Working principle of Squirrel cage Induction motor

  

   An induction motor is the most common type of motor used, in most domestic and industrial applications nowadays. As the name suggests “INDUCTION” this motor works on the principle of Faraday's law of Electromagnetic Induction.

    Like in the case of DC motor and DC generator this type of motor doesn’t incorporate much of the parts in it. Here the stationary and the rotating part are not mentioned as field and armature whereas it is mentioned as stator and rotor, because it is an AC motor, which works on the AC supply. There are generally two types of Induction motor, namely Squirrel cage induction motor and Slip ring Induction motor. In this topic let us discuss the construction and operation of the squirrel cage induction motor. To know the construction and operation of the Slip ring Induction motor, please visit the below article.

https://sciencetopic03.blogspot.com/2021/12/how-does-slip-ring-induction-works.html

SQUIRREL CAGE INDUCTION MOTOR

    The squirrel cage induction motor is the most simple and robust construction motor. It is the motor that is widely used in most domestic and industrial applications. The motor is named as a squirrel cage induction motor because the rotor of this motor more likely looks like a squirrel cage. The stator is made up of several silicon laminations fitted together. The use of several steel plates is to reduce the eddy current loss. The rotor is a complete solid metal, made up of several bars in a skewed manner. The main reason for placing the bars in a skewed manner is to avoid magnetic locking between the stator and rotor; thereby the number of slots in the rotor would never be equal to the number of slots in the rotor. There is no physical contact between the stator and rotor. The rotor is placed in between the stator with a slight gap called the air gap.

Credit (Stator and rotor of Squirrel Cage Induction Motor): By Zureks - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4170132

      When coming into the operation, the induction motor works on the most interesting concept called the Rotating Magnetic Field; shortly called as RMF. The RMF is always a varying magnetic field as it is rotating. As in the case of DC motor two different fluxes are produced, as both the field and the armature have windings. So the armature carries current in the magnetic field produced by the stator. Thus according to Lorentz force law whenever a current-carrying conductor is placed in a magnetic field it experiences a force. So the armature starts to rotate. But in the squirrel cage induction motor, the rotor doesn’t have any windings at all. It is a complete solid metal. So in order to rotate the rotor, a varying magnetic field or rotating magnetic field is required. If an RMF is produced by the stator, it begins to cut the conductor in the rotor. So according to Faraday's law of Electromagnetic Induction, whenever a conductor is placed in a varying magnetic field an EMF gets induced on the conductor. This EMF causes a current to flow if it is a closed conductor loop. As the rotor conductors in the squirrel cage induction motor are short-circuited by end rings at both sides, it creates a closed-loop and causes a current to flow. Again, according to Lorentz force whenever a current-carrying conductor is placed in a magnetic field, a force is experienced by the conductor. So the rotor begins to rotate due to the rotating magnetic field. As in the case of a three-phase induction motor, a three-phase supply is given to the motor and the production of a rotating magnetic field is not a problem, because in a three-phase supply each phase begins its cycle at a different time instance. But in the case of a single-phase induction motor, as only one phase is given, there is a problem in the production of RMF. This problem is described by the concept of double-revolving field theory.

DOUBLE REVOLVING FIELD THEORY

   Consider a single-phase motor that has only one winding (main winding) in the stator, which is given a single-phase supply. When the supply is given to the coil a fluctuating magnetic field is produced. Whenever a current is passed through the coil it produces a magnetic field around it. A fluctuating magnetic field is one; that when a single-phase ac supply is passed through a coil the magnetic field produced by the coil varies as the supply is varied. As the current enters at one point and leaves at another end alternatively, two different fluctuating magnetic fields are produced. This process is depicted below:

Alternating Current passing through a single coil

    These two different fluctuating magnetic fields can be resolved into two oppositely revolving magnetic fields, which is why it is named as double-revolving field theory. As the two fields are also revolving in the stator coils, the two different fields will begin to cut the rotor bars. So, according to Faraday's law of Electromagnetic Induction and Lorentz force, the rotor starts to rotate. But as there are two opposite revolving fields, one field establishes a torque on the rotor at one side and the other establishes the torque on the other side.  So the rotor never rotates and begins to jerk. So to make the rotor rotate, an initial rotation is given to the rotor at any one side. Due to this, one side of the torque will be greater than the other side, and the rotor will try to rotate on that side. This is why single-phase induction motors are not self-starting unless an external torque is given.

Rotor rotating in a clockwise direction

Rotor rotating in an anticlockwise direction

    

    To make the induction motor self-starting, thereby avoiding the initial rotation given to the motor, a capacitor connected with an auxiliary winding (starting winding) is introduced in the stator of the induction motor. The auxiliary winding is also a stator winding which is present perpendicular to the main winding. By using this setup, the double-revolving field is avoided and a rotating magnetic field is produced. The basic operation of a capacitor is that when the supply is given to the capacitor it gets charged and after the maximum charge the capacitor discharges the supply. Due to this property, the supply that is coming out from the capacitor gets delayed and the voltage gets led to the current by 90 degrees. 

Voltage lag in the auxiliary winding

    So according to the diagram given above, the single-phase supply from the source is got split up into two. At one branch the supply passes through the main winding and creates its magnetic field lines. And at another branch, a 90° phase-shifted supply passes to the auxiliary winding due to the capacitor and creates its own magnetic field lines. The field lines produced by the auxiliary are also 90° phase-shifted to the field lines of the main winding. So by combining the fluxes of both the main winding and the auxiliary winding a rotating magnetic field is produced. As seen in the three-phase supply, here also each supply begins its cycle at a different time instance. So a rotating magnetic field is produced, and the motor starts to rotate. So from this, we can understand that the single-phase induction motor operates on two phases even if we give only one phase.

    After the motor has got its initial rotation, there is no need for auxiliary winding and capacitor, as the torque developed on one side will be greater than the other side. So both auxiliary winding and the capacitor can be disconnected by using a centrifugal switch. Such a type of motor where the capacitor is only used for starting purposes is called as capacitor start induction motor. 

Capacitor Start Induction Motor

    A capacitor run motor uses the capacitor for both starting and running purposes. As the capacitor remains connected in this type of motor, the efficiency and the power factor of the motor increase. There is also another special type of motor which includes both capacitor start and capacitor run type. As the name suggests it includes both the starting and running capacitors. During starting, the starting capacitor is used and disconnected, and while running, the running capacitor is used and stays connected in the circuit to increase the efficiency.

Capacitor Run Induction Motor

Capacitor Start Capacitor Run Induction motor

    There is also a special kind of induction motor called the Shaded pole induction motor. This motor does not use any extra windings; rather it makes the alteration in the pole itself. The working of this motor also lies in the principle of Faraday’s law of Electromagnetic Induction.

Construction of shaded pole Induction Motor

    Consider the operation of the transformer which also works in the principle of Electromagnetic induction. When a varying supply is given to the primary winding an EMF and current get induced in the secondary winding. Similarly in a shaded pole motor, whenever a varying supply is given to the stator winding it produces a primary flux over the unshaded part which in turn produces a secondary flux over the shaded part (copper band). As a varying supply is given, the flux produced in the secondary region varies, causing the output magnetic field to alter. Now let’s consider different time instance of a single cycle of supply and see how a rotating magnetic field is produced by this copper band with respect to the time instances.

Considering different time instances of an AC supply

Instance1 (OA)

    At this instance, the supply is increasing which tends to produce more flux in the secondary region (copper band). So according to Lenz law, the flux produced in the copper band will oppose the main field flux. So the flux will get accumulated over the non-shaded part.

Flux gets accumulated over the Non-shaded part

Instance 2 (AB)

    In this instance, the supply will get constant, such that there will be no change in the magnetic field and the field remains constant. So there will be no flux produced in the copper band and the flux remains uniform all over the pole.

Flux gets distributed evenly over the pole

Instance 3 (BC)

    At this instance, the supply will get decreasing, and due to the electromagnetic induction, the decrease in flux causes a secondary flux in the copper band. But this time, the decrease in flux causes the induced current in the copper band to change which aids the main field instead of opposing. So the magnetic field lines get crowded over the shaded part.

Flux gets accumulated over the shaded part

    This change in the magnetic axis at every instant causes a rotating magnetic field and causes the rotor to rotate. All these types of starting a motor can be used for both squirrel cage and slip ring induction motor.