Rockwell Automation 7000A PowerFlex Medium Voltage AC Drive - Air-Cooled (A Frame) - Manual (Page 70)

2-40 Drive Installation
7000A-RM001A-EN-P – January 2011 7000 “A” Frame
• 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 114 pf/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.
3.
Keep cable distances as short as is practically possible. Rockwell
Automation’s recommendation for cable distances are:
a.
For the 20B-ENC encoders, the distance should be kept to a
maximum of 65m (200ft). Longer cable distances could
cause excessive surge currents. The operating frequency of
the encoder has no bearing on this recommended distance
due to AC Termination used. However, if the frequency can
be kept so that the cable’s characteristic impedance is around
348ohms, this will improve the surge currents and may
increase the maximum distance to 100m (330 ft).
b.
For the Universal Encoder Interface, the distance can be
extended out to 200 m (650 ft) @ 100 KHz. This distance
can be increased to 500m at frequencies below 55 Khz. It is
not recommended to exceed this distance since the voltage
drop across the cable could cause a lower than expected
power at the encoder.
Power and Control Wiring
(cont.)