Oil- cooled starters are stepped resistance starters with 3PR3 cast iron resistors in a tank filled with mineral based insulating oil. We acquired the 3PR3 series from Siemens in the year of 2000. The oil-cooled starters store the heat dissipated during the starting process and slowly release it to the atmosphere via the tank surface. Therefore, they are suitable for large drives in applications with low starting frequency. The change of resistance in the wound rotor induction motor (WRIM) rotor circuit is accomplished by successively switching out resistance steps with power contactors. The resistance is defined by the cast elements and the transformer oil acts as an energy storage and insulation medium. The design combines the advantages of a resistor starter with those of a more economical heat carrier (oil) and a high protection class.