Noise - Fuji Electric FRENIC-HVAC User Manual

App. A Advantageous Use of Inverters (Notes on electrical noise)
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 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.
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 is less than approximately 30 to 40 MHz. Therefore,
the noise will affect devices such as AM radios using low frequency band, but will not
virtually affect FM radios and television sets using higher frequency than this
frequency band.
Figure A.1 Outline of Inverter Configuration
A-3