How an Induction motor starts? — real answer from an Engineer.
Answer of a very famous and interesting question from Movie “3 Idiots”.
Hey How are you..? Hope you’re enjoying your studies at home in this lock-down. I’m here with an answer of a very famous and interesting question form movie “3 Idiots” — “How an Induction motor starts?” In the movie scene, VIRUS (Viru-shahastra buddhi) asks Raju rastogi “How an induction motor starts” and drunk Raju answers with a very funny sound.
Here, We’ll see the real answer of that question —
Note — Here I’m only Writing about 3 Phase Induction motor. Single Phase induction motor will be discussed in my upcoming post.
Rotating Magnetic Fields
To understand the starting and working of an induction motor, first we need to understand the “Rotating Magnetic Fields.”
In a 3 phase Induction motor, Stator of the motor has a 3 phase winding of Distributed type. Supply can be given in the winding of the stator. However, supply can also be given in the rotor if it is “SLIP RING INDUCTION MOTOR or WOUND ROTOR INDUCTION MOTOR”.
According to the fact, If a 3 phase supply is given to a 3 phase winding, a Rotating magnetic field is produced. In a 3 phase supply, only one phase at a time has maximum value of current and it changes with time. The sum of currents in all phases is zero at any time in a balanced 3 Phase supply.
In the above animations, You can see how a field is rotating. When current in the phases which colored Red is maximum, it causes maximum flux in the respective direction. After some time, another phase has the maximum value of current and the direction of flux lines changes. Note that direction of rotating magnetic field depends on the phase sequence of supply and the speed of rotating magnetic field depends on the frequency of supplied current.
Generation of Emf in Rotor Winding
All of us know that whenever there is any relative motion between Magnetic field and conductor, an EMF is induced in the conductor. In this case also, the Rotating Magnetic field produces an EMF in the rotor winding and since there is 3 phase winding in the rotor also, the EMF produced is of 3 phase.
Generation of Magnetic Poles in Rotor
“A Current carrying conductor always generates a magnetic field around it.” — This fact will now help us to understand the further process. We’ve discussed that the Rotating Magnetic field produces an EMF in the rotor winding. Since the path of rotor winding is always completed and closed, That induced EMF cause a current to flow in the winding. Due to that current, a magnetic field is generated around the winding.
Rotation of Rotor
Till now, we have discussed all the inner phenomenons of an 3 phase induction motor. Now, Let’s know how the rotor rotates.
We have seen that there is a magnetic field produced around the rotor winding due the current flowing in the rotor. It means there are two poles “N” and “S” is formed around the rotor winding (if it a 2 pole machine. number of such magnetic poles depends on number of poles of the machine. Note that the number of such magnetic poles formed is same for stator and rotor and is equal to the number of poles of machine.). So, now the “N” of rotor will want to concide with “S” of Stator which causes a torque on rotor. As there is a rotating magnetic field i.e, field produced by the supply in the stator is rotating, The poles of rotating magnetic field is rotating (moving) So, the rotor starts rotating about its axis.
Condition of Such Rotation of Rotor
There are some condition for such rotation of rotor. If they are not satisfied, a constant torque can’t be produced. They are -
- The flux produced due to the currents in stator and rotor winding must be of constant magnitude.
- There must be no relative motion between rotor field and stator field.
- There must be some angular displacement between stator field and rotor field.
Thanks for Reading.. Hope you find this Article useful.
