EMC Question of the Week: February 22, 2021

Electric Motor Drive and Motor

The three phase wires connecting a high-voltage power inverter to a remote AC motor should

  1. be unshielded
  2. be individually shielded
  3. have one shield covering all three wires
  4. both b and c.


The best answer is “c.” Shielding is required for safety and to protect nearby components and systems from the strong field coupling that would result from using unshielded cables. However, shields on individual wires block the fields associated with the differential switching voltages and currents on these transmission lines. This induces large switching currents on the three cable shields that circulate between the motor and inverter housings. These large currents generate unwanted heat, and the connection resistance on the shield terminations creates a voltage on the outside of the shields that is a major source of unwanted field coupling.

On the other hand, a shield over all three wires does not block the intentional fields associated with driving the motor. As a result it carries much less current, which reduces unwanted power dissipation and greatly reduces the dependence on ultra-low impedance shield terminations to eliminate unwanted coupling.

Of course, an even better alternative to shielded cables is locating the power inverter in the same metallic housing as the motor.  

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