When have seen that rotor
rotates in the same direction as that of R.M.F. but in steady state
attains a speed less than the synchronous speed. The difference between
the two speeds i.e. synchronous speed of R.M.F. ( Ns ) and rotor speed
(N) is called slip speed. This slip speed is generally expressed as the
percentage of the synchronous speed.
So slip of the induction motor is defined as the difference between the
synchronous speed ( Ns) and actual speed of rotor i.e. motor (N)
expressed as a friction of the synchronous speed ( Ns ). This is also
called absolute slip or fractional slip and is denoted as ‘s’.
value of slip s possible for induction motor which occurs at start.
While s = 0 given us N = Ns which is not possible for an induction
motor. So slip of induction motor can not be zero under any
Practically motor operates in the slip range of 0.01 to 0.05 i.e. 1 % to
5 %. The slip corresponding to full load speed of th motor is called
full load slip.
A 4 pole, 3 phase induction motor is supplied from Hz supply. Determine
its synchronous speed. On full load, its speed is observed to be 1410
r.p.m. calculate its full load slip.