Slip frequency is a fundamental concept in the operation of an induction motor. To understand slip frequency, it's important to have a basic understanding of how an induction motor works.
An induction motor operates on the principle of electromagnetic induction, where a rotating magnetic field produced by the stator windings induces a voltage in the rotor windings. This induced voltage in the rotor windings then causes a current to flow, generating a magnetic field in the rotor. The interaction between the rotating magnetic field of the stator and the magnetic field of the rotor causes the rotor to turn and drive the mechanical load connected to the motor.
However, there's an inherent difference between the speed of the rotating magnetic field produced by the stator and the actual speed at which the rotor turns. This difference in speed is known as "slip." Slip is expressed as a percentage and represents the relative speed difference between the rotating magnetic field and the rotor's actual speed. Mathematically, slip (S) is calculated as:
Slip (S) = (N_s - N_r) / N_s
Where:
N_s is the synchronous speed of the rotating magnetic field (determined by the frequency of the applied voltage and the number of poles in the motor).
N_r is the actual speed of the rotor.
When the motor is running under normal load conditions, the rotor speed is always slightly less than the synchronous speed, resulting in a positive slip. This slip is necessary for the motor to generate torque and overcome the mechanical load it's driving. If the rotor were to turn at exactly the synchronous speed, there would be no relative motion between the stator and rotor magnetic fields, and no electromagnetic induction would occur, meaning no torque would be generated.
Now, the slip frequency is the frequency of the current in the rotor circuit due to the slip-induced voltage. It's calculated as:
Slip Frequency = Slip × Supply Frequency
The slip frequency is important because it determines the frequency of the rotor current and consequently affects the electromagnetic interaction between the stator and rotor, which leads to the generation of torque. As the slip increases, the slip frequency also increases, resulting in a higher frequency rotor current and a greater electromagnetic interaction between the stator and rotor. This increased interaction produces more torque, which is necessary to meet the increasing load demand.
In summary, slip frequency is a critical parameter in the operation of an induction motor. It represents the frequency of the rotor current induced by the speed difference between the rotating magnetic field and the rotor. This frequency governs the electromagnetic interaction that generates torque and enables the motor to drive mechanical loads efficiently.