Baldor MN770 - Manual (Page 45)

Section 1
General Information
6-8 Troublehsooting MN770
Optical Isolation
Isolating electrical circuits with some form of light transmission reduces the electrical
noise that is transmitted from one part of a circuit to another. That is, an electrical signal
is converted to a light signal that is transmitted to a light receiver. This converts the light
back to an electrical signal that has less noise than the input. Two methods are
commonly used; optical couplers and fiber optics.
Optical Couplers Optical couplers, often referred to as opto couplers use a light transmitter and light
receiver in the same unit to transmit data while electrically isolating two circuits. This
isolation rejects some noise. The magnitude of noise rejection is usually specified by the
“common mode rejection, dv/dt rating”. Typically, low cost opto couplers have a common
mode rejection of 100 to 500 V/
m
sec, which is adequate for most control logic signals.
High performance opto couplers with common mode ratings up to 5,000 V/
m
sec are
installed for the most severe noise environments.
Fiber Optics Special plastic fiber strands transmit light over long as well as short distances. Because
the fibers are immune to electromagnetic energy, the use of fiber optic bundles eliminate
the problem of coupling noise into such circuits. These noise–free fiber optic cables can
be run with power or motor conductors because noise cannot be inductively or
capacitively coupled into the fiber optic stands.
Plant Ground Connecting electrical equipment to a good ground is essential for safety and reliable
operation. In many cases, what is perceived as a ground isn’t.
Result: equipment malfunctions or electrical shock hazard exists.
It may be necessary to retain the services of an electrical consultant, who is also a
licensed professional engineer experienced in grounding practices to make the necessary
measurements to establish if the plant ground is really grounded.