EMAC 3

• In an electric motor, the conversion of electrical power to mechanical power occurs in the rotating part of the machine
• In a DC motor and one type of AC motor, electrical power is conducted directly to the rotor through brushes and commutator, making it a conduction motor
• In the most common type of AC motor, electrical power is not conducted directly to the rotor; instead, the rotor receives power inductively, similar to how the secondary of a transformer receives power, hence known as induction motors
• An induction motor can be thought of as a rotating transformer, where a stationary winding is connected to the AC source, and the other winding, mounted on a structure free to turn, receives power by transformer action while it rotates
• The concept of the induction motor was discovered by François Arago, a French physicist, who observed the phenomenon of electromagnetic induction
• Electromagnetic induction is the phenomenon where an electromotive force induces across an electrical conductor when it is placed in a rotating magnetic field
• The stator and rotor are two essential parts of the motor:

  • The stator is the stationary part, carrying the overlapping windings
  • The rotor carries the main or field winding
  • The windings of the stator are equally displaced from each other by an angle of 120°
• The induction motor is a single excited motor where the supply is applied only to the stator
• Excitation in a motor refers to the process of inducing the magnetic field on its parts
• When a three-phase supply is given to the stator, a rotating magnetic field is produced on it
• The three-phase AC induction motor operates on a three-phase supply and is designed to produce a rotating magnetic field
• Three-phase AC induction motors are of two types: squirrel and slip-ring type induction motors
• The construction of three-phase induction motors involves a stator and a rotor with no electrical connection between them
• Stator and rotors are constructed using high-magnetic core materials to reduce hysteresis and eddy current losses
• Three-phase induction motors are widely used in industrial applications to convert electrical energy into mechanical energy
• Three-phase induction motors are the most important type of motor among all others used in industries
• An induction motor consists of two main parts:

  • Stator
  • Rotor
• Stator:

  • The stator is the stationary part of the motor
  • It consists of three main parts:
  • Stator Frame
  • Stator Core
  • Stator Winding
• Stator Frame:

  • The outer part of the motor providing support to the stator core and stator winding
  • It has fins on the outer surface for heat dissipation and cooling
  • Made of die-cast or fabricated steel, aluminum/aluminum alloys, or stainless steel
• Stator Core:

  • Carries the alternating magnetic flux which produces hysteresis and eddy current loss
  • Laminated by high-grade steel stampings insulated from each other by varnish
  • Fixed with the stator frame
• Stator Winding:

  • Placed inside the stator slots within the stator core
  • Consists of a three-phase winding
  • The number of poles of a motor determines the speed of the motor
• The synchronous speed (NS) of the motor is given by the equation: NS = 120f / P

  • f = Supply Frequency
  • P = Total Number of Poles
  • Ns = Synchronous Speed
• The stator winding of motors can be connected in star or delta, done by the connection of terminals in the terminal box
• The stator consists of a cylindrical laminated slotted core placed in the frame
• In the manufacturing process, the armature winding is first wound in slots and properly connected, then dipped into an insulating varnish and baked before being pressed into its frame
• In this construction, the stator laminations are welded together at several places around the outer cylindrical surface
• The number of slots in the stator is 36, varying in every size of motors and its number of poles
• The type of winding used in the stator of polyphase induction motor is the same as that of alternators, with as many coils in the entire windings as there are slots
• In this winding type, the number of coils per phase equals slots per phase, and the number of coils in each pole group per phase equals slots per poles x phases
• Coils in a group must always be connected in series to prevent excessive heating and probable burn out of the winding
• The winding inside the stator is coated with heavy insulation varnish before assembly
• There are three coils per pole per phase in this particular stator because 36 / (4 x 3) = 3