Cutler-Hammer D320 PLC, Руководство пользователя

Страница: 194 / 258

182
D320 PLC User's Manual
Noise Occurrence
Types of Noise
•
Radiation noise is transmitted in the form of a magnetic wave. The amplitude of the magnetic
wave is measured in Gauss.
•
Conduction noise is transmitted through a direct path such as signal wiring or ground
connections as a strong, high-voltage surge. This type of noise is measured as voltage,
current, or power.
• Normal mode (single ended developed) noise can come through the power and/or the signal
cables. This type of noise is not equally distributed across the PLC input terminals.
•
Common mode noise can come through the power and/or the signal cables. In this case the
noise is close to the same amplitude thus the term common on both leads of the cable.
•
Impulse noise is electrical or magnetic energy that has less than a 200 msec pulse duration.
•
Surge noise is electrical energy that has a pulse duration of 200 msec to 2 sec.
•
Transient noise is electrical energy that has an extremely short duration usually lasting only a
few nanoseconds (1 x 10
-9
).
Electrical Noise Fundamental Definitions
•
Isolation means to physically separate the connection between areas. Isolation is effective for
common mode noise.
•
Filters are effective against conduction noise such as impulses. Filtering is used to remove
normal mode noise and common mode noise that has been imprinted onto the signal or power
cables. A low-pass filter passes only low frequency signals. Low-pass filters are classified as
either LC (L = inductor and C = capacitor) filters or RC (R = resistor and C = capacitor)
filters, according to the electrical parts that form the filter.
• Surge absorbers are devices that protect electronic equipment by clamping down extremely
high voltage spikes (lightning strikes) in power cables to a safe level.
•
Charge is an excess or deficiency of electrons in an object. When an object becomes charged,
a magnetic field forms around the object and can radiate noise as the amplitude of the charge
is varied.
•
An inductive load is a device which creates a large magnetic field that opposes any change in
the voltage applied across the device. Devices that act as inductive loads are relay coils, motor
coils, starter coils and actuator coils.
•
Stray capacitance and inductance is created during the installation of an electrical system.
When excess cabling is left wound up this creates stray inductance in the form of a coil. All
cabling inherently has a capacitive rating (so many picofarads per meter). Excessively long
cable runs or untrimmed cable lengths or poorly specified cable types can add large levels of
stray capacitance.