Soft Starter Introduction

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.

The wastage of energy is worse Electrical energy supply tariffs for industrial users almost invariably carry a heavy penalty –i.e.; higher cost per unit- for consumption at low power factors. Further, the cost charged is increased if demand exceeds a maximum limit.

The AC induction motor with a permanently coupled load, when it is operated without the benefit of a soft starter draws a high starting current. Typically, more than 7 times it’s rated full load current. “Reduced voltage-starting decreases the demand”. A Soft starter makes use of this principle.

The mechanical shock delivered to the rotor or to couplings or to any intermediate gearing and the driven load by a high starting current is most severe when the motor is started direction-line. Even reduced voltage starting devices like the star delta starter or Auto transformer starters still impose shock loads because of the very current peaks which cause severe transient torques at the moments of intermediate switching.

The effect of such repeated shocks is to decrease the life of the motor and increase the cost of maintenance.