DPDT stands for double pole double throw switch. By using this we can change the polarity of the applied voltage. It is used in situations where we need different outputs. It has 3 states ON, OFF, ON. The difference between the two ON is that the polarity of both the states is opposite with respect to each other.That’s the reason why it can drive the dc motor in both directions. It can also switch an ac motor but it can’t change the direction of the motor. Since, ac motor will run in same direction irrespective of the polarity therefore, it can only be used for switching ON and OFF for an ac motor but not for changing its direction.