Applications like pumps, conveyers, centrifuges and bandsaws must be started slowly, and sometimes stopped slowly, to prevent mechanical shocks such as water hammer, and strains on bands, couplings and shafts.
Introduction to Soft Starter
An induction motor connected directly to the power supply draws a constant magnetizing current at full speed, no matter what the load is. At anything less than full load, power factor of the induction motor is less than its rated power factor because the torque producing current decreases as the load decreases, but the magnetizing current does not change. The lighter the load, worse the power factor. Motors selected from a standard range are almost always chosen with a rated power in excess of maximum load demand, with the result that in any installation the motors seldom operate at their full rated load. Consequently, they can never achieve their rated power factor even at a maximum load demand. Moreover, if the load is variable.
