Research on Soft Start and Optimized Energy-Saving Control Techniques for AC Asynchronous Motors

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1 Preface At present, a large number of AC asynchronous motors are used in industrial and mining enterprises (including low-voltage motors of 380V/660V and medium-voltage motors of 3KV/6KV). A considerable number of these asynchronous motors and their drive systems are operating in an inefficient manner, resulting in significant waste of electrical energy. The reasons for this are mainly as follows: ① Since most motors are started directly, in addition to causing impacts on the power grid and the drive system as well as leading to accidents, the starting current of 8 to 10 times results in significant energy losses. 　　②When selecting the capacity of electric motors, there is often a one-sided pursuit of a large safety margin, with additional requirements added layer by layer. As a result, the motor capacity becomes excessive, leading to a situation where a large motor is used for a task that requires a smaller motor; this causes the motor to deviate from its optimal operating point, thereby reducing its operational efficiency and power factor. 　　③From the perspective of the operational efficiency of the production machinery driven by electric motors, it is often necessary for electric drive systems to have the capability to adjust voltage and speed; using a fixed-speed and fixed-voltage drive will inevitably result in significant additional energy losses. The inefficient operation of motors has long attracted the attention of **relevant authorities, who formulated and revised a mandatory standard in 1990 and 1995 respectively: ‘Economic Operation of Three-Phase Asynchronous Motors’ (GB12497-1995). It is hoped that this will help to regulate the economical operation of three-phase asynchronous motors. The issuance of national standards has played a significant role in promoting the economical operation of low-voltage motors, but it has had little effect on medium-voltage motors. The reasons are: (1) Medium-voltage motors generally have a large capacity, and once a fault occurs, its impact is significant; therefore, higher reliability is required for power-saving measures ； (2) The energy-saving measures for medium-voltage motors are limited by the voltage tolerance of power electronic devices, making the development of energy-saving products more technically challenging. To date, no mature products for soft starting and energy-saving operation of medium-voltage motors are available on the domestic market. 2 Soft starting of asynchronous motors: With the continuous updating and development of industrial machinery, increasingly higher demands are being placed on the starting performance of motors. These demands can be summarized as follows: ① The motor is required to have a sufficiently large starting torque that can increase smoothly, as well as mechanical characteristic curves that meet specified requirements ； ②As low a starting current as possible ； ③The starting equipment should be as simple, economical, and reliable as possible, with easy starting operations ； ④Power consumption during startup should be as low as possible. Based on these conflicting requirements and the actual conditions of the power grid, two common starting methods are used: one is direct starting at the rated voltage, and the other is reduced-voltage starting. 2.1 Hazards of direct starting Direct starting is the simplest method of starting, in which the motor is connected directly to the power grid through a switch or contactor at the time of startup. The advantage of direct starting is that the starting equipment is simple and the starting speed is fast. But direct startup poses great risks ； ① Grid disturbances: Excessively high starting currents (the no-load starting current can be 4 to 7 times the rated current, while it can reach 8 to 10 times or more during start-up under load) can cause a drop in grid voltage, affecting the proper operation of other electrical devices. They may also trigger under-voltage protection, resulting in harmful shutdowns of the equipment. At the same time, excessive starting current can cause the motor windings to heat up, thereby accelerating insulation aging and affecting the motor’s lifespan. 　　②Mechanical shock: Excessive shock torque often causes fractures in the rotor bars and end rings of the motor, as well as wear and damage to the insulation of the stator end windings, leading to breakdown and engine damage ； Twisted shafts, damaged couplings and drive gears, as well as torn belts, etc. 　　③Impact on production machinery: Sudden pressure changes during startup often cause damage to the pipes and valves in the pump system, reducing its service life ； It affects transmission accuracy and even normal process control. All of these pose a threat to the safe and reliable operation of the equipment, and they also result in excessive starting energy losses, especially when starting and stopping frequently. Therefore, the following constraints apply to the direct starting of electric motors: ① It is a prerequisite to determine whether the production machinery permits the direct starting of the electric motor ； ②The capacity of the motor should not exceed 10~15% of the capacity of the power supply transformer ； ③The voltage drop △U during startup should not exceed 15% of the rated voltage. For medium and high-power motors, direct starting is generally not allowed; instead, certain starting equipment is required to enable normal starting. 2.2 Application scenarios and performance comparison of the traditional voltage-reduction starting method: The purpose of voltage-reduction starting is to reduce the starting current, but it also results in a decrease in the starting torque. For heavy-duty starting, production machinery with high peak loads cannot be started in this way. The traditional voltage-sinking starting methods include the following: (1) Star/delta converter: This method is suitable for motors whose stator windings are connected in delta during normal operation. The stator has six terminals that lead out to the change-over switch; it is connected in star configuration during startup, and then switched to delta configuration after startup is complete. The starting voltage is 220V, and the operating voltage is 380V. The advantage of this starting device is that it is simple in design and consumes less energy during the starting process. The disadvantages include secondary current surges, a high failure rate of the equipment, and the need for frequent maintenance; therefore, it is not suitable for use in devices that start up frequently. During the conversion process, the potentials generated by transient voltages and the residual magnetism of the motor often have a phase difference from the supply voltage; in severe cases, these voltages add together, resulting in excessive inrush currents and electromagnetic torque. Therefore, **this limits its use. Since the starting voltage is of the operating voltage, its starting torque is 1/3 of the rated torque; it can only be used in devices that need to start under no load or with light load (load rate less than 1/3). When the motor is operating under light load or no load, this starting device can also be used for reduced-voltage operation in order to improve the motor’s power factor and efficiency. (2) Reduced-voltage starting with an autotransformer: The high-voltage side of a three-phase autotransformer (also known as a compensator) is connected to the power grid, while the low-voltage side is connected to the motor. It usually has several taps, allowing for different voltage ratios to be selected for loads requiring varying starting torques. It is disconnected after the motor starts. Its advantage is that the starting voltage can be selected, such as 0.65, 0.8, or 0.9UN, to meet the requirements of different loads. The disadvantages are large size, heavy weight, high consumption of non-ferrous metals, high failure rate, and high maintenance costs. (3) Magnetic control soft starter: A magnetic control soft starter utilizes the principle of voltage regulation through magnetic field control; during motor startup, the voltage can rise smoothly from a lower value to full voltage, allowing the torque on the motor shaft to increase gradually. This results in softer starting characteristics and enables soft stopping. However, its switching voltage is around 200V, which cannot be adjusted by the user; it causes significant current surges and is also large in size. (4) For high-voltage motors, a reactor or water resistor can be connected in series in the stator circuit to achieve reduced-voltage starting, and it can then be removed after the starting process is complete. However, reactors are expensive, and water resistance causes significant losses. (5) For wound-rotor asynchronous motors, a frequency-sensitive rheostat or water resistor can be connected in series with the rotor windings to enable starting, and it can then be removed after the starting process is complete. However, frequency-sensitive resistors are expensive, and water resistors incur high losses. Other methods include Y-bonded triangle starting and stator series resistance starting. It is worth noting that although various traditional voltage-sinking starting methods have their own advantages and disadvantages, they share one common advantage: namely, no harmonic pollution. 2.3 New types of electronic soft starters With the development of power electronics technology and microcomputer control technology, a series of electronic starting control devices have been developed at home and abroad for the starting control of asynchronous motors, to replace traditional reduced-voltage starting devices. The main circuit of modern electronic soft starters generally uses a thyristor voltage regulation circuit, which is composed of six thyristors connected in parallel in opposite pairs and connected in series across the motor’s three-phase power supply lines. When the microcomputer control system of the starter receives a start command, it performs the necessary calculations and outputs trigger signals for the thyristors. By controlling the conduction angle β of these thyristors, the starter adjusts the output voltage in accordance with the designed pattern, thereby controlling the starting process of the motor. Once the starting process is complete, a conventional starter will engage the bypass contactor, shorting out all the thyristors so that the motor can operate directly connected to the power grid, thereby avoiding unnecessary energy losses. The control block diagram of the soft starter is shown in Figure 1. Figure 1 Control block diagram of the soft starter. By \"soft starting\", what is meant is a voltage-reduction starting process that follows a pre-set control pattern. Current soft starters generally offer the following starting methods: (1) Current-limiting soft start: As the name implies, current-limiting starting is a soft starting method that limits the starting current of the motor to not exceed a certain set value (Im) during the startup process. It is mainly used for step-down starting of loads that require light-load startup. Its output voltage rises rapidly from zero until the output current reaches the preset current limit Im, after which the output current I remains constant