3-27
IM DR231-01E
Installation and Wiring
3
3.8 Countering Noise
Voltage
waveform
Current
waveform
Voltage/current waveforms of an inverter
Relay
A relay is frequently used to amplify alarm and temperature controller outputs. However, since a
counter-electromotive force (counter e.m.f.) is produced by coil inductance when the relay is
turned off and the e.m.f. becomes noise, care must be taken. Due to chattering at the relay contact,
tens to hundreds of kHz noise occurs mainly in bursts. Thus, the noise energy often becomes high.
Transceiver
In large-scale plants, transceivers are often used for communications between the field and the
control room. Although W/G of the Japanese Electric Measuring Instruments Manufacturers
Association recommends to use transceivers covering a wave band of 27MHz, smaller-sized
transceivers for the 140 or 470MHz wave band are often used.
Noise Simulator
To test the immunity to pulse noise (mainly thyristor noise), a noise simulator is often used. A test
noise of approximately 1kV for 0.8
µ
s is used synchronously with the power supply frequency.
Since data acquisition equipment is often used for measurements of equipment subjected to the
pulse noise test, the influence of noise must be considered.
Propagation of Noise
• Noise is propagated in the following three ways.
Conduction:
noise is conducted through a power line, input wiring, etc.
Electrostatic induction:
noise leaks through capacitances between wires and instruments.
Electromagnetic induction:
a loop in the input line, etc. induces an AC voltage by detecting the
AC magnetic field.
In real applications, the above three propagation paths are not independent of each other. Their
combination propagates noise to the equipment and causes problems.
•Examples of noise propagated through each of these paths are given below.
Conduction:
inverter noise, relay noise, thyristor noise, and noise caused by
surface temperature measurement of a power transistor, etc.
Electrostatic induction:
commercial power supply noise such as the hum of audio
equipment, relay noise, etc.
Electromagnetic induction:
magnetic leakage flux from a power transformer or motor, magnetic
field from a high-frequency induction furnace, the rotating magnetic
field of a power generator (Cu10
Ω
), etc.
The figure on the next page shows the propagation paths using practical examples.
Summary of Contents for DR231
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