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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

B2 中上級

# 誘導電動機の仕組みは？ (How does an Induction Motor work ?)

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home3260 に公開 2021 年 01 月 14 日