Mitsubishi Electric MSC-GE-VB – E Service Technical Manual page 19

MUZ-FD50
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 noise fi lter P.C. board, its external noise is eliminated in NOISE FILTER CIRCUIT.
After noise being eliminated from AC, it is rectifi ed to DC by REACTOR and PFC. If the operating frequency becomes 25
Hz or more, DC voltage rises to 370 V.
DC voltage, to which has AC been rectifi ed by process
The DC (Bus voltage), which has been stabilized in process
PRESSOR.
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 information.
3. Power factor improvement
Booster coil reactor and power factor controller rectify AC to DC and control its voltage.
In the motor drive system of sine wave control, power factor can be improved by reducing harmonics. PFC and reactor that
stabilizes the voltage of DC supplied to inverter circuit and makes its waveform smooth.
4. Power transistor module
IPM consists of the following components.
• Power Transistors (x6) : Converts DC waveform to 3-phase AC waveform and outputs it.
• Drive Circuit
• Protection circuit
Since the above components are all integrated in IPM, IPM has a merit that can get the control circuit simplifi ed and miniatur-
ized.
5. Elimination of electrical noise
NOISE FILTER CIRCUIT, which is formed by *CMC COILS and capacitors placed on the noise fi lter P.C. board, eliminates
electrical noise of AC power that is supplied to main power supply circuit. In short, common mode noise is absorbed in this
circuit.
Moreover, normal mode noise is absorbed in another NOISE FILTER CIRCUIT which is formed by *NMC COILS and capaci-
tors.
Both NOISE FILTER CIRCUITS exist for preventing the electrical noise generated in the inverter circuit from leaking out.
*CMC COILS: Common mode choke coils
*NMC COILS: Normal mode choke coils
2-8-3. 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
motor effi ciently. The general detection method of the polar direction for such a DC motor is to locate it from the induced voltage
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 supplied power voltage 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.
2-8-4. Characteristics of sine wave control in case of brushless DC motor
● Although ordinary three-phase induction motor requires energy to excite the magnetic fi eld of rotor, brushless DC motor does
not need it. So, higher effi ciency and torque are provided.
● This control provides the most effi cient waveform corresponding to the rotation times of compressor motor.
● The rotation can be set to higher compared to the conventional motor drive system. So, the time in which air conditioner can
be operated with saved energy is longer than conventional models. This can save annual electric consumption.
● Compared to square wave control, the torque pulsation is reduced at rotation so that the motor operates more quietly.
● Since response and effi ciency of motor are enhanced in sine wave control, fi ner adjustment can be provided.
Rotor
Rotor Position Signal
In brushless DC motor, permanent magnet is embedded in the rotor. Therefore, it does not require energy to excite the
rotor like AC motor does. However, it is necessary to control the frequency of three-phase AC current supplied to the sta-
tor according to the polar direction of magnet embedded in the rotor so as to drive the motor effi ciently. Controlling three-
phase AC current frequency also means controlling the timing to switch the polarity of stator. Therefore, the polar direction
of rotor needs to be detected.
: Drives transistors.
: Protects transistors from over current.
Permanent magnet is embedded
Necessary
, is stabilized by SMOOTHING CAPACITOR and supplied to IPM.
, is converted to 3 phase AC by IPM and supplied to COM-
DC Motor
Excited by magnetic fi eld of stator
Unnecessary
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AC Motor