Frequency conversion air conditioning in the brushless dc motor control system instance: in the design of air conditioning separate-bodied air-conditioners system, choose 68 Motorola hc908mr24 chip. The chip, low loss, high performance I/O port number, the upgrade of M68HC08 central processor, with 24 k erasable in FLASH, 768 bytes RAM, four 16-bit timer and 10 channel 10 AD conversion functions such as characteristics.
As a dedicated chip, MR24 still has a lot of special excellent properties, making convenient and efficient. For example the chip to control the motor of the AD hoc PWM function, namely the three groups of complementary PWM signal or 6 independent PWM signals; PWM wave polarity control; Software control PWM output; Programmable error protection; Insert the dead zone.
In this air conditioning system, using three groups of PWM wave to control the brushless dc motor, modify the corresponding register, can accurately rapidly change the duty ratio, frequency, etc., make the compressor normal operation as required. Hardware circuit according to the method of back emf, need to use filter on the depth of phase voltage signal filtering, the potential of the actual detection circuit as shown in figure 1 (only draw one phase). Single instead potential detection circuit of the motor phase voltage by partial pressure circuit first, and then into the filter, filter get the square wave signal after the comparator, MR24 I/O ports have the ability to capture the jump in the phase voltage is obtained through zero information.
Designed as a low-pass filter, so that the output signal (called filtering signal) contains no dc component and ac component containing only, and the input signal on the phase lag, then simply use the comparator compared with zero level respectively in every phase; However, in order to in the process of acceleration or deceleration motor can still compare correctly detect the rotor position, actual three-phase filtering signal by the resistance of the three-phase symmetric star connection circuit level where V0 instead of zero level, the center of the voltage comparator output signal contains the rotor position information, through the coupling of light into the CPU and to judge the workings of the motor, control the trigger signal.