Editor’s Note: Much of the information which follows is taken from engineering information provided by Siemens AG in their “Sinamics DCM Converter Units” catalog D 23.1 – 2010. The Sinamics DCM is the line of industrial DC drives in the Sinamics family, which forms a part of Siemens’ “Totally Integrated Automation” concept; learn more at www.siemens.com. However, the concepts discussed herein can generally be applied to any drive application.
Because of the high switching frequencies of their electronic components, variable speed drives are by their nature radiating devices. This radiated energy is termed electromagnetic interference (EMI); measures to reduce EMI during design and installation are intended to ensure electromagnetic compatibility (EMC), which is essentially the ability of a device to function satisfactorily in an electromagnetic environment without itself causing interference unacceptable to other devices in the environment.
In the typical industrial environment, EMI occurs in the range of 150kHz – 30 MHz and can have adverse consequences on the operation of nearby sensitive equipment. When considering measures to ensure EMC, the drive must be looked at as forming part of a system, the other components being minimally the cables and motor. Mitigation involves several design and installation practices involving all of these components. In the US, measures are usually focused on reducing interference to ensure the proper operation of sensitive devices such as PLC’s, sensors, and transducers. In Europe, and in more susceptible US applications, documented compliance with IEC/EN product standard 61800-3 is often required. This standard describes the requirements for EMC compliance for Power Drive Systems (PDS) and is structured to consider the location and sensitivity of equipment likely to be present in a given environment. In any case, the following practical rules will go a long way toward ensuring that the PDS operates without causing undue interference within the industrial environment:
Rule 1: All metal parts of the control cabinet are connected with one another through a large surface area with a good electrical connection (not paint on paint!). If required, contact or serrated washers should be used. The cabinet door must be connected to the cabinet using the shortest possible grounding straps (at the top, center, and bottom).
Do not rely on the cabinet hinge(s) for this purpose.
Rule 2: Contactors, relays, solenoid valves, electromechanical counters, etc., in the cabinet and – where applicable, in neighboring cabinets – must be provided with surge protection, e.g. RC elements, varistors, and diodes. The protective circuit must be directly connected to the particular coil.
Rule 3: Signal cables should only be routed at just one level in the cabinet, if at all possible.
Rule 4: Unshielded cables in the same circuit (outgoing/incoming conductors) must be twisted wherever possible, or the area between them minimized, to prevent the unnecessary formation of frame antennae.
This also can reduce the effects of capacitive coupling at higher frequencies. Cables designed specifically for drive applications, whether shielded or not, are symmetrically fabricated to take this into account.
Rule 5: Connect spare wires to the cabinet ground at both ends . This achieves an additional shielding effect.
This also reduces their ability to radiate if not low-impedance grounded.
Rule 6: Avoid unnecessary cable lengths. This keeps coupling capacitances and inductances low.
Rule 7: Crosstalk is generally reduced if cables are routed close to the control cabinet ground. Therefore, do not route cables freely around the cabinet, but route them as close as possible to the cabinet enclosure or to the mounting plates. This also applies to spare cables.
Rule 8: Signal and power cables must be physically separated to prevent coupling paths. A minimum distance of 20 cm must be observed.
If it is not physically possible to separate the conductors by distance, encase them in separate metallic conduits, with each conduit being solidly bonded throughout its length. (Note that other manufacturers have differing recommendations for conductor spacing, but the same general rule applies – route the line, load, and signal conductors at a distance from one another.) Also, if routed at or near the recommended separation distances, avoid routing them in parallel for any distance; if they must cross, they are to do so at 90-degree angles.
Rule 9: Ground the shields of digital signal cables at both ends (source and destination), ensuring maximum contact area and good conductivity. In the event of poor equipotential bonding between the shield connections, run an additional equipotential bonding conductor with a cross-section of at least 10 sq. mm parallel to the shield for the purpose of reducing the shield current. Generally speaking, the shields may also be connected to the cabinet enclosure (ground) at several points. The shields can be connected several times even outside the control cabinet.
In each case, however, 360-degree shield bonding is required for good conductivity. Also bear in mind that the best way to ensure shield integrity is not to break the shield – so minimize cable terminations wherever possible. Foil-type shields should be avoided, as they are at least 5 times less effective than braided shields.
Rule 10: Connect shields for analog signal cables at one end to prevent low-frequency, capacitive interference from being coupled in (e.g. 50/60 Hz hum). In this case, the shield should be connected in the control cabinet (i.e. at the source end).
Also keep in mind that the power conductors, particularly the motor leads, are typically the most severe source of EMI and should always be considered first when assessing EMC. Shielded cables are usually recommended, especially for smaller motors and/or in sensitive applications. Unlike signal cable shields, power cable shields should be connected at both ends.
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