Rockwell Automation 7000 PowerFlex Medium Voltage Air-Cooled (B Frame) Installation - ForGe Control - Manual (Page 59)

Rockwell Automation Publication 7000-IN007E-EN-P - June 2014 59
Drive Installation Chapter 3
To avoid radiated and/or conducted noise, run power and signal lines separately
with a minimum distance between them of at least 75 mm (3 in.). If they must
overlap somewhere in the system, run the power lines at 90° to the signal lines.
Signal lines should also use twisted pair shielded cable and run in separate
conduit grounded to the building ground.
Encoder wires and shields should maintain continuity from the encoder to the
drive. Avoid using a terminal block in a junction box. This has the potential of
creating radiated noise and ground loops.
Ground the encoder case to the building ground to insure proper and reliable
operation. Most encoders provide for a case ground connection through the
connector/cable pair if you cannot make a ground connection through the
mounting bracket/machine ground. DO NOT ground the encoder case through
both the machine and cable wiring. Use low capacitance wires (≤ 40 pF/ft) with
100% shield coverage for long cable runs and connect the shield only at the drive
end.
Figure 51 - Detail power terminal dimensions
For more protection against electrical noise, specify an encoder with
complementary outputs and connect with twisted pair cable. With this type of
cabling, the induced currents will self-cancel.
As a final precaution, ground the shield together with all other parts of the system
that require grounding to a single point ground. This reduces varying ground
potentials caused by high current fluxes from motors, remote control switches,
and magnetic fields.
Signal Distortion
The primary causes of signal distortion are cable length and capacitance. The
longer the cable, the greater chance of signal distortion at the receiving end. The
receiving end responds to either a logical ‘0’ or a logical ‘1’. Anywhere in between
is undefined and the transition through this region should be < 1.0 μs. If the
leading edge of the waveform is distorted it causes the transition time through
this region to increase. At some point, the receiver can become unstable and
either gain or lose encoder counts.
Wrong Shield
Connection
Encoder
Drive