Oil-cooled starters are starters with oil-cooled resistors. For switching the starter stages, switching rollers which are installed in the oil-filled resistor housing, cam switches or air contactors, both not installed in the oil tank, are used. Switching rollers have the advantage that the connections to the resistor are short and routed under oil. The disadvantage is the complex mechanics, which in the case of automatically running starters require a motor drive with a stepping mechanism, the switching time of which is also the same from stage to stage, so that an optimum ramp-up time is difficult to achieve. What has been said for the switching drum also applies to the cam controller. This is not installed under oil, so that complex wiring to the resistor is necessary. The advantage of the cam controller is the accessibility to the contacts, so that in case of malfunction or maintenance the oil does not have to be drained. Air contactors for switching the stages avoid the disadvantages of the switching roller and cam controller. They can be individually switched optimally by time stages or PLC (PLC), current or speed dependent and can be replaced individually in case of malfunction. It is not necessary to use the original make, but a contactor of the same design and performance fulfills its purpose exactly the same way. This simplifies the procurement of spare parts.

Oil-cooled starters must not be transported with oil filling, they are always supplied without oil filling and are only to be filled with insulating oil by the customer at the installation site. Just like oil-cooled resistors, they should always be mounted above or in an oil drip pan.

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