EMI related problems are more effectively alleviated during the design and installation phases rather than after the system is in service. Many of the steps listed

here can be implemented at a relatively low cost when compared to the cost for identifying and fixing the problem later in the field.

Earthing

The frequency converter and motor should be solidly earthed to the equipment frame. A good high frequency connection is necessary to allow the high frequency

currents to return back to the frequency converter rather than to travel thorough the power network. The earth connection will be ineffective if it has high

impedance to high frequency currents, therefore it should be as short and direct as practical. Flat braided cable has lower high frequency impedance than round

cable. Simply mounting the frequency converter or motor onto a painted surface will not create an effective earth connection. In addition, running a separate

earth conductor directly between the frequency converter and the running motor is recommended.

Cable routing

Avoid routing motor wiring, mains wiring, and signal wiring in parallel. If parallel routing is unavoidable, try to maintain a separation of 200 mm (6–8 inches)

between the cables or separate them with a earthed conductive partition. Avoid routing cables through free air.

Signal cable selection

Signal cable selection. Single conductor 600 volt rated wires provide the least protection from EMI. Twisted-pair and screened twist-pair cables are available which

are specifically designed to minimise the effects of EMI. While unscreened twisted-pair cables are often adequate, screened twisted-pair cables provide another

degree of protection. The signal cable's screen should be terminated in a manner that is appropriate for the connected equipment. Avoid terminating the screen

through a pigtail connection as this increases the high frequency impedance and spoils the effectiveness of the screen. Refer to Section Earthing Screened

Cables.

A simple alternative is to twist the existing single conductors to provide a balanced capacitive and inductive coupling thus cancelling out differential mode

interference. While not as effective as true twisted-pair cable, it can be implemented in the field using the materials on hand.

Motor cable selection

The management of the motor conductors has the greatest influence on the EMI characteristics of the system. These conductors should receive the highest

attention whenever EMI is a problem. Single conductor wires provide the least protection from EMI emissions. Often if these conductors are routed separately

from the signal and mains wiring, then no further consideration is needed. If the conductors are routed close to other susceptible conductors, or if the system is

suspected of causing EMI problems then alternate motor wiring methods should be considered.

Installing screened power cable is the most effective means to alleviate EMI problems. The cable's screen forces the noise current to flow directly back to the

frequency converter before it gets back into the power network or takes other undesirable and unpredictable high frequency paths. Unlike most signal wiring,

the screening on the motor cable should be terminated at both ends.

If screened motor cable is not available, then 3 phase conductors plus earth in a conduit will provide some degree of protection. This technique will not be as

effective as screened cable due to the unavoidable contact of the conduit with various points within the equipment.

Serial communications cable selection

There are various serial communication interfaces and protocols on the market. Each of these recommends one or more specific types of twisted-pair, screened

twisted-pair, or proprietary cables. Refer to the manufacturer's documentation when selecting these cables. Similar recommendations apply to serial communi-

cation cables as to other signal cables. Using twisted-pair cables and routing them away from power conductors is encouraged. While screened cable provides

additional EMI protection, the screen capacitance may reduce the maximum allowable cable length at high data rates.

High Power Service Manual for Unit Sizes 6x

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