to flow through the entire coil.
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Otherwise it would take the shortest path possible and we'll see why that's important a little later in this video.
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Now, this is a three phase induction motor, so we have three separated sets of coils in the stator.
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The ends of each set will connect with the terminals within the electrical terminal box.
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We will also see how these are connected a little later in this video.
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When connected to the electrical supply, the stator generates a rotating electromagnetic field.
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Connected to the shaft is the rotor.
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In this case, it's a squirrel cage type rotor.
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It's called a squirrel cage because it has two end rings which are connected by some bars
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and these will all rotate together.
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This design is similar to a small cage or an exercise wheel used by a pet hamster or even a squirrel.
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The squirrel cage is fitted with a number of laminated steel sheets.
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These sheets will help concentrate the magnetic field into the bars.
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Sheets are used instead of a solid piece of metal as
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this improves the efficiency by reducing the size of the eddy currents in the rotor.
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When the rotor is placed inside the stator and the stator is connected to an electrical power supply, the rotor will begin to rotate.
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So how does this work?
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work.
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When electricity passes through a wire an electromagnetic field is generated around the wire.
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We can see this by placing some compasses around the wire.
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The compasses will rotate to align with this magnetic field.
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If the direction of current is reversed the magnetic field also reverses and so the compasses will change direction.
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The magnetic field of the wire is pulling
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and pushing the compass dials just like
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if we slide two bar magnets towards each other they will either be attracted or repelled.
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We can even use one magnet to rotate another
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or we could also rotate the magnet by changing the intensity of the magnetic field around it.
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If we place a wire in a magnetic field
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and pass a current through it the magnetic field of the wire will interact with the permanent magnets magnetic field
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and the wire will therefore experience a force this force will move the wire either upwards or downwards
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depending on the direction of current and the polarity of the magnetic fields.
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If we wrap the wire into a coil, the electromagnetic field becomes stronger.
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The coil will produce a north and south pole just like a permanent magnet.
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We call these coils of wire an inductor.
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When we pass an alternating current through the wire, the electrons will be constantly changing direction between flowing forwards
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and backwards so the magnetic field will also expand and collapse and the polarity reverses each time.
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When we place another separated coil in close proximity and complete the circuit,
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the electromagnetic field will induce a current in the second coil.
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We can connect two coils together and place them opposite each other to create a larger magnetic field field.
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If we placed a closed loop of wire inside
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this large magnetic field we will induce a current in the loop as we know
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when we pass a current through a wire it generates a magnetic field
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and we also know the magnetic fields will push or pull each other when they interact.
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So this loop of wire will also generate a magnetic field and this will interact with the larger magnetic field.
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Each side of the coil will experience opposing forces which causes it to rotate.
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This loop is therefore our rotor and the coils are therefore our stator.
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The rotor will only rotate until it aligns with the stator coils.
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At this point it will likely get stuck as the induced current reverses with the coil.
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To overcome this we need to introduce another set of coils in the stator and we must connect these to another phase.
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The electrons flow in this phase at a slightly different time.
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So the electromagnetic field will also therefore change in strength as well as polarity at a slightly different time.
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This will force the rotor to rotate.
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Inside the induction motor we have three separated coils which are used to produce a rotational electromagnetic field.
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When we pass an alternating current through each coil,
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the coils will produce an electromagnetic field which changes in intensity as well as polarity as the electrons change direction.
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But if we were to connect each coil to a different phase, then the electrons in each coil will change direction at a different time.
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This means the polarity and intensity of the magnetic field will also occur at a different time.
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To distribute this magnetic field we need to rotate the coil sets 120 degrees from the previous phase.
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We then combine these into the stator.
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The magnetic field varies in strength and polarity between the coils which combine to produce the effect of a rotating magnetic field.
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We saw earlier in this video that current can be induced into a second coil when in close proximity.
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The bars of the squirrel cage are shorted at each end, which therefore creates multiple loops or coils.
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Each bar therefore induces a current and creates a magnetic field.
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The magnetic field of the rotor bars interacts with the magnetic field of the stator.
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The rotor bar's magnetic field is attracted to the magnetic field of the stator.
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As the magnetic field is rotating,
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the rotor will therefore also rotate in the same direction as the magnetic field to try and align with it,
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but it will never be able to fully catch up.
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The bars of the rotor are often skewed.
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This helps distribute the magnetic field across multiple bars and stops the motor being able to align and jam.
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the stator contains all of the coils
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or windings used to create the rotating electromagnetic field
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when electricity is passed through the wires to power the coils we find an electrical terminal box on the top
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or sometimes on the side inside this box we have six electrical terminals each terminal has a corresponding letter
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and number we have u1 v1 and w1 then w2 u2
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and v2 we have our phase 1 coil connected to the 2u terminals then the phase 2 coils
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which are connected to the 2v terminals and lastly the phase 3 coil
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which is connected to the 2w terminals notice
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that the electrical terminals are arranged in a different configuration on one side to the other.
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We will see why that is in just a moment.
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We now bring in our three phase power supply and connect these to their respective terminals.
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For the motor to run, we need to complete the circuit.
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And there are two ways to do this.
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The first way is the Delta configuration.
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For this, we connect across the terminals U1 to W2, V1 to U2, and W1 to V2.
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This will give us our delta configuration.
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Now, when we provide AC current through the phases, we see
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that electricity flows from one phase to another as the direction of AC power reverses in each phase at a different time.
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That is why we have the terminals in different arrangements in the terminal box.
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Because we can easily connect across and allow electricity to flow between the phases as the electrons reverse at different times.
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The other way we can connect the terminals is to use the star or Y configuration.
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In this method, we connect between W2, U2 and V2 on only one side.
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This will give us our star or Y equivalent connection.
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Now, when we pass electricity through the phases, we see the electrons are shared between the terminals of the phases.
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Due to their design differences, the amount of current flowing in the star and delta configuration is very different.
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And we're going to see some calculations for these towards the end of this video.
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Let's have a look at the difference between the star and delta configurations.
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Let's say we have the motor connected in delta with a supply voltage of 400 volts.
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That means when we use a multimeter to measure the voltage between any two phases, we will get a reading of 400 volts.
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We call this our line -to -line voltage.
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Now, if we measure across the two ends of a coil, we again see the line -to -line voltage of 400 volts.
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Let's say each coil has a resistance, or impedance as this is alternating current, of 20 ohms.
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That means we will get a current reading on the coil of 20 amps.
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We can calculate that from 400 volts divided by 20 ohms, which is 20 amps.
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But the current in the line will be different.
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It will be 34 .6 amps.
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We get that from 20 amps multiplied by the square root of 3, which is 34 .6 amps.
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That's because each phase is connected to two coils.
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Now, if we look at the star or wire configuration, we again have a line -to -line voltage of 400 volts.
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We see that if we measure between any two phases, but with the star configuration,
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all our coils are connected together and meet at the star point or neutral point.
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It's from this point that we can run a neutral wire if needed.
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So this time, when we measure the voltage across the ends of any coil we get a lower voltage of 230 volts.
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That's because the phase isn't directly connected to two coils like in the delta configuration.
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One end of the coil is connected to a phase
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but the other is connected to a shared point so the voltage is therefore shared.
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The voltage is less as one phase is always in reverse.
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We can calculate this by 400 volts divided by the square root of three, which is 230 volts.
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As the voltage is less, the current will be two.
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If this coil also has an impedance of 20 ohms, then 230 volts divided by 20 amps I'll see you guys equals 11 .5 amps.
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The line current will also therefore be the same at 11 .5 amps.
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So we can see from the Delta configuration the coil is exposed to the full 400 volts between two phases.
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But the star configuration is only exposed to 230 volts between the phase and the neutral point.
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So the star uses less voltage and less current compared to the Delta version.
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Okay, that's it for this video, but to continue learning about electrical engineering,
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check out one of the videos on screen now and I'll catch you there for the next lesson.
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Don't forget to follow us on Facebook, Instagram, LinkedIn, as well as theengineeringmindset .com.
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Vocabulaire et conseils d’expression pour cette leçon
Cette leçon d’expression orale s’appuie sur la vidéo « How Electric Motors Work - 3 phase AC induction motors ac motor ». Les mots qui reviennent le plus souvent : coil, field, magnetic, phase, wire. Cette vidéo contient 161 phrases et 2253 mots à répéter en shadowing. La partie parlée dure 15:29. Le locuteur parle à un rythme régulier d’environ 145 mots par minute, confortable pour le shadowing. Seuls 75 % des mots font partie des 3 000 mots les plus courants en anglais, le vocabulaire est donc exigeant.
Vocabulaire clé de cette vidéo
15 mots de la vidéo à retenir, avec leur prononciation et leur sens :
| Mot | Prononciation | Sens |
|---|---|---|
| coil nom | /kɔɪl/ | spire |
| magnetic adjectif | /mæɡˈnɛtɪk/ | magnétique |
| connect verbe | /kəˈnɛkt/ | accoupler, connecter |
| wire nom | /ˈwaɪɚ/ | fil |
| rotate verbe | /ˈɹoʊ.teɪt/ | tourner, rotationner |
| terminal nom | /ˈtɚmɪnəl/ | terminal, aérogare |
| voltage nom | /ˈvəʊltɪd͡ʒ/ | tension, voltage |
| configuration nom | /kənˌfɪɡ.əˈɹeɪ̯.ʃən/ | configuration |
| delta nom | /ˈdɛltə/ | delta |
| electromagnetic adjectif | /ɪˌlɛk.tɹoʊ.mæɡˈnɛ.tɪk/ | électromagnétique |
| reverse nom | /ɹɪˈvɝs/ | contraire |
| loop nom | /luːp/ | boucle |
| cage nom | /keɪd͡ʒ/ | cage |
| shaft nom | /ˈʃɑːft/ | hampe |
| magnet nom | /ˈmæɡ.nɪt/ | aimant |
Les verbes à particule que vous entendrez
| Mot | Sens |
|---|---|
| pass through verbe | traverser |
| check out verbe | mater, regarder |
La grammaire de cette vidéo
Les structures que le locuteur utilise le plus, avec les mots exacts de la vidéo :
| Structure | Dans la vidéo |
|---|---|
| Voix passive be + participe passé — l’accent est mis sur ce qui arrive, pas sur qui le fait | be invented · is sponsored · are held |
| Propositions relatives who / which + proposition — une précision sur une personne ou une chose | wires which are · rings which are · coils which are |
Prononciation à surveiller
Le locuteur utilise 8 contractions et formes réduites, comme I'll, don't, I'm. Prononcez-les sous leur forme courte, telles que vous les entendez.
- Les sons « th »: mathematics /mæθ(.ə)ˈmæt.ɪks/, smoothly /ˈsmuːðli/
- Les sons « sh » et « zh »: configuration /kənˌfɪɡ.əˈɹeɪ̯.ʃən/, shaft /ˈʃɑːft/, induction /ɪnˈdʌkʃən/, sheet /ʃit/, efficiency /ɪˈfɪʃ.ən.si/
- Mots longs — placez bien l’accent: configuration /kənˌfɪɡ.əˈɹeɪ̯.ʃən/, electrical /ɪˈlɛktɹɪkəl/, electromagnetic /ɪˌlɛk.tɹoʊ.mæɡˈnɛ.tɪk/, electricity /ɪˌlɛkˈtɹɪsɪti/, intensity /ɪnˈtɛnsɪti/
Les sons difficiles pour les francophones :
- /tʃ/ et /dʒ/ — à ne pas adoucir en « ch » et « j »: voltage /ˈvəʊltɪd͡ʒ/, cage /keɪd͡ʒ/, generate /ˈd͡ʒɛn.ə.ɹeɪt/, lecture /ˈlɛk.t͡ʃə/, arrangement /əˈɹeɪnd͡ʒmənt/
- /r/ anglais — langue recourbée, sans frotter la gorge: rotate /ˈɹoʊ.teɪt/, configuration /kənˌfɪɡ.əˈɹeɪ̯.ʃən/, electrical /ɪˈlɛktɹɪkəl/, electromagnetic /ɪˌlɛk.tɹoʊ.mæɡˈnɛ.tɪk/, reverse /ɹɪˈvɝs/
- /ŋ/ final — sans prononcer de /g/ après: bearing /ˈbɛɹɪŋ/, winding /ˈwaɪndɪŋ/
Comment s’entraîner avec cette vidéo
- Écoutez la vidéo en entier une fois sans parler et notez les mots que vous ne connaissez pas.
- Répétez phrase par phrase à vitesse normale, en reprenant chacune jusqu’à ce que votre rythme corresponde à celui du locuteur.
- Enregistrez-vous et comparez avec l’original, en faisant attention à des mots comme coil, magnetic, connect.
Qu'est-ce que la technique du Shadowing ?
Le Shadowing est une technique d'apprentissage des langues fondée sur la science, développée à l'origine pour la formation des interprètes professionnels. Le principe est simple mais puissant : vous écoutez de l'anglais natif et le répétez immédiatement à voix haute — comme une ombre suivant le locuteur avec un décalage de 1 à 2 secondes. Les recherches montrent une amélioration significative de la précision de la prononciation, de l'intonation, du rythme, des liaisons, de la compréhension orale et de la fluidité.