How much distance should I leave between the motor cable and a signal or RF cable?
Leave at least 50 mm of separation between motor and supply cables and analogue signal cables; where they must cross, cross them at 90 degrees.
The practical rule is to leave at least 50 mm of separation between motor and supply cables and analogue or low-level signal cables. If they have to cross, cross them at 90 degrees and do not share a long parallel route. As the separation distance grows, magnetic coupling falls away quickly.
Interference arrives by two paths
The first is capacitive coupling between conductors, fed by fast voltage changes. The second is magnetic coupling, fed by fast current changes. Both are high at the output of a switching DC motor driver. What is critical is not the distance so much as the length run in parallel; two cables running side by side for 30 cm cause far less trouble than ones running side by side for 3 m.
Layout in the panel and in the vehicle
- Use separate ducts or spiral wrap for power and signal, and do not let them share the same terminal row. The reason for running them separately is not tidiness but reducing the coupling area.
- Use a 90° angle at unavoidable crossings; with this geometry the coupling area is at its minimum.
- Place the RF receiver antenna as far as possible from the motor cable and from contactor and relay coils, on top of a metal mass. Antenna placement affects the range more than the choice of cable does.
- Run the analogue command line (for example the PT500's E analogue input or the KS250's IN5 analogue input) with twisted and, where needed, shielded cable.
- Separate the signal return from the power return and join them at a single point.
| Cable | Its role | Measure |
|---|---|---|
| Motor output and supply | Noise source | Twist it, run it in its own duct and keep it short |
| Analogue reference (pot, joystick) | The most sensitive line | Twisted and shielded, shield grounded at one end |
| Digital input (button, relay trigger) | Medium sensitivity | A separate duct is enough in most installations |
| RF antenna cable | Handled separately | Away from the motor and the coils, on a metal mass |
If the separation cannot be achieved
In a cramped panel 50 mm is not always available. In that case apply the following in order: twist the signal pair, use shielded cable and ground the shield at one end only. Grounding at both ends creates an ground loop and usually makes the problem worse. A metal divider or a duct lid also acts as a physical barrier between power and signal. The order matters: put the route right first, then shield; working the other way round usually complicates the shielding for nothing.
Where 50 mm is not enough
- There is an inverter nearby. The output of a frequency converter is a far harsher noise source than the motor cable; an inverter reducing the remote control range is not solved by separation distance.
- A welding machine is running. The arc current returns through the chassis; what decides is not separation but the chassis path and isolating the supply.
- The cable is long and entirely parallel. On a route running side by side for metres, 50 mm is not enough; shielding or a change of route is needed.
- The signal level is very low. On analogue references of the order of millivolts, shielded and twisted cable has to be taken as mandatory from the start.
What to check during commissioning
Even a few hundred millivolts of noise on the analogue reference can cause jitter in the speed setting or false triggering of the current-threshold stop function. When you commission the system, start and stop the motor and watch that the analogue reference does not drift and the remote control does not cut out. If you see a problem, put the route right first; solving the separation at the wiring stage is more effective and cheaper than adding filters afterwards.
The full topic: DC Motor Driver Selection Guide