Common Mode Noise Impact in AC drive Applications

Radiated noise from an AC drive cable is a source of interference with adjacent systems that is often easier to identify and rectify than common mode noise. In the latter, high levels of noise across a broad frequency range, often from 60 Hz to 30 MHz, can capacitatively couple from the windings of the motor to the motor frame, and then to ground.

Common-mode noise can also capacitatively couple from unshielded motor leads in a conduit to ground via conduit ground straps, supports or other adjacent, unintentional grounding paths. This common-mode ground current is particularly troublesome because digital systems are susceptible to the high-frequency noise generated by AC drives.

Signals susceptible to common-mode noise include those from proximity sensors, and signals from thermocouples or encoders, as well as low-level communication signals in general. Because this type of noise takes the path of least resistance, it finds unpredictable grounding paths that become intermittent as humidity, temperature, and load change over time.
    
One way to control common-mode noise is to provide a known path to ground for noise captured at the motor's frame. A low-impedance path, such as a properly designed cable ground/shield system, can provide the noise with an easier way to get back to the AC drive than using the building ground grid, steel, equipment, etc.

In the study presented in this paper, tests were conducted on the five cable types to determine the ground path impedance of the shield and grounding system of each cable. The tests were conducted across a broad frequency spectrum. Results are outlined in the Figure. Lower impedance implies a more robust ground path, and therefore relatively lower noise coupled to the building ground. Lower building ground noise means a reduced need for troubleshooting of nearby adjacent systems and components.

A cable should never be the weak link in an AC drive system. It must be able to stand up to the operating conditions, and maintain the life of other components in the system. Selecting an appropriate AC drive cable can improve overall AC drive system longevity and reliability by mitigating the impact of reflected waves.

Special attention should be paid to the cable's insulation type, impedance, and shield/ground system.

Cables employing a heavy wall of thermoset insulation are recommended because of the proven electrical benefits and improved high temperature stability it offers. Shielding systems, including copper tape, combination foil/braid, and continuous armoring types, are the most appropriate for AC drive applications because of the low impedance path they provide for common-mode noise to return to the AC drive.

When AC drive cables are installed in close proximity to low-level communications cables and other susceptible devices, shielded instrumentation cable should be used. It would also be prudent to limit the run length of AC drive cable parallel to instrumentation cables to 10 ft. or less to reduce the likelihood of radiated noise issues.