Appendix A Trouble-free Use of Inverters (Notes on Electrical Noise)
Appendix-2
A.2 Noise
This section gives a summary of noises generated in inverters and their effects on devices subject to noise.
[ 1 ] Inverter Operating Principle and Noise
Fig. A.2-1 shows an Outline of inverter configuration. The inverter converts AC to DC (rectification) in a converter
unit, and converts DC to AC (inversion) with 3-phase variable voltage and variable frequency. The conversion
(inversion) is performed by PWM implemented by switching six transistors (IGBT: Insulated Gate Bipolar Transistor,
etc.), and is used for variable speed motor control.
Switching noise is generated by high-speed on/off switching of the six transistors. Noise current (i) is emitted and
at each high-speed on/off switching, the noise current flows through stray capacitance (C) of the inverter, cable and
motor to the ground. The amount of the noise current is expressed as follows:
i = C·dv/dt
It is related to the stray capacitance (C) and dv/dt (switching speed of the transistors). Further, this noise current is
related to the carrier frequency since the noise current flows each time the transistors are switched on or off.
Noise is generated by the DC/DC power supply converter for the control circuit during transistor switching.
These noise frequency bands extend across several tens of MHz, and may interfere with communication devices
such as AM radios, factory wireless networks, and telephones.
Fig. A.2-1 Outline of inverter configuration
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