12LRN
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2. Outline of PAM circuit
2.1. PAM (Pulse Amplitude Modulation)
The PAM circuit varies the compressor drive voltage and controls the rotation speed of the compressor.
The IGBT shown in the block diagram charges the energy (electromotive force) generated by the reactor to the electrolytic capacitor for the inverter
by turning ON and OFF.
When the IGBT is ON, an electric current flows to the IGBT via the reactor (L5) and diode bridge (DB2).
When the IGBT turns OFF, the energy stored while the IGBT was ON is charged to the capacitor via the diode bridge (DB1).
As such, by varying the ON/OFF duty of the IGBT, the output voltage is varied.
2.2. High power factor control circuit
This circuit brings the operating current waveform closer to the waveform of commercial power supply voltage to maintain a high power factor.
Because of the capacitor input, when the PAM circuit is OFF, the phase of the current waveform deviates from the voltage waveform as shown below.
To prevent this deviation, a current is supplied during the periods indicated by "O" in the diagram.
To determine the length of period to supply a current, the zero-cross timing of the AC input voltage is input to the microcomputer via the clock circuit.
The power source frequency is also determined at the same time.
The IGBT turns ON after the time length determined by the zero-cross point to supply a current to the IGBT via the reactor.
This brings the current waveform closer to the voltage waveform in phase.
As described above, the ON/OFF operation of the IGBT controls the increase/decrease of the compressor power supply voltage (DC voltage) to
improve the compressor efficiency and maintain a high power factor by keeping the current phase closer to that of the supply voltage.
PAM drive circuit block diagram
Reactor L5
[PAM drive circuit]
+
Microcomputer (IC1)
AC
230V
Compressor
Noise
filter
AC clock
detection
circuit
DB1
IPM
DB2
Compressor
position
detector
IGBT
drive
circuit
IGBT
Overvoltage
detection
circuit
Stored energy
Reactor
L5
DB1
DB2
IGBT
IGBT ON
IGBT OFF
AC voltage waveform
AC voltage and current waveform when PAM is ON
AC current waveform
IGBT ON period
Zero-cross detection
AC voltage waveform
AC current waveform
AC voltage and current waveforms when PAM is OFF
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