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Electromagnetic Compatibility (EMC) and Variable Speed Drives

Editors 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 atwww.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 consideringmeasures 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 PLCs,

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 largesurface 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.

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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 ofdigital 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 cablesare usually recommended, especially for smaller motors and/or in sensitive applications. Unlike

signal cable shields, power cable shields should be connected at bothends.

If youd like additional information or assistance, please contact us atinfo@joliettech.com, drop by

our web site (www.joliettech.com), or visit the Comments section to drop off some

thoughts/questions. And check back next week for another installment.

(Posted on behalf of one of our readers):

In the past I sent a problem on a RTD over load from a VFD drive.

We have found a solution.

First the problem:The person who wired up the VFD at the factory, feed the ground to the RTD cards and Relay

panel.

On the same wire. Feeding through one RTD board to the next and then to the relay panel.

This caused a different ground load on the RTD cards causing them to fail.

Solution:

We removed the ground wire from the ground lug at the RTD cards and the relay panel.

This will allow the board ground to keep the same as rest of the VFD.

RTD boards are grounded on the back side to the VFD back board. The ground lug is connected to

this connection after it is runs through the board (picking up the shields from the RTD wires from

the motor) and is a bleed off for the RTD wires and the board the Back plate ground connection is

for the board then the RTD wires at the opposite end of the board from the ground lug.

Figured some one might have the same problem.

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