The debate as to whether to use screened cabling, which incurs a higher cost, wages on. However, the risks of not using screened cables can generate significant electro-magnetic compatibility (EMC) issues.
When using variable-speed drives (VSDs), grounding schemes are critical to avoid effects such as high-frequency bearing currents - caused by induced motor shaft and frame voltages - or to reduce the effects of radio frequency interference (RFI).
To meet VSD EMC requirements, high-frequency earthing is used: a technique not needed for standard installations such as lighting circuits or direct-on-line motors. The reason is that VSDs emit high frequencies as a result of the fast switching of power components such as IGBTs and control electronics. This high-frequency emission can propagate by conduction and radiation.
To prevent radiated emissions, it is necessary that the entire drive system, including the converter, motor and motor cable, forms a Faraday cage. Using a shielded cable with 360-degree grounding ensures continuity of this cage. There are several ways to implement such high-frequency earthing and installers must read the installation manuals that accompany AC drive products.
Particular attention should be taken when motor junction boxes are used, as experience shows that poor earthing practice in these applications can result in significant challenges with RFI.
For example, screened or shielded cable that ensures noise is not radiated from the motor cable should always be used. The screen can be terminated using special glands, which - depending on the exact installation - may be insulated or may use a conductive sleeve. It is recommended to use 3-core symmetrical cable which is screened, with the earth being a separate conductor. It is important that the earth conductor is returned from the motor directly to the VSD earth.
However, even with the main grounding system in place, VSDs and motors may still need special attention. The main problem can occur if the motor and the VSD have different ground potentials. It is then important that the grounding is done via the frequency converter to bring both the drive and motor up to the same potential.
The point of good installation practice is not only to guarantee the functionality of the equipment, but also to ensure the safety of any personnel working in the vicinity of the plant and equipment. It is essential that all personnel involved with the installation of VSDs are familiar with local electrical installation regulations as well as familiarising themselves with the user manuals and documentation relating to the drive being installed. For example, local rules may dictate PE grounding. But some manufacturers may stipulate the ability to use shielded or screened cables as part of the grounding. However, the conductivity of the shield may not be good enough for PE grounding. If this occurs, the installer should first check that the seal/shield is good enough to act as a PE conductor and then follow the local safety rules and regulations.
