Why use a bypass contractor with a soft starter?

The Bypass contactor opens when the motor reaches full speed to reduce power (heat) losses in the solid-state power section. This translates to much smaller enclosures without need for special cooling.

• Also, the bypass contactor is internal to the soft starter, eliminating the need for additional devices, further reducing enclosure sizes and minimizing installation time.
• By-pass contactor significantly reduces heat generated by the starter, which minimizes enclosure sizes and costs.
• With ramp start (the most commonly used form of soft start), the motor is accelerated using an initial torque value of 10 to 85 per cent of locked rotor torque, corresponding to 10 to 92 per cent of the motor’s locked rotor current. Motor voltage then increases over the ramp time; at 90 per cent of rated speed the built-in bypass contractor closes which reduces voltage drop and cuts heat build-up in the starter.
• The soft start motor accelerator is an electronic, self-contained, panel-or enclosure-mounted motor starting device. It is intended to provide three-phase induction motors with a smooth start, both mechanically and electrically. Soft starters utilize thyristors in a full-wave power bridge configuration. By varying the thyristor conduction period, the
  starter controls the voltage applied to the motor. This, in turn, controls the torque developed by the motor. After the motor reaches its designed speed, contacts are closed to bypass the thyristors.