DC Motor Bi-directional Speed Control

Why not just get a DC motor controller rather than playing around with switches and simple control elements that limit your ability to control and protect the motor and whatever it is connected to (and are unsafe at this voltage level).

For example Dart Controls . They aren't expensive and this is the most common method of motor control unless you want really simple on/off/reverse control, in which case you use a changeover switch with a centre off position and wait for the motor to stop before reversing direction (although I would NOT recommend this method for a 300V DC motor).

_{Jack - Used to sell and service Dart controllers.}

Using switches for starting across the line will cause in rush of about 50 amps - 10 x the max allowed.