Abstract: Asynchronous motors are widely used in industrial and agricultural production. During motor startup, a large current is generated, significantly impacting the system itself and the power grid, resulting in energy waste. When the motor load is below 75% of its rated load, efficiency is low, leading to further energy waste. Based on theoretical analysis, an intelligent energy-saving controller for motors is designed for energy saving, soft starting, and operational protection. Keywords: DSP, soft starting, energy saving, adaptive control 1 Introduction China's energy policy emphasizes the conservation and rational utilization of energy resources. To alleviate the contradiction between China's energy resources and economic and social development, it is essential to focus on domestic resources and significantly improve the efficiency of energy resource utilization. A firm policy of simultaneous development and conservation, prioritizing conservation, is crucial. The development and application of new energy-saving and consumption-reducing technologies are encouraged. In 2004, China paid tens of billions of US dollars for rising prices of electricity, coal, and oil, while the economic losses indirectly caused by energy shortages are difficult to quantify. Energy conservation is a long-term strategic policy for China's economic and social development and an extremely urgent task at present. Electric motors are the largest consumers of electricity and also the users with the greatest potential for energy saving. In industrial production, electric motors are the most important prime movers, accounting for over 50% of total electricity generation, according to statistics. Motors operate at their highest efficiency near rated load, typically above 80%, but efficiency drops significantly as the load decreases. Since motor selection is based on the maximum required load and the power needed under worst-case conditions, motors typically operate under light loads. Under light or uneven loads, motor efficiency is lower. Therefore, improving the efficiency of these motors can significantly save energy. Asynchronous motors have poor starting performance; the full-voltage starting current is approximately 4 to 8 times the rated current. For high-power motors, this can significantly impact the power grid, affecting the normal operation of other electrical equipment on the same grid. Simultaneously, full-voltage starting also puts a significant strain on the motor's mechanical components, shortening their lifespan. Using soft-start measures, gradually increasing the starting voltage until normal operation, reduces the impact on the power grid during motor starting and decreases the stress on the mechanical components. The biggest problem with AC induction motors is that their output torque cannot match the load torque during starting and operation. When a motor starts, it typically generates 150% to 200% of its normal torque within a fraction of a second, increasing the load to normal speed. This can cause significant impact torque damage to the drive structure. Simultaneously, the motor generates a starting current 4 to 8 times higher than normal, affecting the stability of the power supply system. When the motor is under half-load for an extended period, its copper coil windings generate excessive magnetic flux, leading to a decrease in motor efficiency. This current (usually called induced current) is constant, causing the motor to waste approximately 30% to 50% of its electrical energy. 2. Theoretical Analysis of Voltage Regulation Energy Saving and Current Limiting Soft Start 2.1 Basic Principle of Voltage Regulation Energy Saving The basic principle of voltage regulation energy saving is to utilize the low efficiency of asynchronous motors under light loads, reducing the input terminal voltage to the motor to reduce no-load losses and improve efficiency. After the motor terminal voltage decreases, the air gap main magnetic flux also decreases proportionally, from Φ∝E2∝U2; the excitation component Io in the motor stator current also decreases accordingly. Due to the decrease in saturation, the value of Io decreases with E2 by a power greater than 1. However, when Φ decreases, if the motor's load torque remains unchanged, the rotor current I2 will increase, with I2 ∝ 1/Φ ∝ 1/E2. These changes affect motor losses as follows: rotor copper loss PAI ∝ I2² ∝ 1/E2², stator iron loss PFE ∝ φ2 ∝ E1², and stator current I1 is the vector sum of I2 and Io. When the voltage decreases appropriately, current I1 can decrease, and copper loss will decrease accordingly. Mechanical losses generally do not change much, while miscellaneous losses vary with stator and rotor currents. Therefore, whether the total loss can be reduced depends on the relationship between iron loss, stator copper loss, and rotor copper loss, and importantly, on whether the stator current can be reduced. When the motor is lightly loaded or unloaded, the proportion of Io in I1 is larger, and the proportion of I2 is smaller, so the stator current can be reduced. Reduced voltage operation can achieve the purpose of energy saving. 2.2 Current-limiting soft-start principle. The simplified Γ-type model of the asynchronous motor is shown in Figure 1. Figure 1. Simplified Γ-type model of asynchronous motor. r1 and r2 are the resistance and leakage reactance of the stator winding, respectively, and and are the resistance and leakage reactance of the rotor after conversion, respectively. rm and xm are the excitation resistance and excitation impedance, respectively, and s is the slip. The mathematical expression of the mechanical characteristics of asynchronous motor can be obtained from the simplified model: At startup, the rotor speed n2 = 0, and the slip S = 1. At this time, the mechanical characteristic equation is: Since the motor starts, from the simplified model of the motor discussed above, at startup, S = 1, which is much larger. Ignoring the excitation current, and considering Zm as an open circuit, then: The relationship between the starting torque MST and the stator current per phase is as follows: Where: MN: rated electromagnetic torque of asynchronous motor; IN: rated current per phase of asynchronous motor; SN: rated slip of asynchronous motor. Usually, the SN of induction motor is very small, generally 0.01 to 0.05, so to obtain a larger Mst multiple, a larger starting current multiple is required. During current-limited starting, a larger current-limiting value Ist results in a larger starting torque Mst for the motor. Therefore, the shorter the time it takes for the motor to reach a stable speed, the faster the start-up. However, to ensure a sufficient starting torque, the starting current cannot be too small, and the current-limiting value must be appropriately chosen. In high-torque starting applications, current-limited starting may not be suitable. This is because a large starting current Ist is required to meet the large starting torque. When the starting current Ist is too large, it can cause a significant impact on the power grid. Therefore, current-limited starting is not always suitable for high-torque starting applications. 3. Control Strategy We selected a current-limited soft-start method, allowing for real-time monitoring of the current using an embedded chip, reducing the large inrush current generated during motor starting and minimizing equipment losses. To achieve fast response and zero steady-state error, a digital PI controller can be selected for the control system. Due to the integral action, the integral controller (I) will always have a delay in output response, but as long as an error exists, the integral process will not stop and will eventually stabilize at the expected output value. While proportional regulators (P) have a fast response speed, they never stabilize at the given value. Therefore, combining both leverages their respective strengths, achieving both speed and eliminating errors. The adaptive operation process is as follows: The voltage regulation coefficient K corresponding to the load rate m and other operating parameters of the asynchronous motor, such as power factor and energy saving rate, are calculated beforehand according to the optimal control scheme. When the reference input (m, K) is simultaneously applied to the inputs of both the asynchronous motor and the reference model, due to the uncertainty of some initial parameters of the motor, the initial output response (power factor) of the 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 signal e(t) enters the adaptive adjustment loop, it undergoes calculations defined by the adaptive law, generating an appropriate additional control action ΔK, automatically changing the stator terminal voltage of the asynchronous motor. This gradually brings the motor's output response closer to the output response under optimal control, meaning that the power factor of the motor after adaptive voltage regulation is consistent with the power factor under the optimal control scheme. Finally, when the deviation signal e(t) = 0, the adaptive adjustment process stops. 4 Hardware Design This controller mainly includes a main circuit, a control circuit, and a 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 applied to the stator of the motor is changed; while 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 stop is required or the motor load changes, the soft start device is connected to the motor circuit to complete the soft stop or energy saving function. The control circuit and drive protection circuit include 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, etc. The schematic diagram of the DSP control system is shown in Figure 2. Here is a brief introduction. (1) Voltage detection: Two functions are implemented in the voltage detection circuit. One is the detection function of the synchronization signal. It samples at the zero-crossing moment of the three-phase voltage and serves as the synchronization signal for the thyristor pulse trigger signal. The other is to convert the three-phase power supply voltage signal into a DC signal after being stepped down by a transformer, and then send it to the DSP after being converted by AD, for voltage negative feedback regulation, fault detection, overvoltage and undervoltage protection. (2) Current detection: The three-phase current signal is converted into a voltage signal by a Hall sensor, and then this voltage signal is sent to the DSP after being converted by AD, for current negative feedback regulation, fault detection and overcurrent protection. (3) Thyristor trigger circuit: Using the control signal given by the DSP, a pulse signal with a certain pulse width is sent out after the pulse transformer to drive the thyristor to conduct, and the voltage applied across the motor is changed by controlling the conduction angle. (4) Main control microcomputer TMS320LF2407 chip: It is the core of the system and is mainly responsible for processing the detection signal, adjusting the phase shift range, giving the drive signal of the thyristor and contactor, receiving the input control signal and outputting data, etc. Figure 2 DSP Control System Schematic Diagram 5 Software Design Figure 3 System Software Flowchart As shown in Figure 3, the system software consists of the following parts: system initialization module, serial communication between the PC and RS-232, and closed-loop control subroutine design. The main function of the system initialization module is to perform system self-test and initialization. This mainly includes system memory and I/O port detection; if a fault is detected, it immediately returns an alarm to the host computer. The main function of the serial communication module between the PC and RS-232 is to transmit control parameters and display data. The closed-loop control subroutine mainly includes current-limiting start program design, pulse-triggered synchronous signal interrupt design, and pulse-delayed trigger interrupt program design. 6 Experimental Results Analysis An experiment was conducted using this energy-saving controller on a 3.0kW J0 series 4-pole motor. The experimental data is shown in the attached table. The attached table shows the experimental results of the motor running under different loads. From the table, it can be seen that when the motor is running near the rated load for energy saving, its efficiency and power factor do not change much, indicating that the energy-saving effect is not obvious at this time; as the load rate decreases, the motor's operating efficiency gradually improves, and the power factor increases more significantly. Theoretical analysis and experimental results show that when the motor load rate is greater than 0.5, the energy-saving effect of voltage reduction is not significant; when it is between 0.3 and 0.5, the motor efficiency does not change much, but the power factor is significantly improved, resulting in some energy-saving effect; and when it is less than 0.3, the energy-saving effect of voltage reduction is significant, with both motor efficiency and power factor greatly improved. Analysis also reveals that the motor operating efficiency is relatively high in the load rate range of 0.75 to 1, but not at its highest at 1. This energy-saving operation scheme allows the asynchronous motor to exhibit high efficiency under different loads while also improving the power factor. In practical applications, since the stator voltage and stator current of the asynchronous motor are easier to measure accurately than the power factor, the energy-saving scheme of adjusting the voltage under light load for asynchronous motors using stator voltage as the control variable has high practical value. Research results show that when the motor load rate is less than 0.3, the energy-saving effect of installing an energy-saving controller is significant. When the changes in motor parameters caused by load changes are ignored, the torque of the asynchronous motor decreases as the stator voltage decreases. Therefore, when taking voltage reduction measures for energy saving, the voltage reduction behavior is also constrained by whether the motor can drive the load normally. To ensure that the electromagnetic torque of the motor can overcome the no-load torque and drive the load normally, torque verification is required. The overload multiple of an asynchronous motor is typically 1.8–3.7, and the minimum voltage reduction range is approximately (0.27–0.56). 7 Conclusion The intelligent controller, an embedded system, controls the firing angle of the thyristors, thereby controlling the starting voltage of the motor, which is unmatched by other traditional starting methods. It ensures smooth starting under the required starting characteristics of the load, reducing the impact on the power grid, and guarantees reliable motor starting. It has protections against overcurrent, phase loss, and phase sequence detection, and also saves energy. This product has stable performance, high reliability, and a power saving rate of over 33%, a power factor improvement of more than 2 times, smooth soft starting, and improved power grid quality. This achievement is a utility model product, integrating motor soft starting, energy-saving operation, and multiple protections. This device is suitable not only for applications with slow load changes but also for applications with rapid and large load changes, such as hoists, water supply stations, and deep water wells in oil fields. It has been applied to oilfield pumping units and, after a period of operation, has demonstrated stable performance and significant energy savings, resulting in good economic benefits.