Rockwell Automation 7000L PowerFlex 7000 Medium Voltage AC Drive C Frame - Marine - Manual (Page 89)

Drive Installation 2-59
7000 “C” Frame - Marine 7000L-UM302B-EN-P – June 2013
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
will reduce varying ground potentials caused by high current fluxes
created by motors, remote control switches and magnetic fields.
• Signal Distortion
The primary cause of signal distortion is cable length, more
specifically cable capacitance. Generally speaking the longer the
cable length the more there is a 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 < 1.0us. If the leading edge of the
waveform is distorted it causes the transition time through this region
to increase. At some point, the receiver may become unstable and
either gain or lose encoder counts.
To reduce the effects of signal distortion at the encoder receiving
electronics, the following guidelines should be considered:
1. Use a low capacitance cable. Purchase a cable that has a
capacitance of < 120pf/m (40pf/ft). As an example Belden
1529A is an 18Awg 3pair cable having a capacitance of 114pf/m
(35pf/ft).
2. Use twisted pair cabling with a shield that covers 100% of the
cable. This is especially true in the case of quadrature encoders.
It is still a better choice for absolute encoders, although the data
in these encoders will not exhibit the same frequency spectrum
as quadrature encoders and single wire cabling can be used. In
either case, always check with the encoder manufacturer for the
recommended cable.