Mitsubishi Electronics MS-AWA, MUZ-ANA, MUY-ANA, MU-AWA, MSY-ANA, MSZ-ANA Power factor improvement

3. Power factor improvement

Booster coil L (Reactor) and PFC 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 L (Reactor) stabilize the voltage of DC supplied to inverter circuit and make its waveform smooth.

4. Power transistor module

IPM consists of the following components.

· Power Transistors (x6): Converts DC waveform to three-phase AC waveform and outputs it.

· Drive Circuit	: Drives transistors.
· Protection circuit	: Protects transistors from over current.

Since the above components are all integrated in IPM, IPM has a merit that can get the control circuit simplified and mini- aturized.

5. Smoothing capacitor

CB1, CB2 and CB3 stabilize the DC voltage and supply it to IPM.

6. Elimination of electrical noise

Noise filter circuit, which is formed by *CMC COILS and capacitors placed on the noise filter 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 capacitors. Both noise filter circuit exists 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

3-1-3. Sine wave control

In these air conditioners, compressor equips brushless DC motor which doesn'thave 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.

3-1-4. Characteristics of sine wave control in case of brushless DC motor

●Although ordinary three-phase induction motor requires energy to excite the magnetic field of rotor, brushless DC motor doesn't need it. So, higher efficiency and torque are provided.

●This control provides the most efficient 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 energy saved 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 efficiency are enhanced in sine wave control, finer adjustment can be provided.

	DC Motor	AC Motor
Rotor	Permanent magnet is embedded.	Excited by magnetic field of stator
Rotor Position Signal	Necessary	Unnecessary

wIn brushless DC motor, permanent magnet is embedded in the rotor. Therefore, it doesn't require energy to excite the rotor like AC motor does. However, it's necessary to control the frequency of three-phase AC current supplied to the stator according to the polar direction of magnet embedded in the rotor so as to drive the motor efficiently. 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.

3-1-5. Control Method of Rotation Times

Sine wave control makes the current transformers conduct real time detection of the value of the current running into the motor, locates the rotor position from the detected value and decides if voltage should be impressed and if frequency should be chan- ged.

Compared to the conventional control and rotor position detection method, sine wave control can provide finer adjustment of the voltage of supplied power. The value of the current running into the motor is determined by each motor characteristic.

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