Three phase motors can be started Direct on Line (DOL), using a Star Delta Starter (SDS) contactor set or using a Motor Starting Auto Transformer.
DOL is the most basic form of starting an electric motor. This with connect the motor directly with the power suppply via a contactor.
SDS requires the motor to have with a "Star" winding and a "Delta" winding and will normally have six wires from the motor. The contact set starts the motor in "Star" connection and after a set time (seconds) it switches to the "Delta" winding. These units come in open style or enclosed with push buttons.
Enclosed type Open type
A Motor Starting Auto Transformer starts the motor on a lower voltage. This requires an appropriate control system to start the motor on 50%, 65% or 80% reduced voltage for a short time. After a set time the contactor set switches to direct on line.
These transformers can be fitted with thermisters (sensor resistance varies with temperature) or thermic sensors (switch opens at set temperature). The type required will depend on the control system you have.
Motor Starting Auto Transformer