三相电和相位

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我们来谈谈关于相位在电力学的应用。我们通常以两种不同的方式来谈论相位。第一,当电压的相位和其他相位不同,例如三相电。第二,当电压的相位与电流不同。

如果你有两个不同的发电机,即使它们以相同的频率运作,例如60赫兹,如果你把它们放在一起,你必需确保它们的相位是一样。简单来说,电压需要一起上升和下降。如果它们的相位不同步,他们便会互相对抗。有时候,如果你的做法正确,你会希望你的电压相位变得不同步。在工业环境中,尤其是马达,你可以找到一种被称为“三相电”的交流电。这里有三根电压相差120度的电线,如你所见。第二个正弦波的顶部出现120度的情况比第一个正弦波更迟出现,并且第三个正弦波的顶部在此情况后也出现120度。第四条电线通常是用来接地,令它比普遍的只有一根电压变化的电线和接地线的“单相电”更有效率。

除了效能之外,三相电比单相电更好的原因,是因为功率是持续输出。使用单相电,可能有很好的平均功率,但它会不断地变化,可能是在一瞬间或者。一秒内功率的输出便会多次地变为零如果你的马达是用三相电,就可以用更小和更高效能的马达,并且由于功率持续输入,马达的扭矩不会震动。三相电令到马达不需要个别的启动器电路,并在起动时为它们提供更多的扭矩。从三相电取得单相电比你想像中更简单 - 不连接另外两个输入便可以做到。

另外一种相位你需要考虑到的就是如果电压和电流的相位是否一样。对于纯电阻负载,随着电压的增加,电流亦会在相同的时间增加。但是因为某些原因,稍后我们会在教学影片中解释。电感性或电容性负载可能会导致你的电流比电压靠前或留后。在住宅环境中较少见,如果有一个电感性负载或是电容性负载,例如是马达在你的搅拌机或吸尘器中,电流和电压并不同步。有趣的是,如果你记得,功率是电压乘电流。因此,每次电压或电流的值为0时,都不会有电力输出。你可以从视觉上看到当电压和电流越不同步,你实际获得的功率就越小。讽刺的是,即使没有将它全部使用,但仍然需要同样多的功来产生电。 当它像这样不同步时,它被称为无功功率,以伏安无功来衡量。电子工程师喜欢用虚数和相位角来形容这些,虽然它看起来很难理解,但这些只是用数学方法来描述相位的差异。其实并不是真的很难,只要你了解这个原则你就会明白正在发生的事。

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If you connect two sources, they must be synchronized in order to be an efficient power source.

If you have two different electrical generators, even if they operate at the same frequency, 60 hertz for example, if you tie them together, you need to make sure they’re in phase. In the simplest way, it just means that the voltages need to rise together and fall together. If they aren’t synchronized, they’re going to be fighting against each other.

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3-Phase Power Waveforms

Sometimes, if you do it right, you want your voltages to be out of sync. In industrial situations, particularly with motors, you can get what is called “three phase” power. This is where you have three wires with the voltages off from each other by 120 degrees. The peak of the second sine wave occurs 120 degrees later than the first and the top of the third sine wave occurs another 120 degrees after that. A fourth wire typically provides a reference to ground, making this more efficient than a typical single-phase or “monophase” power source, where you only have one wire with a varying voltage and a ground wire.

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Single-Phase Power Waveform

Beside efficiency, three phase power is better than monophase because there is constant power output. With only a single phase, you may have good average power, but it is constantly changing and you have moments, many times a second, where the power output is zero. If you have three phase power to motors, the motors can be smaller and more efficient and they don’t pulsate in their torque due to the constant power input. These three phases also make it so motors don’t require separate starter circuits and give them more torque when they’re starting. Finally, getting monophase power out of three phase power is extremely simple - you simply don’t connect the other two inputs.

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Current is leading when the load is capacitive.

The other type of phase you need to think about is if voltage and current are in phase. With a purely resistive load, as voltage increases, current increases at the same exact time. But for reasons that we’ll explain in a later video, an inductive or capacitive load can cause your current to *lead* or *lag* behind the voltage. So, if you have an inductive load like a motor in your blender or vacuum cleaner, or even a capacitive load, which is less common in residential circumstances, the current and voltage will not be synchronized.

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Current is lagging when the load is inductive.

If you remember, power is voltage times current so each time either voltage or current is 0, there is no power being output. You can see visually how the further out of sync voltage and current are, the less power you are actually getting. Ironically, or annoyingly, it still takes just as much work to generate that power, even if you don't get to use all of it. When it’s out of phase like this, it’s called reactive power and it’s measured in volt-amp reactive or VARs. Engineers like to use imaginary numbers and phase angles to describe this and while it may seem scary, those are just mathematical ways to describe that difference in phase. It’s not actually that bad, as long as you understand the principle of what’s going on.

摘要:

  • 将来自不同来源的电压的相位同步
  • 三相电的好处
  • 电压或电流的相位如何變得不同步
  • 当电压或电流的相位不同时,功率会流失

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