Mitsubishi MSZ-FA-VA Service Technical Manual page 32

3. Purpose of PAM adoption
PAM : Pulse Amplitude Modulation
PAM has been adopted for the efficiency improvement and the adaptation to IEC harmonic current emission standard.
Outline of simple partial switching method
In conventional inverter models, diode module rectifies AC voltage to DC voltage, smoothing capacitor makes its DC waveform
smooth, and IPM converts its DC voltage to imitated AC voltage again in order to drive the compressor motor.
However, it has been difficult to meet IEC harmonic current emission standard by above circuit because harmonic gets
generated in the input current waveform and power factor gets down. The simple partial switching method with PAM, which has
been adopted this time, places and utilizes the booster chopper circuit (L61, DB65 and TR821) before rectifying AC voltage in
the general passive-method converter circuit. As harmonic gets suppressed and the peak of waveform gets lower by adding
booster chopper circuit as mentioned above and by synchronizing the timing of one-time switching with the zero-cross point of
waveform, the input current waveform can be improved and the requirement of IEC harmonic current emission standard can be
satisfied. Since the switching times is just once by synchronizing with the zero cross point, this simple partial switching method
has the feature of lower energy loss compared to active filter method. In addition, output and efficiency is enhanced by
combining with vector-controlled inverter in order to boost the voltage of power supplied to IPM.
Input current waveform without PAM
Due to the time of no electricity;
· Power factor gets worse.
· Harmonic gets increased.
Input current
Input voltage
Energized time is short in
case L inductance is small.
No electricity runs into
diode module because the
voltage at both sides of smoothing
capacitor is higher than input voltage.
4. Intelligent power module
IPM consists of the following components
· IGBT (x6)
· Drive Circuit
· Protection circuit
Since the above components are all integrated in IPM, IPM has a merit to make the control circuit simplify and miniaturize.
5. Smoothing capacitor
C63A, C63B and C63C stabilize the DC voltage and supply it to IPM.
6. Elimination of electrical noice
Noise filter circuit, which is formed by *CMC COILS capacitors placed on the POWER P.C. board, eliminates electrical
noise of AC power that is supplied to main power supply circuit. And this circuit prevents the electrical noise generated in
the inverter circuit from leaking out.
*CMC COILS; Common mode choke coils
Sine wave control
In these air conditioners, compressor equips brushless DC motor which doesn't have Hall element.
In short, the motor is sensorless. However, it's necessary to locate the polar direction of rotor in order to drive brushless DC
motor efficiently. The general detection method of the polar direction for such a DC motor is to locate it from the voltage induced
by unenergized stator.
Therefore, It is necessary to have a certain period of time in which the stator is being unenergized for the rotor position
detection when the voltage of supplied power is impressed.
So the motor has been driven by square wave control (the conventional motor drive system) which energizes the motor only
when the range of electrical angle is within 120_ because it is forced to be unenergized within 30_ at start & end of one heap
in one waveform cycle (180_) when the voltage is impressed.
However, torque pulsation occurs at rotation in this method when the current-carrying phases are switched over to other
phases in sequence. Therefore, sine wave control system is adopted for these air conditioners because it can make the
phase-to-phase current waveform smoother (sine wave) in order to drive the motor more efficiently and smoothly.
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Input current waveform with PAM
Owing to the increase of energized time;
· Power factor gets better.
· Harmonic gets suppressed.
: Converts DC waveform to three-phase AC waveform and outputs it.
: Drives transistors.
: Protects transistors from overcurrent.
Release of energy stored in L
Peak gets down.
Energized time is
extended by optimization
of L inductance.
Compulsory energizing
by switching.
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