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  • induction motors are the most commonly used electrical machines

  • they are cheaper rugged

  • and easier to maintain compared to other alternatives

  • in this video

  • we will learn the working of a three-phase squirrel cage induction

  • motor

  • it has two main parts

  • stator

  • and rotor

  • stator is a stationary part

  • and rotor is the rotating part

  • stator is made by stacking thin slotted

  • highly permeable steel lamination inside a steel or cast-iron frame

  • winding passes through slots of stator

  • when a three-phase AC current passes through it

  • something very interesting happens

  • it produces a rotating magnetic field

  • to understand this phenomenon much better

  • consider a simplified three-phase winding

  • with just three coils

  • A wire carrying current produces magnetic field around it

  • Now, for this special arrangement magnetic field produced by a three phase

  • A.C current will be as shown

  • at a particular instant

  • with variation in a c current

  • magnetic field takes a different orientation as shown

  • From these three positions it's clear that,

  • it's like a magnetic field of uniform strength rotating

  • The speed of rotation of a magnetic field is known as synchronous speed

  • Assume you're putting a closed conductor inside it

  • since the magnetic field is fluctuating, an E.M.F will be induced in the loop

  • according to the Faraday's law

  • The E.M.F will produce a current through the loop

  • So, the situation has become like a current carrying loop is situated in the

  • magnetic field

  • this will produce magnetic force in loop

  • according to the Lawrence law

  • so the loop will start to rotate

  • A similar phenomenon happens inside an induction motor also

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

  • Three-phase A.C current passing through stator winding produces a

  • rotating magnetic field

  • so

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

  • is shorted by the end rings. And will start to start rotate

  • That's why it's called an induction motor electricity is inducted in the

  • rotor by magnetic induction rather than direct electric connection

  • To aid such electromagnetic induction

  • insulated iron core lamina are packed inside the rotor

  • such small slices of iron make sure that eddy current losses are minimal

  • This is another big advantage of a three-phase induction motor

  • it is inherently self starting

  • You can see here that both magnetic field and the rotor are rotating

  • but at what speed the rotor rotate ?

  • to obtain this answer, let's consider different cases

  • consider a case where the rotor speed is the same as the magnetic field speed

  • since both the rotating at the same speed the rotating loop or always

  • experience constant magnetic field

  • so there won't be any induced E.M.F and current

  • this means zero force on the rotor bars

  • so the rotor will gradually slowdown

  • but as a slows down rotor loops will experience of varying magnetic field

  • so induced current and force will rise again

  • and the rotor will speed up

  • in short the rotor will never be able to catch up with the speed of the magnetic

  • field

  • it rotates at a specific speed which is slightly less than synchronous speed

  • The difference between synchronous and rotor speed is known as slip

  • rotational mechanical powers transferred through a powershaft

  • in short in an induction motor electrical energy is entered via stator

  • and output from motor, mechanical rotation is received from rotor

  • energy lost during motor operation is dissipated as heat

  • a fan at the other end helps in cooling down the motor

  • hope you've got a good introduction on the working of induction motors

induction motors are the most commonly used electrical machines

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B2 rotor magnetic magnetic field induction field motor

How does an Induction Motor work ?

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    home3260 posted on 2015/01/30
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