シャドーイング練習: How Electric Motors Work - 3 phase AC induction motors ac motor - 動画で英語スピーキングを学ぶ

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This is an electrical motor is one of the most important

devices ever to be invented these motors are used everywhere from pumping the water we drink to powering elevators

and cranes even cooling nuclear power stations

so we are going to look inside one

and learn in detail exactly how they work in this video

which is sponsored by the Great Courses Plus visit the link in the video description to start a free trial

and get access to some truly great online courses.

The induction motor will look something like this.

They turn electrical energy into mechanical energy which we can use to drive pumps,

fans, compressors, gears, pulleys etc. Almost all the parts are held inside the main housing.

At the front we find the shaft.

This is the part that rotates and we can connect things like pumps, gears and pulleys to this to do work for us.

At the back we find the fan and a protective cover.

The fan is connected to the shaft and so it rotates whenever the motor operates.

That's because the induction motor can produce a lot of heat

when in operation so the fan blows ambient air over the casing to cool it down.

If the induction motor became too hot

The insulation of the internal electrical coils will melt.

This will cause a short circuit and the motor will destroy itself.

The fins on the side of the enclosure help to increase the surface area and that lets us remove more unwanted heat.

The shaft is supported by some bearings, which sit inside the front as well as the rear shields.

The bearings help the shaft rotate smoothly and also hold it in position.

Inside the housing we find the stator.

The stator is stationary and does not rotate.

This consists of a number of copper wires which are wrapped into coils between the slots positioned around the inner perimeter.

The copper wires are coated with a special enamel which electrically insulates the wires from each other.

This means electricity has to flow through the entire coil.

Otherwise it would take the shortest path possible and we'll see why that's important a little later in this video.

Now, this is a three phase induction motor, so we have three separated sets of coils in the stator.

The ends of each set will connect with the terminals within the electrical terminal box.

We will also see how these are connected a little later in this video.

When connected to the electrical supply, the stator generates a rotating electromagnetic field.

Connected to the shaft is the rotor.

In this case, it's a squirrel cage type rotor.

It's called a squirrel cage because it has two end rings which are connected by some bars

and these will all rotate together.

This design is similar to a small cage or an exercise wheel used by a pet hamster or even a squirrel.

The squirrel cage is fitted with a number of laminated steel sheets.

These sheets will help concentrate the magnetic field into the bars.

Sheets are used instead of a solid piece of metal as

this improves the efficiency by reducing the size of the eddy currents in the rotor.

When the rotor is placed inside the stator and the stator is connected to an electrical power supply, the rotor will begin to rotate.

So how does this work?

work.

When electricity passes through a wire an electromagnetic field is generated around the wire.

We can see this by placing some compasses around the wire.

The compasses will rotate to align with this magnetic field.

If the direction of current is reversed the magnetic field also reverses and so the compasses will change direction.

The magnetic field of the wire is pulling

and pushing the compass dials just like

if we slide two bar magnets towards each other they will either be attracted or repelled.

We can even use one magnet to rotate another

or we could also rotate the magnet by changing the intensity of the magnetic field around it.

If we place a wire in a magnetic field

and pass a current through it the magnetic field of the wire will interact with the permanent magnets magnetic field

and the wire will therefore experience a force this force will move the wire either upwards or downwards

depending on the direction of current and the polarity of the magnetic fields.

If we wrap the wire into a coil, the electromagnetic field becomes stronger.

The coil will produce a north and south pole just like a permanent magnet.

We call these coils of wire an inductor.

When we pass an alternating current through the wire, the electrons will be constantly changing direction between flowing forwards

and backwards so the magnetic field will also expand and collapse and the polarity reverses each time.

When we place another separated coil in close proximity and complete the circuit,

the electromagnetic field will induce a current in the second coil.

We can connect two coils together and place them opposite each other to create a larger magnetic field field.

If we placed a closed loop of wire inside

this large magnetic field we will induce a current in the loop as we know

when we pass a current through a wire it generates a magnetic field

and we also know the magnetic fields will push or pull each other when they interact.

So this loop of wire will also generate a magnetic field and this will interact with the larger magnetic field.

Each side of the coil will experience opposing forces which causes it to rotate.

This loop is therefore our rotor and the coils are therefore our stator.

The rotor will only rotate until it aligns with the stator coils.

At this point it will likely get stuck as the induced current reverses with the coil.

To overcome this we need to introduce another set of coils in the stator and we must connect these to another phase.

The electrons flow in this phase at a slightly different time.

So the electromagnetic field will also therefore change in strength as well as polarity at a slightly different time.

This will force the rotor to rotate.

Inside the induction motor we have three separated coils which are used to produce a rotational electromagnetic field.

When we pass an alternating current through each coil,

the coils will produce an electromagnetic field which changes in intensity as well as polarity as the electrons change direction.

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.

This means the polarity and intensity of the magnetic field will also occur at a different time.

To distribute this magnetic field we need to rotate the coil sets 120 degrees from the previous phase.

We then combine these into the stator.

The magnetic field varies in strength and polarity between the coils which combine to produce the effect of a rotating magnetic field.

We saw earlier in this video that current can be induced into a second coil when in close proximity.

The bars of the squirrel cage are shorted at each end, which therefore creates multiple loops or coils.

Each bar therefore induces a current and creates a magnetic field.

The magnetic field of the rotor bars interacts with the magnetic field of the stator.

The rotor bar's magnetic field is attracted to the magnetic field of the stator.

As the magnetic field is rotating,

the rotor will therefore also rotate in the same direction as the magnetic field to try and align with it,

but it will never be able to fully catch up.

The bars of the rotor are often skewed.

This helps distribute the magnetic field across multiple bars and stops the motor being able to align and jam.

the stator contains all of the coils

or windings used to create the rotating electromagnetic field

when electricity is passed through the wires to power the coils we find an electrical terminal box on the top

100

or sometimes on the side inside this box we have six electrical terminals each terminal has a corresponding letter

101

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.

123

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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.

159

Okay, that's it for this video, but to continue learning about electrical engineering,

160

check out one of the videos on screen now and I'll catch you there for the next lesson.

161

Don't forget to follow us on Facebook, Instagram, LinkedIn, as well as theengineeringmindset .com.

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このレッスンの語彙とスピーキングのポイント

このスピーキングレッスンは、動画「How Electric Motors Work - 3 phase AC induction motors ac motor」を教材にしています。繰り返し出てくる語は次のとおりです：coil, field, magnetic, phase, wire。この動画には、シャドーイング用の文が161文、単語が2253語あります。音声の長さは15:29です。話す速さは1分あたり約145語で安定しており、シャドーイングしやすいペースです。英語の頻出3,000語に含まれる単語は75%だけなので、語彙は難しめです。

この動画の重要語彙

動画に出てくる覚えておきたい単語15語を、発音と意味つきで紹介します。

単語	発音	意味
coil 名詞	/kɔɪl/	とぐろ, 渦状
magnetic 形容詞	/mæɡˈnɛtɪk/	磁気の
connect 動詞	/kəˈnɛkt/	繋げる
wire 名詞	/ˈwaɪɚ/	針金, 線
rotate 動詞	/ˈɹoʊ.teɪt/	回る
terminal 名詞	/ˈtɚmɪnəl/	ターミナル, エアターミナル
voltage 名詞	/ˈvəʊltɪd͡ʒ/	電圧
configuration 名詞	/kənˌfɪɡ.əˈɹeɪ̯.ʃən/	構成, 配置
delta 名詞	/ˈdɛltə/	デルタ
electromagnetic 形容詞	/ɪˌlɛk.tɹoʊ.mæɡˈnɛ.tɪk/	電磁気の, 電磁の
loop 名詞	/luːp/	ループ
cage 名詞	/keɪd͡ʒ/	檻, 篭
magnet 名詞	/ˈmæɡ.nɪt/	磁石
squirrel 名詞	/ˈskwɪ.ɹ(ə)l/	栗鼠, リス
generate 動詞	/ˈd͡ʒɛn.ə.ɹeɪt/	生む, 生み出す

動画に出てくる句動詞

単語	意味
check out 動詞	試験する, 検査する

この動画の文法

話し手がよく使っている文型を、動画の実際の表現とともに紹介します。

文型	動画での表現
受動態 be + 過去分詞 — 誰がするかより、何が起きるかに焦点を当てる	be invented · is sponsored · are held
関係詞節 who / which + 節 — 人や物について情報を加える	wires which are · rings which are · coils which are

注意したい発音

話し手はI'll, don't, I'mなど、短縮形や弱形を8回使っています。聞こえたとおりの短い形で発音しましょう。

「th」の音: mathematics /mæθ(.ə)ˈmæt.ɪks/, smoothly /ˈsmuːðli/
「sh」と「zh」の音: configuration /kənˌfɪɡ.əˈɹeɪ̯.ʃən/, shaft /ˈʃɑːft/, induction /ɪnˈdʌkʃən/, sheet /ʃit/, efficiency /ɪˈfɪʃ.ən.si/
長い単語（アクセントの位置に注意）: configuration /kənˌfɪɡ.əˈɹeɪ̯.ʃən/, electrical /ɪˈlɛktɹɪkəl/, electromagnetic /ɪˌlɛk.tɹoʊ.mæɡˈnɛ.tɪk/, electricity /ɪˌlɛkˈtɹɪsɪti/, intensity /ɪnˈtɛnsɪti/

日本語話者が苦手な音：

/r/ と /l/ の区別 — /l/ は舌先を歯茎につけ、/r/ はどこにもつけない: electrical /ɪˈlɛktɹɪkəl/, electromagnetic /ɪˌlɛk.tɹoʊ.mæɡˈnɛ.tɪk/, electricity /ɪˌlɛkˈtɹɪsɪti/, squirrel /ˈskwɪ.ɹ(ə)l/, neutral /ˈnuː.tɹəl/
/v/ — /b/ にならないように、上の歯を下唇に当てる: voltage /ˈvəʊltɪd͡ʒ/, reverse /ɹɪˈvɝs/, divide /dɪˈvaɪd/, overcome /ˌəʊ.vəˈkʌm/, vary /ˈvɛəɹi/
/f/ — 「フ」ではなく、上の歯と下唇で出す: configuration /kənˌfɪɡ.əˈɹeɪ̯.ʃən/, shaft /ˈʃɑːft/, efficiency /ɪˈfɪʃ.ən.si/, fin /fɪn/

この動画での練習方法

まず声を出さずに動画を最後まで聞き、知らない単語をメモします。
通常の速度で一文ずつシャドーイングし、話し手のリズムに合うまで繰り返します。
自分の声を録音して元の音声と比べます。coil, magnetic, connectなどの単語に特に注意しましょう。

シャドーイングとは？英語上達に効果的な理由

シャドーイング（Shadowing）は、もともとプロの通訳者養成プログラムで開発された言語学習法で、多言語習得者として知られるDr. Alexander Arguelles によって広く普及されました。方法はシンプルですが非常に効果的：ネイティブスピーカーの英語を聞きながら、1〜2秒の遅延で声に出してすぐに繰り返す——まるで「影（shadow）」のように話者を追いかけます。文法ドリルや受動的なリスニングと異なり、シャドーイングは脳と口の筋肉が同時にリアルタイムで英語を処理・再現することを強制します。研究により、発音精度、抑揚、リズム、連音、リスニング力、そして会話の流暢さが大幅に向上することが確認されています。IELTSスピーキング対策や自然な英語コミュニケーションを目指す方に特におすすめです。

シャドーイングのやり方: ステップ別の完全ガイドを読む →

シャドーイング練習: How Electric Motors Work - 3 phase AC induction motors ac motor - 動画で英語スピーキングを学ぶ

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