Adding Speed Control for a DC Motor
Option 1: do away with the relay completely:







simulate this circuit &ndash; Schematic created using CircuitLab

The relay isn't really required unless you want to be able to change direction using some electrical signal from a microcontroller for example. Since you're using a speed controller with a manual knob, I assume direction will be manual also.



Option 2, now using a 3PDT relay. Imagine SW3 being part of the relay:







simulate this circuit

Basically SW2 and SW3 are in parallel. SW3 'latches' the relay until it's released by breaking SW1. This way the relay is powered directly of the power source while the motor source comes from the speed controller.