Mitsubishi Electric MSC-GE-VB Service Manual page 27

MXZ-6C120VA
1. At the start of operation
Main power supply circuit is formed when RELAY is turned ON at COMPRESSOR startup.
To prevent rush current from running into the circuit when power supply is turned ON, RESISTOR are placed in sub circuit.
2. At normal operation
When AC runs into P.C. board, its external noise is eliminated in NOISE FILTER CIRCUIT.
After noise is eliminated from AC, it is rectifi ed to DC by DIODE BRIDGE.
Then REACTOR ,SWITCHING POWER TRANSISTOR and DIODE boost the DC voltage to 350V.
DC voltage, to which AC has been rectifi ed by process
POWER TRANSISTOR BRIDGE.
The DC (Bus voltage), which has been stabilized in process
BRIDGE and supplied to COMPRESSOR.
CURRENT TRANSFORMER which is placed in the power supply circuit to COMPRESSOR, is used to measure the value
of phase current and locates the polar direction of rotor with algorithm. PWM (Pulse width modulation) controls impressed
voltage and frequency with those pieces of information.
3. Power factor improvement
An AC/DC converter circuit made up of DIODE BRIDGE and SMOOTHING CAPACITOR causes waveform distortion of the
input current. Waveform distortion leads to lower input power factor and higher input current.
It also causes increase of harmonic current.
A circuit made up of REACTOR, SWITCHING POWER TRANSISTOR and DIODE controls the input current and
thus smoothes the input current waveform. This circuit improves the power factor and reduces the input current.
Also, the input current waveform will satisfy the harmonic current standard.
Input current waveform without power factor
improvement
Input current
Input voltage
4. Power module
Power module consists of the following components.
• Diode bridge (Diode x4): It converts AC voltage to DC votage.
• Switching power transistor, Diode: It improves the power factor and controls the bus-bar voltage with the reactor.
• Power transistor bridge (Transistor x6): It converts DC voltage to 3-phase AC voltage.
5. Elimination of electrical noise
NOISE FILTER CIRCUIT, which is formed by choke coils and capacitors placed on 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.
3-1-3. Sine wave control
In these air conditioners, compressor equips brushless DC motor which does not have Hall element.
In short, the motor is sensorless. However, it is necessary to locate the polar direction of rotor in order to drive brushless DC mo-
tor effi ciently. 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 and 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 effi ciently and smoothly.
, is stabilized by SMOOTHING CAPACITOR and supplied to
, is converted to 3-phase AC by POWER TRANSISTOR
Input current waveform with power factor
improvement
Input current
Input voltage
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