Why should the motor cable be run twisted?
Because current flows in opposite directions in the outgoing and returning conductors, the magnetic fields they produce cancel each other; twisting the two wires repeats this cancellation at every twist pitch and greatly reduces radiated noise. For the same reason a twisted pair is also more resistant to magnetic interference coming from outside.
In a brushed DC motor the brush arc produces broadband noise, while the switching driver creates fast current changes. Two cables run straight and parallel behave like an efficient antenna, because the field between them stays one-directional.
What twisting solves:
- Radiated noise: At every twist pitch the field direction reverses and cancels against the neighbouring pitch.
- Received interference: An external magnetic field induces voltages of opposite sign in successive pitches, and the sum converges to zero.
Application recommendations:
- Twist the two motor leads as a single bundle with a regular pitch; twist the supply pair (+/−) the same way.
- Keep motor cables at least 50 mm away from analogue and RF signal cables; where that is unavoidable, cross them at a 90° angle.
- If you use shielded cable, ground the shield at one end only, preferably on the driver side; grounding at both ends creates a ground loop and 50/60 Hz interference.
- You can suppress the brush arc with a capacitor or a ferrite bead fitted at the motor terminals.
In installations with an RF receiver (the ST42 and PT500 can be used with a remote control) these details directly affect the range; an untwisted long motor cable raises the receiver's noise floor if it runs near the antenna.
Full topic: DC Motor Driver Selection Guide