- 3 Phase Induction Motor Definition: A 3 phase induction motor is a self-starting motor that converts three-phase AC electrical energy into mechanical energy without extra starting mechanisms.
- Main Components: The motor consists of a stator that generates a rotating magnetic field and a rotor that turns within this field.
- Magnetic Field Creation: The stator’s three-phase winding arrangement produces a rotating magnetic field essential for the motor’s operation.
- Induction Operation: As the rotor conducts through the stator’s magnetic field, it induces an emf that generates current and causes the rotor to turn, following the principle of electromagnetic induction.
- Slip Importance: The difference in speed between the stator’s magnetic field and the rotor (slip) is crucial, as it allows for torque generation and prevents the rotor from reaching synchronous speed.
An electrical motor converts electrical energy into mechanical energy. A balanced three-phase Alternating Current supply creates a rotating stator field, so a 3 phase induction motor develops starting torque without an auxiliary phase-shifting winding. A real installation still needs suitable switching, overload protection and, where required, a method that limits starting current.
The operating principle follows from the construction of a 3 phase induction motor. Its two main assemblies are:
- A stator
- A rotor
Stator of 3 Phase Induction Motor
The stator has a laminated magnetic core with slots that hold a distributed three-phase winding. When a balanced three-phase supply energises windings displaced in space, their combined magnetomotive force produces a rotating magnetic field across the air gap.
Rotor of 3 Phase Induction Motor
The common squirrel-cage rotor has a cylindrical laminated core with slots containing aluminium or copper conductors. End rings connect the bars to form a closed rotor circuit. Manufacturers often skew the slots to reduce cogging, torque ripple and magnetic noise. A wound-rotor induction motor instead has insulated rotor windings connected to slip rings.
Working of Three Phase Induction Motor
Production of Rotating Magnetic Field
The three stator phase windings are displaced by 120 electrical degrees, shown here as 120o. Balanced currents that are also separated by 120 electrical degrees combine to form a nearly constant-magnitude rotating magnetic field. Its synchronous speed is 120f/P revolutions per minute, where f is supply frequency in hertz and P is the number of poles.
How Rotation Begins:
Under Faraday’s law, changing flux linkage induces an electromotive force. At standstill, the rotating stator field has relative motion with the rotor, so the rotor conductors experience changing magnetic flux
. In a squirrel-cage induction motor, end rings short the bars. In a wound-rotor motor, slip rings can connect the rotor winding to external resistance during starting. The induced emf drives rotor current.
The rotor current creates its own magnetic field. Interaction between the stator and rotor fields produces electromagnetic torque. In line with Lenz’s law, the resulting torque opposes the relative motion that produced the rotor current, so the rotor accelerates in the direction of the stator field.
In steady motoring operation, the rotor runs below synchronous speed. If the rotor reached the same speed as the rotating field, the relative speed and rotor frequency would fall to zero. The rotor would then have no induced emf or current to sustain electromagnetic torque. Under load, the motor therefore settles at a speed that leaves enough slip to produce the required torque.
Slip is the difference between synchronous speed and rotor speed, usually expressed as a fraction or percentage of synchronous speed: s = (Ns – Nr)/Ns. A squirrel-cage induction motor needs no external electrical connection to its rotor because its closed conductors carry current induced by the stator magnetic field. Wound-rotor designs are the exception because they use slip rings for access to the rotor circuit.
For a standard squirrel-cage three phase induction motor, the usual characteristics are:
- Self-starting from a balanced three-phase supply, subject to adequate starting torque and suitable control equipment.
- No commutator or brushes, so there is no brush sparking; armature reaction in synchronous machines is a separate topic.
- Mechanically robust because the cage rotor has no commutator or wound electrical connection.
- Often cost-effective for constant-speed industrial drives.
- Usually low-maintenance, although bearings, cooling, insulation and connected equipment still need inspection.
Very very very nice………thnks for this.
No worries Swati! Happy to hear it helped.
No worries at all Paul – glad you found it to be useful.