Questions: 12. Why can an induction motor never operate at synchronous speed?

Solution Steps

Synchronous speed is the speed at which the magnetic field in the stator of an induction motor rotates. It is determined by the supply frequency and the number of poles in the motor, given by the formula:

\[ N_s = \frac{120 \times f}{P} \]

where \(N_s\) is the synchronous speed in revolutions per minute (RPM), \(f\) is the frequency in hertz, and \(P\) is the number of poles.

Induction motors operate based on the principle of electromagnetic induction. For the rotor to experience a torque, there must be a relative motion between the rotating magnetic field and the rotor. This relative motion is quantified by "slip," which is the difference between the synchronous speed and the actual rotor speed. Slip is defined as:

\[ s = \frac{N_s - N_r}{N_s} \]

where \(N_r\) is the rotor speed.

If an induction motor were to reach synchronous speed (\(N_r = N_s\)), the slip (\(s\)) would become zero. At zero slip, there would be no relative motion between the stator's magnetic field and the rotor. Consequently, no induced current would be generated in the rotor, and thus no torque would be produced to maintain motion. Without torque, the rotor would slow down, increasing slip and allowing the motor to continue operating below synchronous speed.

Final Answer