Mitsubishi Electric MS-A•WA Service Technical Manual page 15

2-7-3. 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.
2-7-4. 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 of motor are enhanced in sine wave control, finer adjustment can be provided.
Rotor
Rotor Position Signal
w In 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.
2-7-5. 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.
DC Motor
Permanent magnet is embedded.
Necessary
15
AC Motor
Excited by magnetic field of stator
Unnecessary