跟读练习: 26 - How does harmonic phase shifting work including a detailed mathematical explanation to reduce s - 通过视频学习英语口语

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In this video we're going to talk about a subject that a lot of people ask questions about, harmonic cancellation with phase shifting transformers.

So with two variable frequency drives connected to a common bus and motors connected to those drives,

the harmonics that come out of those typical six pulse drives will have a current waveform that look like this.

So the drives will look like this and what happens is the total current is basically just two times that.

So it's one plus one equals two.

Before we put isolation transformers and phase shifting transformers in here to get cancellation, we're going to demonstrate how harmonics add up on power systems.

So we're going to go to a part of the system

over here where we're actually going to show you how that works.

And we're going to actually combine these harmonics from drives.

We're going to put two different drives on and show how those harmonics add up together.

So we'll turn one on.

We'll take a look at the waveform and you see the harmonics coming out of one of the drives.

And now if we take a look at the harmonic spectrum,

the harmonic spectrum from that has your typical fifth and seventh harmonics, what you'd expect.

And so the fifth harmonic right now is about 10 .9 amps.

Now if I turn on the second drive what we end up with is about twice that amount of harmonic current.

So as expected one plus one equals two and that's the harmonics that come out of both drives.

Now, what we're going to do is we're going to turn those drives off.

And we're going to go back to the paper

and we're going to describe what happens when we put two drives on but with phase shifting transformers.

OK.

So now with phase shifting transformers, we would put in this situation where we have,

for example, a delta y on one side, delta y on one side.

And this could be a delta delta, or in fact, it could be a delta zigzag, which effectively gives you a zero degree phase shift.

So you get zero degrees here, and you get 30 degrees here.

So the same harmonics come out of these drives that we saw in the other picture.

OK?

But what happens up here is this transformer has zero degree phase shift.

The harmonics come out the same.

This transformer, the harmonics kind of look like this, or the waveform kind of looks like this.

And so when you add these two together, we get what's not now a six -pulse system,

but rather a 12 -pulse system and we end up with a current that looks fairly linear.

So let me show you that in the actual waveforms that we can see from the two drives coming on.

So I'm going to turn this over to a combination where we put one drive on

and if we look at that situation, we have the isolation transformer where we're going around.

It's a delta -delta transformer.

Now if we look at the actual waveforms from that, the harmonics look very similar.

But if we look at the actual waveforms, the harmonics look the same.

But then when we put on the fourth drive, or the other drive, where we have the phase shifting, or the 30 degree phase shift,

you can see that now we have what's similar to a 12 pulse situation.

So now if I go to my harmonic spectrum, we actually have the fifth and seventh go away.

Now if we turn drive number one off,

what we're going to see is the harmonic currents come back from that drive that's now on,

the one that went through the delta Y transformer, and we can now see that the wave shape is,

as I showed you, a little bit different.

Now what's interesting about this is let's just look at one of the harmonics, the fifth harmonic.

Okay, and what we're going to do is we're going to look at the fifth harmonic here on phase A.

And the fifth harmonic is 9 amps at minus 62 degrees.

Now what we're going to do is we're going to turn that drive off, and we're going to turn drive number one back on.

and we'll see that the harmonic currents from that drive come in at about the same amount or magnitude,

about almost 9 amps again, but the phase angle is about 114 degrees.

So now if we put both of them on together, we put the other drive on with it,

now we get that fifth and seventh to cancel out, but your eleventh and thirteenth basically adds up together.

So going back to the paper now, let's talk about how that worked.

Again, we had no phase shift here, so we expect the wave shape to add up to be similar,

or to go through the transformer and look very similar.

This one here, the wave shape changed.

And this is a 30 degree phase shift.

So we're saying the 5th and 7th from here cancel with the 5th and 7th from here.

And what happens is we don't end up with 5th and 7th flowing this way.

So no 5th and 7th going this way, but we do end up with 11th and 13th, which ends up being this 12 -pulse system.

How does that work?

Well, if you take each harmonic order, the positive, negative, and zero sequence harmonics for each, we say plus, minus, zero,

plus, minus, zero, and that pattern continues.

Plus, minus, zero, plus, minus.

So if we look at just the fundamental, the fundamental we say is a vector,

a phasor, that rotates in this direction.

So phase A is at zero, rotates this direction, phase B follows, and phase C follows that.

So a negative sequence harmonic would actually rotate the opposite direction.

So how do we get the fifth and seventh to cancel from one side to the other?

First of all, all of these harmonics on the left side stay exactly the same.

The fifth harmonic on the right side is five times as fast as the 60 hertz, so the phase shift is 5 times 30 degrees,

but it's actually in the opposite direction.

It's a negative sequence harmonic.

It's in the opposite direction of the 60 Hertz.

So if the 60 Hertz is going ahead 30 degrees, and this one goes the other direction 30 degrees,

but times 5, we end up with 150 degrees this way and 30 degrees this way the net effect is 180 degrees.

It's like we always talk about two vehicles leaving at the same time, going opposite directions.

One goes 30 this way, 30 miles this way or whatever, and we say the other one goes 150 miles this way.

The difference in that is 180 miles.

In this case, we're 180 degrees out of phase.

And so what that actually means is when the fifth harmonic comes up from the bottom on this side, the fifth harmonic looks like this.

And when the fifth harmonic comes up from this side, it looks like exactly the opposite.

So those two added together in this path up here go away because they add up to zero.

And the seventh harmonic, because the seventh harmonic is a positive sequence harmonic,

it actually rotates the same direction as the 60 hertz.

So imagine 210 degrees or 7 times 30 going this way, 210 degrees.

And we're going the same direction or 30 degrees with the fundamentals.

So the net difference again is 180 degrees.

So as we look at the 5th and 7th coming up through power system, we're actually canceling out the 5th and 7th from both sides.

So as we wrap up, the key point here is we take two six -pulse drives, we put them through phase -shifting transformers,

and we create a 12 -pulse system.

Seems complicated, but at the end of the day, as we saw with the minus 61 or so degrees and 114,

115 degrees positive, you saw the fifth harmonic was exactly almost equal and opposite or 180 degrees apart, and that's how it works.

Thank you.

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本课的词汇与口语要点

这节口语课以视频“26 - How does harmonic phase shifting work including a detailed mathematical explanation to reduce s”为素材。视频中反复出现的词有：harmonic, drive, degree, phase, fifth。这段视频共有 95 个句子、1342 个单词可供跟读。讲话部分时长为 9:00。说话人语速平稳，每分钟约 149 个词，很适合跟读。 85% 的单词属于英语最常用的 3,000 词，其余的词建议在练习前先学一下。

视频中的重点词汇

视频中 15 个值得学习的单词，附发音和释义：

单词	发音	释义
delta 名词	/ˈdɛltə/	德爾塔 /德尔塔
pulse 名词	/pʌls/	脈 /脉, 脈搏 /脉搏
seventh 形容词	/ˈsɛvənθ/	第七
minus	/ˈmaɪ.nəs/	減 /减
sequence 名词	/ˈsiː.kwəns/	序列, 順序 /顺序
cancel 动词	/ˈkæn.sl̩/	划, 勾
rotate 动词	/ˈɹoʊ.teɪt/	旋转, 转动
fundamental 形容词	/ˌfʌn.dəˈmɛn.təl/	基本
spectrum 名词	/ˈspɛktɹəm/	範圍 /范围, 光譜 /光谱
connect 动词	/kəˈnɛkt/	連接 /连接
isolation 名词	/ˌaɪsəˈleɪʃən/	隔離 /隔离, 隔斷 /隔断
cancellation 名词	/kænsəˈleɪʃən/	取消
complicated 形容词	/ˈkɑm.plɪˌkeɪ.tɪd/	複雜 /复杂
angle 名词	/ˈæŋ.ɡəl/	角, 角度
frequency 名词	/ˈfɹiːkwənsi/	頻率 /频率, 頻度 /频度

视频中出现的短语动词

单词	发音	释义
go away 动词		離開 /离开, 走
go through 动词		通過 /通过
come out 动词	/ˌkʌm ˈaʊt/	出, 出來 /出来
come up 动词		上來 /上来
end up 动词		到达
go ahead 动词	/ˌɡoʊ əˈhɛd/	請便 /请便

需要注意的发音

说话人用了 27 次缩略和弱读形式，例如 we're, we'll, don't。请按听到的简短形式来说。

“th” 音: seventh /ˈsɛvənθ/, eleventh /ɪˈlɛv.ənθ/, thirteenth /ˌθɝˈtiːnθ/
“sh” 和 “zh” 音: isolation /ˌaɪsəˈleɪʃən/, cancellation /kænsəˈleɪʃən/
长单词——注意重音位置: fundamental /ˌfʌn.dəˈmɛn.təl/, isolation /ˌaɪsəˈleɪʃən/, cancellation /kænsəˈleɪʃən/, complicated /ˈkɑm.plɪˌkeɪ.tɪd/

中文母语者容易读错的音：

/v/ — 不要读成 /w/，上齿轻触下唇: seventh /ˈsɛvənθ/, variable [ˈvæɹ.i.ə.bl̩], vector /ˈvɛktɚ/, eleventh /ɪˈlɛv.ənθ/
词尾辅音 — 要读清楚，后面不要加元音: pulse /pʌls/, seventh /ˈsɛvənθ/, minus /ˈmaɪ.nəs/, sequence /ˈsiː.kwəns/, rotate /ˈɹoʊ.teɪt/
/r/ 和 /l/ 的区别: linear /ˈlɪn.i.əɹ/, variable [ˈvæɹ.i.ə.bl̩]

如何用这段视频练习

先完整听一遍视频，不要开口，记下不认识的单词。
用正常速度逐句跟读，每句重复到你的节奏与说话人一致为止。
录下自己的声音并与原声对比，特别注意 delta, pulse, seventh 这类单词。

什么是跟读法？

跟读法 (Shadowing) 是一种有科学依据的语言学习技巧，最初开发用于专业口译员的培训，并由多语言者Alexander Arguelles博士普及。这个方法简单而强大：您在听英语母语原声的同时立即大声重复——就像是一个延迟1-2秒紧跟说话者的影子。与被动听力或语法练习不同，跟读法强迫您的大脑和口腔肌肉同时处理并模仿真实的讲话模式。研究表明它能显着提高发音准确性，语调，节奏，连读，听力理解和口语流利度——使其成为雅思口语备考和真实英语交流最有效的方法之一。

影子跟读法: 阅读完整分步指南 →

跟读练习: 26 - How does harmonic phase shifting work including a detailed mathematical explanation to reduce s - 通过视频学习英语口语

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