Emerson Control Techniques Epsilon Eb Series Reference Manual page 97

Epsilon Eb and EN Drives Reference Manual
Achieving Low Impedance Connections
Noise immunity can be improved and emissions reduced by making sure that all the
components have a low impedance connection to the same ground point. A low impedance
connection is one that conducts high frequency current with very little resistance. Impedance
cannot be accurately measured with a standard ohmmeter, because an ohmmeter measures
DC resistance. For example, a 12 inch long 8 gauge round wire has a significantly higher
impedance than a 12 inch long 12 gauge flat braided conductor. A short wire has less
impedance than a long one.
Low impedance connections can be achieved by bringing large areas of conductive surfaces
into direct contact with each other. In most cases this requires paint removal because a ground
connection through bolt threads is not sufficient. However, component materials should be
conductive, compatible and exhibit good atmospheric corrosion resistance to prevent loss
through corrosion which will hinder the low impedance connection. Enclosure manufacturers
offer corrosion resistant, unpainted mounting plates to help.
Bringing components into direct contact cannot always be achieved. In these situations a
conductor must be relied upon to provide a low impedance path between components.
Remember a flat braided wire has lower impedance than a round wire of a large gauge rating.
A low impedance connection should exist between the following components, but not limited
to:
• Enclosure and mounting plate
• Servo amplifier chassis and mounting plate
• EMI/RFI AC line filter chassis and mounting plate
• Other interface equipment chassis and mounting plate
• Other interface equipment chassis and electrical connectors
• Enclosure and conduit fittings or electrical connectors
• Enclosure mounting plate and earth ground
• Motor frame and conduit fittings or electrical connectors
• Encoder chassis and electrical connector
A good rule to follow when specifying conductors for high frequency applications is to use a
metal strap with a length to width ratio that is less than 3:1.
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