Induction motors are the simplest and most rugged electric motor and consists of two basic electrical assemblies: the wound stator and the rotor assembly. The induction motor derives its name from currents flowing in the secondary member (rotor) that are induced by alternating currents flowing in the primary member (stator). The combined electromagnetic effects of the stator and rotor currents produce the force to create rotation.
Rotors typically consist of a laminated, cylindrical iron core with slots for receiving the conductors. The most common type of rotor has cast-aluminum conductors and short-circuiting end rings. This "squirrel cage" rotates when the moving magnetic field induces a current in the shorted conductors. The speed at which the magnetic field rotates is the synchronous speed of the motor and is determined by the number of poles in the stator and the frequency of the power supply: ns = 120f/p, where ns = synchronous speed, f = frequency, and p = the number of poles.
Synchronous speed is the absolute upper limit of motor speed. If the rotor turns exactly as fast as the rotating magnetic field, then no lines of force are cut by the rotor conductors, and torque is zero. When running, the rotor always rotates slower than the magnetic field. The rotor speed is just slow enough to cause the proper amount of rotor current to flow, so that the resulting torque is sufficient to overcome windage and friction losses, and drive the load. The speed difference between the rotor and magnetic field, called slip, is normally referred to as a percentage of synchronous speed: s = 100 (ns - na)/ns, where s = slip, ns = synchronous speed, and na = actual speed
Induction motors are the simplest and most rugged electric motor and consists of two basic electrical assemblies: the wound stator and the rotor assembly. The induction motor derives its name from currents flowing in the secondary member (rotor) that are induced by alternating currents flowing in the primary member (stator). The combined electromagnetic effects of the stator and rotor currents produce the force to create rotation. Rotors typically consist of a laminated, cylindrical iron core with slots for receiving the conductors. The most common type of rotor has cast-aluminum conductors and short-circuiting end rings. This "squirrel cage" rotates when the moving magnetic field induces a current in the shorted conductors. The speed at which the magnetic field rotates is the synchronous speed of the motor and is determined by the number of poles in the stator and the frequency of the power supply: ns = 120f/p, where ns = synchronous speed, f = frequency, and p = the number of poles. Synchronous speed is the absolute upper limit of motor speed. If the rotor turns exactly as fast as the rotating magnetic field, then no lines of force are cut by the rotor conductors, and torque is zero. When running, the rotor always rotates slower than the magnetic field. The rotor speed is just slow enough to cause the proper amount of rotor current to flow, so that the resulting torque is sufficient to overcome windage and friction losses, and drive the load. The speed difference between the rotor and magnetic field, called slip, is normally referred to as a percentage of synchronous speed: s = 100 (ns - na)/ns, where s = slip, ns = synchronous speed, and na = actual speed
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