App. A Advantageous Use of Inverters (Notes on Electrical Noise)
A-2
A.2 Noise
This section gives a summary of noises generated in inverters and their effects on devices subject to noise.
[1] Inverter noise
Figure A-1 shows an outline of the 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
Gated Bipolar Transistor).
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, I/O wires 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.
In addition to the main circuit of the inverter, the DC-to-DC switching power regulator (DC/DC converter),
which is the power source for the control circuit of the inverter, may be a noise source in the same principles
as stated above.
The frequency band of this noise spans several tens of MHz and may affect the AM radio, wireless
communication, and/or telephone.
Figure A-1 Outline of Inverter Configuration
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