How does an Induction Motor work ?

How does an Induction Motor work ?


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 Inside a steel or cast iron frame When a three-phase current passes through this winding something very interesting happens It produces a rotating magnetic field This RMF is what causes the rotor to turn To understand how the rotating magnetic field is generated as well as its properties 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 an EMF will be induced in the loop 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 and the loop will start to rotate 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 To aid such electromagnetic induction 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 But at what speed will the rotor rotate? 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 The rotor will then speed up 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 Induction motors are also self started 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 We hope you will support us at patreon.com So that we can continue our educational service Thank you!

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