Motion Control - ElmoMC

Motor Cable Length

The Problem:Screened \ twisted \ bundled cables have significant parasitic capacitance between the conductors. The capacitance increases with the increase of the cable length. A typical value is 200 pF per meter or 60pF per feet.
Using long cables for the motor - amplifier connection might cause short circuit condition in the power stage for short durations of time. The magnitude of the short circuit depends on the capacitance value and the operating voltage.

Those short circuits cause:

  • Excessive heat dissipation in the amplifier.
  • In extreme cases, activating the short circuit protection.
  • Disturbances in the current loop operation.
  • Electro magnetic disturbances.
  • As a rule of thumb we assume that cable length under 10 meters (30 feet) do not cause any significant problem.
  • In those cases where a long cable is required, additional inductance must be connected between the cable and each of the amplifier outputs. An inductance value of 150-200 µH is typically sufficient. This can also be achieved by winding each of the cable conductor around a separate toroid.
  • Using separate conductor/cable for each amplifier's output. The conductors must not be twisted or bundled together.
  • Use a cable with minimum parasitic capacitance. This is not a practical solution as it is difficult to source such a cable and its cost is very high.