Asynchronous motor is widely used in engineering industry. When the motor starts, it will produce a larger current, which will have a greater impact on the system itself and the power grid, and cause a waste of electricity. When the motor load is less than 75% of the rated load, the efficiency is low, resulting in energy waste. After theoretical analysis, an intelligent energy-saving controller for motor, i.e. three-phase asynchronous motor, is designed for energy-saving, soft starting and operation protection of motor. China’s energy policy is to focus on energy resources conservation and fair use. To alleviate the contradiction between China’s energy resources and economic and social development, we must base ourselves on our country and improve the efficiency of energy resources utilization. We should resolutely implement the policy of developing and saving at the same time and put saving first. We will encourage the development and application of new technologies for energy conservation and consumption reduction. Energy conservation is a long-term strategic policy for China’s economic and social development, and it is also an extremely urgent task at present. Motor is the biggest user of power consumption, and also the biggest user of power saving potential. Motor is the most important power equipment in industrial production. According to statistics, the electricity consumption of motor accounts for more than 50% of the total power generation. The efficiency of the motor is the highest around the rated load, usually more than 80%.
When the load decreases, the efficiency decreases significantly. The selection of motor is based on the maximum load and the power required in the worst case. Therefore, in most cases, the motor runs under light load, and in light load or uneven load, the efficiency of the motor is low. Therefore, improving the operation efficiency of these motors can significantly save electricity. The basic principle of voltage regulation and energy saving is that the efficiency of asynchronous motor under light load is very low, and the end voltage of the input motor is reduced to improve the efficiency by reducing the no-load loss. When the motor terminal voltage decreases, the air gap main flux decreases proportionally from_E2_U2, and the excitation component Io in the motor stator current also decreases. Due to the decrease of saturation, the power of Io decreasing with E2 is greater than 1. But when_decreases, if the load torque of the motor remains unchanged, the rotor current I2 will rise, with I2_1/_1/E2. The effects of these changes on motor losses are as follows: rotor copper loss PAI_I22_1/E22, stator iron loss PFE_phi 2_E12, stator current I1 is the vector sum of I 2 and Io. When the voltage drops properly, the current I1 can be reduced and the copper consumption can be reduced accordingly. In general, the mechanical loss does not change much, and the stray loss varies with the stator and rotor currents. Therefore, whether the total loss can be reduced depends on the relationship between the iron loss `stator copper loss’and the rotor copper loss, and whether the stator current can be reduced is very important. When the motor is under light or no load, the proportion of Io component in I1 is larger than that of I2 component.
The stator current can be reduced, and the step-down operation can achieve the purpose of reducing voltage and saving energy. We choose the current-limiting soft-start mode, so we can use embedded chip to monitor the current in real time, reduce the large impact current generated in the process of motor starting, and reduce the loss of equipment. In order to achieve fast response and no static error, the digital PI regulator can be selected to adjust the control system. Integral regulator (I) always has a delay in output response due to the effect of integration, but as long as there are errors, the integration process will not stop, and will eventually stabilize in the expected output value. The proportional regulator (P) has a fast response speed, but will never be stable at a given value. Therefore, the combination of the two, giving full play to their respective advantages, not only achieves the rapidity, but also eliminates the error. The process of adaptive operation of the system is as follows: the corresponding voltage regulation coefficient K and other parameters of asynchronous motor, such as power factor and energy saving rate, are calculated in advance according to the optimal control scheme when the load rate is m. When the reference input (m, K) is added to the induction motor and the reference model at the same time, the initial parameters of the motor are uncertain, so the output response (power factor) of the initial motor will not be completely consistent with the output response (power factor) required by the optimal control scheme, resulting in a deviation signal e (t), when the signal e (t) enters. After entering the adaptive adjusting circuit, the appropriate additional control effect K is generated by the operation prescribed by the adaptive law, and the stator terminal voltage of the induction motor is automatically changed, so that the output response of the motor is gradually close to the output response under the optimal control, that is, the power factor of the motor and the optimal control scheme after the adaptive voltage regulation. The power factor is the same. Finally, when the deviation signal e(t)=0, the adaptive adjustment process stops. The controller mainly includes the main circuit, control circuit and drive protection circuit. The main circuit is mainly composed of three pairs of bidirectional thyristors and contactors. By controlling the conduction of the bidirectional thyristors, the voltage added to the stator end of the motor can be changed. The main function of the contactor is to bypass the bidirectional thyristors from the three-phase circuit after the soft start process is completed. When soft parking or motor load changes are needed, the soft start device is connected to the motor circuit to complete soft parking or energy saving functions. In the control circuit and drive protection circuit, it includes voltage detection, current detection, main control chip (TMS320LF2407), thyristor trigger circuit, contactor drive, RS-232 host computer serial communication circuit and auxiliary switching power supply.
Voltage detection: In the voltage detection circuit, two functions are realized. One is the detection function of synchronous signal, which sampled at zero-crossing time of three-phase voltage as synchronous signal of thyristor pulse trigger signal; the other is transforming three-phase power supply voltage signal into DC signal after transformer step-down, and then sent to DSP after AD conversion as voltage negative feedback regulation, fault detection, over-voltage and under-voltage. Pressure protection. Current detection: Three-phase current signal is converted into voltage signal by Hall sensor, and then the voltage signal is converted by AD and sent to DSP as current negative feedback regulation, fault detection and over-current protection. Thyristor trigger circuit: Using the control signal given by the DSP, the pulse signal with a certain pulse width is sent out after the pulse transformer to drive the thyristor to turn on, and the magnitude of the voltage added to the motor is changed by controlling the conduction angle. The main control computer chip, TMS320LF2407, is the core of the system. It is mainly responsible for processing the detection signal, adjusting the phase shift range, giving the driving signal of thyristor and contactor, receiving the input control signal and output data, thermostatic element etc. The system software consists of the following parts: system initialization module, serial communication between PC and RS-232 and the design of closed-loop control subroutine. The function of the system initialization module is mainly to check and initialize the system. It mainly includes system memory and I / O port detection, and returns to the upper computer to alarm if the fault is found. The main function of serial communication module between PC and RS-232 is to control parameter transmission and data display. Closed-loop control subroutines mainly include current-limiting startup program design and pulse trigger synchronous signal interrupt design, as well as pulse delay trigger interrupt program design. The energy-saving controller is used to test a J0 series 4-pole motor with 3.0kW. When the motor runs around rated load, its efficiency and power factor change little, which indicates that the energy-saving effect is not obvious at this time. With the decrease of load rate, the operation efficiency of the motor improves gradually, and the power factor increases more obviously. Theoretical analysis and experimental results show that when the load ratio of motor is greater than 0.5, the effect of reducing voltage and saving energy is not obvious; when the load ratio of motor is between 0.3 and 0.5, the efficiency of motor does not change much, but the power factor improves obviously and has a certain energy-saving effect; when the load ratio is less than 0.3, the effect of reducing voltage and saving energy is obvious, and the efficiency and power factor of motor are improved greatly. Through analysis, it can be found that the motor operating efficiency is higher in the range of 0.75-1 load rate, but the efficiency is not the highest when the load rate is 1. This energy-saving operation scheme can make the asynchronous motor show higher efficiency under different loads, and take into account the progressive power factor. In practical application, because the stator voltage and stator current of asynchronous motor are easier to measure correctly than the power factor, the energy-saving scheme of selecting the stator voltage as the control variable of asynchronous motor has high practical value. The results show that the energy-saving effect of installing energy-saving controller is obvious when the load rate of motor is less than 0.3. When the change of motor parameters caused by load change is neglected, the torque of asynchronous motor will decrease with the decrease of stator voltage. Therefore, when taking step-down measures to save energy, the step-down behavior is also restricted by whether the electrical performance can drive the normal operation of the load. In order to ensure that the electromagnetic torque of the motor must be able to overcome the no-load torque and drive the normal operation of the load, it is necessary to verify its torque. Generally, the overload ratio of asynchronous motor is 1.8-3.7, and the minimum range of voltage reduction is approximately (0.27-0.56). In summary, with the development of society and economy, human demand for energy is increasing. How to better achieve energy conservation and emission reduction has become a topic of worldwide concern. Asynchronous motor plays a very important role in daily life, especially in industrial production, and is the main consumer of electric energy. In practical applications, for various reasons, asynchronous motors often operate under no-load or light-load conditions, which results in low power factor, low efficiency and large power waste. Therefore, the development of a practical energy-saving controller for asynchronous motor is of great practical significance to reduce energy consumption and production costs.