Application of Digital Signal Controller in Frequency Conversion Home Appliances

Microchip introduced the dsPIC digital signal controller series, which optimizes the design of hardware and software for the needs of embedded variable frequency control. It combines the ease of use, low cost and powerful processing power of DSP to provide frequency conversion for home appliances. The best solution.

Compared with the previous traditional home appliances, frequency conversion appliances generally have the characteristics of energy saving, mute and enhanced performance.

Microchip introduced the dsPIC digital signal controller series, which optimizes the design of hardware and software for the needs of embedded variable frequency control. It combines the ease of use, low cost and powerful processing power of DSP to provide frequency conversion for home appliances. The best solution.

Microchip's dsPIC30F digital signal controllers deliver all the power of a powerful 16-bit microcontroller: fast, complex and flexible interrupt handling, rich digital and analog on-chip peripherals, flexible power management, and flexible use of multiple clocks Mode, power-on reset and power-down protection, watchdog timer, code encryption, full-speed real-time emulation and full-speed online debugging.

Currently, Microchip's dsPIC30F family has three series, the Universal Series, the Motor Control and Power Conversion Series, and the Sensor Series. Among them, there are seven products in the motor control series, which support a variety of motor control applications, such as BLDC motors, single-phase and three-phase induction motors (ACIM) and switched reluctance motors.

The series is also suitable for power conversion applications such as uninterruptible power supplies (UPS), inverters, switching power supplies and power factor correction (PFC).

BLDC motor is the first choice for inverter appliances

Various frequency conversion appliances, although specific applications have their own characteristics, but the core part is the inverter control of the motor. Although all kinds of motors have applications, DC brushless motors (BLDC motors) have become a variable frequency home appliance because of their large starting torque, small starting current, high operating efficiency, good speed regulation performance, low noise, small size and many other advantages. Preferred motor type.

The structure of the BLDC motor is different from that of a conventional DC motor. It has a rotor composed of permanent magnets and a stator composed of windings. For a common three-phase BLDC motor, the stator is composed of three-phase windings and is connected in a star shape. To make the BLDC motor rotate, a rotating magnetic field is required. The mechanical structure of the BLDC determines that the mechanical rotational speed of the rotor is synchronized with the rotational flux of the stator. Thus, to control the BLDC, it is necessary to energize the stator windings in a specific energization sequence, knowing the exact position of the rotor, which is commonly referred to as "commutation."

The key to making the BLDC motor properly commutated is to know exactly where the rotor is. There are several different ways to know the exact position of the rotor. Some form of position sensor can be used to measure the position of the rotor. A commonly used photoelectric shaft encoder mounted on the motor shaft or a Hall sensor that detects the position of the rotor magnet. Such a control method is often referred to as a sensored BLDC control method. The use of position sensors is convenient and straightforward, but the cost and mechanical complexity of the control system is high and in some cases difficult to use. For example, in applications such as air conditioners or refrigerators that use compressors, the motor needs to be immersed in the liquid, and mechanical assembly and other factors are also very limited, making the control method using the sensor less suitable. In such an application environment, sensorless control methods are required.

The sensorless control method, although not using a real sensor, actually requires measuring the exact position of the rotor, except that it uses other means to measure indirectly. It can be implemented by several methods such as back electromotive force method, phase current method and freewheeling diode method, among which the back electromotive force method (BEMF) is the most commonly used.

For a normal 120° energized three-phase BLDC motor, one of the three stator windings is not energized at any one time. The position of the rotor is estimated from the counter electromotive force generated when the stator winding that is not energized cuts the magnetic field lines generated by the rotating rotor. By analyzing the magnitude, phase, and trend of this back EMF, the exact position of the rotor can be known. The following scheme is based on the BEMF zero-crossing detection method of the unpowered phase, using Microchip's dsPIC30F digital signal controller to perform all control functions.

BEMF's zero-crossing detection technology was chosen because it has a number of advantages that are especially important for home appliance applications. This technology is highly adaptable to motors and is suitable for a wide range of motors. It is suitable for Y-connected and delta-connected motors. It does not require precise knowledge of the motor characteristics. The manufacturing tolerances of the motor are not critical. Voltage control and current. Control is effective.

For the BEMF phase voltage, its mathematical expression is as follows: To make the BLDC motor correctly commutate, the key is to know the position of the rotor accurately. There are several different ways to know the exact position of the rotor. Some form of position sensor can be used to measure the position of the rotor. A commonly used photoelectric shaft encoder mounted on the motor shaft or a Hall sensor that detects the position of the rotor magnet. Such a control method is often referred to as a sensored BLDC control method. The use of position sensors is convenient and straightforward, but the cost and mechanical complexity of the control system is high and in some cases difficult to use. For example, in applications such as air conditioners or refrigerators that use compressors, the motor needs to be immersed in the liquid, and mechanical assembly and other factors are also very limited, making the control method using the sensor less suitable. In such an application environment, sensorless control methods are required.

The sensorless control method, although not using a real sensor, actually requires measuring the exact position of the rotor, except that it uses other means to measure indirectly. It can be implemented by several methods such as back electromotive force method, phase current method and freewheeling diode method, among which the back electromotive force method (BEMF) is the most commonly used.

For a normal 120° energized three-phase BLDC motor, one of the three stator windings is not energized at any one time. The position of the rotor is estimated from the counter electromotive force generated when the stator winding that is not energized cuts the magnetic field lines generated by the rotating rotor. By analyzing the magnitude, phase, and trend of this back EMF, the exact position of the rotor can be known. The following scheme is based on the BEMF zero-crossing detection method of the unpowered phase, using Microchip's dsPIC30F digital signal controller to perform all control functions.

BEMF's zero-crossing detection technology was chosen because it has a number of advantages that are especially important for home appliance applications. This technology is highly adaptable to motors and is suitable for a wide range of motors. It is suitable for Y-connected and delta-connected motors. It does not require precise knowledge of the motor characteristics. The manufacturing tolerances of the motor are not critical. Voltage control and current. Control is effective.

For the BEMF phase voltage, its mathematical expression is as follows:

When the speed is greater than zero, there is only two positions where the BEMF of a phase in each electrical cycle is zero. The two positions can be distinguished by the slope of the zero crossing in the following figure. Each of these segments corresponds to a 60° portion of the electrical cycle, and commutation occurs at the boundary of each segment, so the boundary of the segment needs to be detected. There is a 30° offset between the BEMF zero crossing and the position that requires commutation, which must be compensated to ensure that the motor is flat

Runs steadily and efficiently. The above description is for a Y-connected motor. The specific implementation method is different for the Δ-connected motor. From the hardware implementation point of view, the following circuits can be used. The specific zero-crossing detection can be implemented by first monitoring all three-phase terminal voltages and VDCs through a voltage divider and dsPIC's on-chip AD converter; then detecting when the phase BEMF passes 1/2 VDC in the corresponding time period. For a specific period of time, only one of the phases is monitored. A timer on the dsPIC chip is used to measure the time between the second zero crossings, then this time is divided by 2 and loaded into another timer to perform the required 30° compensation.


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