AC MACHINES-1 (66761) Theory
Completion requirements
10. Realize the principle and construction of 3-phase induction motor.
10.9. Express the derivation of the equation r f = s f and (1 s) P 120f
What is "Slip Speed in an Induction Motor"? - its Importance
Slip Speed of the Induction Motor is defined as the difference between the synchronous speed and the actual rotor speed.
SPEED The speed of a rotating fields depends upon the frequency of the source and the number of poles on the stator.
Synchronous speed is given by: ns = 120f / p where: ns = synchronous speed (r/min) f = frequency of the source (Hz) p = number of poles This equation shows that the synchronous speed a) increase with frequency and b) decreases with the number of poles
Example: Calculate the synchronous speed of a 3 phase induction motor having 10 poles when it is connected to a 60 Hz source:
Solution: ns = 120f / p =120 X 60 / 10 = 720 r/min Slip and Slip Speed The slip s of induction motor is the difference between the synchronous speed and the rotor speed. Expressed in per unit: s = 𝑛𝑠− 𝑛 𝑛𝑠 where: s = slip ns = synchronous speed (r/min) n= rotor speed (r/min) The slip is practically zero at no load And equal to 1 when the rotor is locked.
Example: A 0.5 hp , 6-pole induction motor is excited by 3 phase, 60 Hz source. If the full load speed is 1140 r/min, calculate the slip.
Solution The synchronous speed is