ACH550-UH User’s Manual
233
Technical Data
Grounding and Routing
Background
Motor cables require extra care in grounding and routing. The reasons have to do
with the following factors:
•
Parasitic capacitance – Capacitors are, essentially, conductors that don’t touch,
but are in close proximity to each other. So, for example, there is a weak
capacitive connection between cables and any conductors they are near. Such
unintentional, but inevitable conductive paths are called parasitic capacitors.
Currents flowing through these paths often create problems. For example, current
leaks to control cables can create noise interference, leaks to the motor can
damage bearings, and leaks to the drive or other electronic cabinets can damage
components.
•
Proximity – As the conductors get closer together, capacitance increases.
•
Proximal area – As the area in close proximity increases, the capacitance
increases, e.g. close parallel paths increase parasitic capacitance between
conductors.
•
AC frequency – For a given capacitance, increased AC frequency increases
current conductance. Hence, capacitive paths that are negligible at 50/60 Hz can
be very significant conductors at 8,000 Hz. Motor cable signals are pulses at up to
8,000 Hz and the common mode frequency can reach 48,000 Hz (8k Hz x 3
phases x 2 pulse edges).
•
Alternate paths – Where multiple paths exist, the most conductive path draws the
most current. So, the ground wiring must be a significantly better path, in order to
reduce the current in the alternate paths, the paths through parasitic capacitors.
The high frequencies associated with motor cables also increase the potential for
electromagnetic noise radiation. See "Motor Cable Requirements for CE & C-Tick
Compliance" below.
Motor Cable Shielding
Motor cables require shielding using conduit, armored cable or power shield cable.
•
Conduit – When using conduit:
– Bridge joints with a ground conductor bonded to the conduit on each side of
the joint.
– Bond conduit run to the drive enclosure.
– Use a separate conduit run for motor cables (also separate input power and
control cables).
– Use a separate conduit run for each drive.
•
Armored Cable – When using armored cable:
– Use six-conductor (3 phases and 3 grounds), type MC continuous corrugated
aluminum armor cable with symmetrical grounds.
– Armored motor cable can share a cable tray with input power cables, but not
with control cables.
