Name: 感應電機是如何工作的？ (How does an Induction Motor work ?)
Uploaded: 2021-01-14T08:24:59.000Z
Duration: 6 min 46 s
Description: 【看影片學英語】數萬部 YouTube 影片，搭配英漢字典即點即查，輕鬆掌握單字發音與用法，長久累積看電影不必再看字幕。

The invention of Induction Motors permanently altered the course of human civilization

This hundred-Year-old motor invented by the great scientist Nikola Tesla is the most common motor type even today

In fact about 50% of global electric power consumption is due to induction motors

Let's get into the workings of induction motors or more specifically into Nikola Tesla's genius thinking

The induction Motor has two main parts the stator and rotor

The stator is basically a three coil winding and three-phase AC power input is given to it

The winding passes through the slots of the stator

which are made by stacking thin highly permeable steel laminations

When a three-phase current passes through this winding something very interesting happens

This RMF is what causes the rotor to turn

To understand how the rotating magnetic field is generated

let's consider a simplified stator winding

Here the three coils are connected 120º apart

A wire carrying current produces a magnetic field around it

When a three-phase power is applied to this special arrangement

the magnetic field produced will be as shown at a particular instant

With variations in AC current the magnetic field takes different orientations

If you compare these three instances you can see that it is like a magnetic field of uniform strength rotating

The rotational speed of the magnetic field is known as the synchronous speed

Assume that you are putting a closed conductor inside it

According to Faraday's law because the Magnetic field is varying

The EMF will produce a current in the loop

thus, the situation has become like a current carrying loop situated in the magnetic field

according to the Lorentz Force law an electromagnetic force will be produced on the loop

The same phenomenon occurs inside an induction motor as well

Here instead of a simple loop something very similar to a squirrel cage is used

The three-phase AC current passing through the stator produces a rotating magnetic field

So as in the previous case current will be induced in the bars of the squirrel cage

Which is shorted by end rings, so the rotor will start to rotate

That's why the motor is called an induction motor

Electricity is induced on the rotor with help of electromagnetic induction rather than direct connection

insulated iron core lamina are packed inside the rotor

Such small sizes of iron make sure the Eddy current losses are minimum

You can see that the induction motor has a big advantage. It is inherently self-starting

As you can see both the magnetic field and rotor are rotating

To obtain the answer to this question let's consider different cases

Consider a case where the rotor speed is the same as that of the magnetic field

Due to the fact that both are rotating at the same speed

The magnetic field will never cut the loop

Thus there will not be any induced EMF and current

This translates to zero force on the rotor bar and the rotor will gradually slow down

As it slows down the magnetic field will cut the rotor loop

So the induced current and force will rise again

In short the rotor will never be able to catch up to the speed of the magnetic field

It rotates at a specific speed which is slightly less than the synchronous speed

The difference between the synchronous and rotor speeds is known as slip

Now let's understand why induction motors rule both the industrial and domestic worlds

You can note that induction motors do not require a permanent magnet

They do not even have brushes commutator rings or position sensor

like other electrical machine counterparts

The most important advantage is that induction motor speed

can be controlled easily by controlling the input power frequency

To understand it properly let's once again consider the simple coil arrangement

We learned that a rotating magnetic field is produced due to the three-phase input power

It is quite clear that the speed of the RMF is proportional to the frequency of the input power

Because the rotor always tries to catch up with the RMF

the rotor speed is also proportional to frequency of the AC power

Thus by using a variable frequency drive one can control the speed of the induction motor very easily

This property of the induction motor makes them an attractive choice

for elevators, cranes even in electric cars

Due to the high-speed band of induction motors electric cars are capable to run with a single speed transmission

another interesting property of the induction motor  is that

when the rotor is moved by a prime mover it can also act like a generator

In this case you have to make sure that the RMF speed is always less than the rotor speed

We believe that you have now developed a clear understanding

of the ingenious operation principles behind an induction motor

as well as why it is still ruling the domestic and industrial worlds

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