Research on an Intelligent Power Saver Based on Single Chip Microcomputer

The energy-saving protection of three-phase AC asynchronous motors has always been a hot topic in the field of motor research. In particular, the tight power supply in the country in recent years has made the research and promotion of energy-saving equipment for AC asynchronous motors more urgent. Three-phase asynchronous motors are widely used due to their simple structure and convenient and reliable use of intelligent energy-saving devices , but they are the largest user of electric energy. It is estimated that more than 50% of the electric energy is consumed by them, and 20% of the electric energy is consumed without doing any useful work.

In actual use, three-phase asynchronous motors often do not work at rated power, but often run under light load, which forms the so-called "big horse pulling a small cart" phenomenon. This phenomenon not only causes a large amount of energy waste, but also causes the motor power factor to decrease (the power factor is 9000 when fully loaded and only 2000 when unloaded), so that the consumed electrical energy is converted into heat, causing the motor temperature to increase, which has a serious impact on the service life of the motor. Generally speaking, reducing the motor temperature by 10°C can double the service life of the motor.

To this end, the author has developed a three-phase asynchronous motor intelligent energy saver. Compared with the traditional capacitor power compensator, it can automatically adjust according to the random load changes; compared with the energy saver using mechanical or electronic soft start, it has the characteristics of continuous operation in the energy saving state and low cost; compared with the frequency converter, it does not require precise speed regulation, is suitable for working conditions with large random load changes, and has low cost. Therefore, it has a very broad application prospect.

2 The principle of motor energy saver

2.1 Relationship between power saving rate and motor efficiency   

According to the three-phase motor power formula P - Ulcos}P, reducing power consumption can be achieved by reducing the motor's U, I and cos}P. Since the power factor will increase while reducing the voltage and the excitation current (the motor's current is mainly the excitation current), the power increase must be less than the current reduction to achieve energy saving.   

The lower the motor efficiency, the greater the power consumption of the motor, and the greater the power saving space of the motor's intelligent energy saver. The power Ps absorbed by the motor from the power grid is equal to the sum of the motor's rated power Pn and the total loss Pn (the motor efficiency is the knife), that is: The relationship between the motor's total loss and the rated power and efficiency is Pn - Pn, Guandao (1) Ps-Pn+Pn (2) Pn-Ps-Pn-(C1/,-')}Pn-C3) The lower the speed of the motor with the same power, the lower the efficiency, and the higher the speed, the higher the efficiency. The smaller the motor power, the lower the efficiency and the greater the loss. Since the light-load voltage regulation intelligent energy saver is adopted to save power mainly to reduce the power loss of the motor when it is lightly loaded, thereby improving the motor efficiency. It can be seen that the lower the efficiency and the greater the no-load loss of the motor, the greater the space for voltage regulation to save power.

2.2 Relationship between power saving rate and load rate   

No-load loss accounts for a large proportion of the total loss. The main components of no-load loss are iron loss and mechanical loss, which are called constant loss; copper loss and stray loss increase in a square relationship with the increase of current, so they are called variable loss. "With the increase of motor output power, the efficiency initially shows an obvious upward trend. The lower the load rate, the greater the proportion of no-load loss, and the higher the voltage regulation power saving rate", but to achieve the best intelligent energy saver power saving rate, it also depends on the voltage regulation amplitude. Although voltage reduction can reduce iron loss, when the voltage drops to a certain level, if it continues to drop, the current will increase again, thereby increasing copper loss. Therefore, to achieve the best energy saving effect, a reasonable voltage regulation coefficient must be achieved.   

As the voltage decreases, the motor load remains unchanged, the slip rate increases, and the motor output power also decreases. Therefore, in the actual measurement process, the active power saved will be larger than the theoretical calculation. Since the torque of the motor is proportional to the square of the voltage, if the torque of the motor remains unchanged, the slip rate is approximately proportional to the square of the voltage.   

From the above analysis, we can see that the energy saving rate of the energy saver depends on the efficiency of the motor and the load characteristics. When the motor is lightly loaded, the voltage reduction range is adopted. Therefore, when the motor is subjected to voltage reduction and energy saving transformation, careful analysis of the motor's operating characteristics is conducive to estimating the energy saving rate, and will be conducive to cost control and recovery.

2.3 Energy-saving principle based on thyristor   

The voltage is controlled by using a triac to "cut" the phase angle between the motor voltage and current. The triac only allows part of the positive and negative half-cycles of the supply voltage to be supplied to the motor.

As shown in Figure 1, the result is a reduction in the RMS voltage supplied to the motor, which results in the minimization of hysteresis losses, the return of the phase angle to its original size, and the improvement of the efficiency of the smart energy saver motor. The current that maintains the operation of the motor is composed of two different parts: load or resistive current and inductive or excitation current. Inductive current depends on voltage and flux density. To a certain extent, resistive current is also a function of voltage. This saves energy by reducing the supply voltage to the motor. The hardware part of the motor electrical design control system based on a single-chip microcomputer is shown in Figure 2.

It consists of an AT89S52 single-chip computer system, a synchronous detection circuit, a thyristor trigger circuit and an external signal processing interface.

3.1 Single-Chip Microcomputer System   

The system uses ATMEL's AT89S52 microcontroller, which contains 3 timers, 2 external interrupt sources, 8KB Flash memory, 256B internal data memory, and multiple protection functions. There is no need to expand the interrupt and timing control chip in the system to fully meet the working requirements of this system.   

The circuit is provided with two intelligent power saver function setting ports and a fault output port. One port can set the setting mode of the thyristor. When the port is closed, it is given by an external signal. The other port is used to control the trigger pulse. When the port is closed, there is a pulse output. When the port is disconnected, the thyristor pulse is forced to be turned off. When the system is not working properly, the fault output port is closed and an external alarm device can be connected. The LED signal light is used to display the load working status and the cause of the circuit failure.

3.2 Synchronous detection circuit   

The synchronous detection circuit is shown in Figure 3, which consists of a synchronous transformer, a comparator and an optoelectronic isolation device. When the power supply voltage passes through zero in the positive direction, the microcontroller system is interrupted.

3.3 Thyristor drive circuit   

The thyristor drive circuit is shown in Figure 4. The trigger signal sent by the single-chip microcomputer P1.0 is photoelectrically isolated, amplified by the transistor, and transformed by the pulse transformer to form the thyristor trigger signal G1 K1G2 K2. The circuit is similar to this circuit.

3.4 External signal interface circuit   

When the given signal is output by other regulators or given by external potentiometers, the given analog signal (CO-10V) or (C1-SV) is set by the program and enters the single-chip computer through A/D conversion. The A/D converter uses the high-speed 12-fold A/D converter AD574 with a conversion speed of 251}s. The intelligent power saver fully meets the requirements of the system. After current detection, the voltage detection signal is rectified, filtered, etc., and the signal enters the single-chip computer after A/D conversion.

4. Software Design   

After the system is powered on, the microcontroller first initializes the programming chip. After the initialization is completed, the port data is collected, and the given method of the trigger angle (external port voltage given) is determined according to the port data. The given value is converted into a corresponding time value and stored in the specified unit. The interruption caused by the synchronous detection circuit sends the trigger signal of the thyristor, and according to the external setting, the corresponding content (such as load voltage, current, thyristor trigger angle given value, etc.) is displayed. The main program flow chart is shown in Figure 5.

According to the program flow, the software is functionally organized into the following modules: (1) Initialization module: completes the initialization of the A/D chip, the setting of the control registers in the microcontroller, etc. (2) Input module: determines the external input, and calls the corresponding functional subroutines according to the requirements of different functions, such as circuit fault display, current cutoff feedback setting, etc. (3) Display subroutine: displays the circuit fault and working status. (4) Synchronous detection subroutine: after the hardware circuit detects that the grid voltage passes through zero, it causes an interrupt to the microcontroller. After entering the interrupt subroutine, the time value corresponding to the trigger angle is loaded into the counter to start the counter. When the counting time is reached, the microcontroller timing interrupt is triggered to send out the corresponding trigger pulse.   

Through various functional modules, the load voltage (current) is continuously adjustable, and the actual voltage (current) is compared with the given value. When designing the energy saver, fuzzy control is introduced into the control system. Fuzzy control does not rely on the precise mathematical model of the controlled object and can overcome the influence of nonlinear factors. Using the fuzzy intelligent energy saver controller in the control system of the AC motor energy saver can fully reflect its characteristics of adapting to nonlinearity and time-varying rapid response, and can achieve significant power saving effects.

5 Conclusion   

The developed motor energy saver has a significant power saving effect. Especially for those motors that are often under low load or have frequent load changes, the average power saving rate is 1600-3000, which improves the power factor and reduces the line loss of the power grid and the copper loss of the transformer. It also has perfect soft start and soft stop functions, which can ensure the continuous and smooth start of the motor, eliminate the mechanical noise and large starting current generated by the conventional starting method, effectively reduce the wear of the bearings and belts, and reduce the mechanical stress of the rack and gear, thereby extending the service life of the motor. It adopts intelligent energy saver microprocessor control, without manual adjustment. Under light load conditions, the motor voltage automatically drops to the minimum demand while the speed remains constant.

references:

[1] He Limin, Selected Microcontroller Application Technology (1) [M], Beijing: Beijing Aerospace Press, 1993.

[2] Lu Anding, Chen Wei, et al., Practical Application of Energy Saving Reconstruction of Electric Motors [M], Shanghai: Shanghai Science and Technology Press, 1994.

[3] Wu Renrong, Electrical Engineering[M], Beijing: Water Resources and Electric Power Press,

[4] Xu Zhihong et al., Motor voltage reduction technology [J], Energy Conservation and Environmental Protection 2003, (9): 40-41.