March 2021

Drives & Controls & BACK TO BASICS n SPONSORED BY Cabling best practice Part 2: Grounding Improving installation techniques for variable speed drives can dramatically reduce the likelihood of equipment failure and increase system reliability. Here, ABB's Martin Richardson considers the importance of earthing. T he 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 pa rt of the grounding. However, the conduct ivity of the shield may not be good enoug h for PE grounding. If this occurs, the installe r 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. n motion plastics ® Versatile ... D1dryve l For stepper, EC/BLDC and DC motors l Digital I/O, analogue inputs, CANopen and Modbus TCP l Energy-saving closed-loop motor control l Easy to program via an integrated web server interface l 24 hour delivery Motor control made easy 10 pieces from £410 per piece: £450.00 UK-1225-dryve D1 94x270N3_CC.indd 1 08.01.21 1