Abstract This article reviews the status of motor starting and protection methods before 2007 and forecasts the application of motor starting and protection in 2008. Keywords: Traditional methods, hard starting, soft starting, improved protection, liquid resistance . 1. Introduction Traditional motor starting methods mainly use autotransformer reduction and star-delta starting. However, the latter has limitations; it can only be used when the starting torque is not too high. In recent years, low-voltage motor starting has evolved into electronic soft starting and frequency converter starting, while high-voltage motor starting has also evolved into starting primarily using liquid resistance voltage reduction, and some have adopted high-voltage frequency converter starting. Previously, low-voltage motor protection mainly used thermal relays with phase loss protection for overload protection and phase loss protection, while high-voltage protection used traditional relays to achieve overcurrent, overload, and locked-rotor protection, generally achieving good motor starting and protection. 2. Review of starting and protection methods in 2007 In 2007, the starting methods of motors have changed from the traditional hard starting methods such as autotransformer reduction and star-delta starting to a situation where hard starting and soft starting each account for half. The protection methods are also constantly being improved, evolving from traditional thermal element protection to new electronic protection with various functions. This is a relatively rapid technological advancement. 2.1 Current Status of Hard Starting Methods Traditional hard starting methods, such as autotransformer reduction and star-delta starting, have advantages such as simple circuits, convenient maintenance, and low initial cost. However, their starting current is still relatively large, posing a significant impact on the motor and its connected mechanical equipment. Although two starting tap voltages are provided to select according to the actual load conditions, if the starting torque is large, selecting a lower voltage tap will prevent the starting process from completing, meaning the starting current will not decrease. If a higher voltage is selected, the starting current will be even larger, resulting in a greater impact on the motor and power grid. This often leads to insulation breakdown of the motor and transformer, accidental tripping of some main switches in the power grid, and makes setting certain protection parameters of the power grid difficult. The AC contactor, a key component of the starter, is also prone to burnout. In particular, the secondary control circuit of traditional national standard hard starters uses a time relay to achieve automatic switching between starting and running. Over time, if this time relay fails, it will no longer be able to perform its automatic switching function. Thus, prolonged operation in the starting state can cause the motor to overload and be damaged due to undervoltage. Furthermore, the autotransformer coil, unable to deactivate, will burn out due to continuous operating current, affecting the safe operation of electrical equipment. This hard-start method was commonly used in early starting methods, primarily due to its lower cost, while soft-start is more expensive. Traditional protection in all hard-start systems typically uses thermal relays with phase-loss protection for overload and phase-loss protection. However, this method has many drawbacks and is becoming an obsolete starting and protection method. 2.2 Basic Situation of Soft Start In recent years, electronic soft starters have become a successful alternative to hard starters. They offer advantages such as low starting current, continuous and smooth starting and acceleration, and a continuous change in starting torque from small to large. They also add several practical protections compared to hard starters, effectively solving the problems existing in traditional starting and protection methods. A soft starter is a novel motor control device integrating soft starting, soft stopping, light-load energy saving, and multiple protection functions. Its main components are three anti-parallel thyristors connected in series between the power supply and the controlled motor, along with their electronic control circuit. By using different methods to control the conduction angle of three anti-parallel thyristors, the input voltage of the controlled motor can be varied according to different requirements, thus adapting to the starting of motors with different loads. A frequency converter is also a type of soft starter, but it has more advantages than ordinary soft starters: it can not only achieve slow soft starts and has better energy-saving effects, but it can also be effectively combined with other automatic control devices to form a mature automatic control system; its output changes not only the voltage but also the frequency; a soft starter is actually a voltage regulator, used for motor starting, its output only changes the voltage and not the frequency. A frequency converter has all the functions of a soft starter, but it is much more expensive and has a much more complex structure. A soft starter, used in series between the power supply and the controlled motor, controls the conduction angle of its internal thyristors, causing the motor input voltage to gradually rise from zero according to a preset function until the start-up is complete, providing the motor with full voltage; this is soft starting. During soft starting, the motor's starting torque and speed gradually increase. Generally, the working principle of a soft starter has the following starting methods. 2.2.1 Ramp-up Soft Start: This starting method is the simplest, lacking closed-loop current control. It only adjusts the thyristor conduction angle to increase it in a function of time. Its disadvantage is that, due to the lack of current limiting, a large inrush current can sometimes be generated during motor startup, potentially damaging the thyristor and significantly impacting the power grid. Therefore, it is rarely used in practice. 2.2.2 Ramp-up Constant Current Soft Start: In this starting method, the starting current gradually increases during the initial stage of motor startup. Once the current reaches a preset value, it remains constant (from t1 to t2) until startup is complete. During startup, the rate of current increase can be adjusted according to the motor load. A higher current increase rate results in a larger starting torque and a shorter startup time. This is the most widely used starting method, especially suitable for starting fan and pump loads. 2.2.3 Step Start: Step start aims to rapidly reach the set starting current value in the shortest possible time. By adjusting the starting current setting, a rapid startup effect can be achieved. 2.2.4 Pulse Impact Starting: In the initial stage of starting, the thyristor conducts with a large current for a very short time, then drops back, and then rises linearly according to the original set value, connecting to constant current starting. This method is rarely used in general loads and is suitable for heavy loads and starting applications that need to overcome large static friction. 3. Outlook for Starting and Protection Methods in 2008: In 2008, motor starting will mainly use electronic soft starting, while frequency conversion starting will be appropriately adopted. High-voltage starting will also gradually promote the use of new liquid resistance reduced-voltage starting. Hard starting will be basically eliminated in new projects. For some existing hard starting systems, maintenance and modification plans will be made according to actual conditions, and soft starters will be gradually replaced when conditions are ripe. 3.1 Comparison of Soft Starting and Traditional Reduced-Voltage Starting Methods: Traditional reduced-voltage starting methods for squirrel-cage motors include star-delta starting, autotransformer reduced-voltage starting, and reactor starting. These starting methods all belong to stepped reduced-voltage hard starting, which has obvious disadvantages, namely, the occurrence of secondary inrush current during the starting process. The differences between soft starting and traditional reduced-voltage starting methods are as follows. 3.1.1 The inrush current-free soft starter gradually increases the thyristor conduction angle during motor startup, causing the motor starting current to linearly rise from zero to the set value. 3.1.2 The constant current starting soft starter can introduce current closed-loop control, maintaining a constant current during motor startup and ensuring smooth starting. 3.1.3 The starting current can be freely and steplessly adjusted to the optimal starting current based on load conditions and grid relay protection characteristics. 3.1.4 Achieving soft stop: Traditional motor shutdown methods involve instantaneous power cuts, but in many applications, instantaneous motor shutdown is unacceptable. For example, in water pump systems supplying high-rise buildings, instantaneous shutdown can cause a significant "water hammer" effect, damaging pipes and even the pumps. To reduce and prevent water hammer, the motor needs to be gradually and slowly stopped—a soft stop. In pumping stations, soft starters meet this requirement, and soft stop technology avoids damage to the pumping station's "flip gate," reducing maintenance costs and workload. The soft-stop function in a soft starter involves the thyristor gradually reducing its conduction angle from full conduction after receiving a stop command, transitioning to full shutdown after a certain time. The stopping time can be adjusted between 0 and 120 seconds according to actual needs. 3.1.5 Achieving Light-Load Energy Saving A squirrel-cage induction motor is an inductive load, and its operating current lags behind the voltage. If the motor's operating voltage remains constant, the power factor is lower under light load and higher under heavy load. A soft starter can achieve energy saving under light load by reducing the motor terminal voltage, thereby increasing the power factor and reducing copper and iron losses; under heavy load, it automatically increases the motor terminal voltage to ensure normal motor operation. 3.2 Protection Functions of Soft Starters The protection functions of soft starters are much better than those of traditional hard starters. 3.2.1 Overload Protection Function The soft starter introduces a current control loop, which can continuously track and monitor changes in the motor current. Overload protection is achieved by adding an overload current setting and an inverse time control mode, enabling the thyristor to be shut off and an alarm signal to be issued when the motor is overloaded. 3.2.2 Phase Loss Protection Function: During operation, the soft starter continuously monitors changes in the three-phase line current. If a current interruption occurs, it will trigger a phase loss protection response. 3.2.3 Overheat Protection Function: The soft starter uses an internal thermal relay to detect the temperature of the thyristor heatsink. Once the heatsink temperature exceeds the allowable value, it automatically shuts off the thyristor and issues an alarm signal. 3.2.4 Other Functions: The soft starter, combined with other protection components, can achieve functions such as stall protection. Furthermore, through the combination of electronic circuits, various other interlocking protections can be implemented in the system. 3.3 Soft Starter MCC Control Cabinet: The MCC (Motor Control Center) control cabinet is the motor control center. Soft starters can be easily integrated into MCC control cabinets, which consist of the following parts: ① an input circuit breaker; ② a soft starter (including electronic control circuitry and three-phase thyristors); ③ a bypass contactor for the soft starter; ④ a secondary control circuit (which selects and operates manual start, remote start, soft start, and direct start functions), with voltage and current displays and indications for fault, operation, and working status. Most soft starters have bypass contactor contacts on both sides of the thyristors, which has the following advantages: ① The control cabinet has two starting methods (direct start and soft start); ② When the soft start ends, the bypass contactor closes, causing the soft starter to exit operation until it is restarted when the machine stops. This extends the life of the soft starter, avoids harmonic pollution to the power grid, and reduces the heat loss of the thyristors in the soft starter. 3.4 Extended Functions of Soft-Start MCC Control Cabinets Further combining soft-start MCC control cabinets can achieve various composite functions. By adding control logic to two control cabinets, a "one-in-one-out" solution can be formed for building fire protection systems and sprinkler pumps, domestic water pumps, and other systems. If equipped with a PLC (Programmable Logic Controller), the fire pump can be automatically detected at set intervals (e.g., every half month) and automatically shut down at set intervals. With corresponding control logic, the normal operation of the fire pump and various systems can be monitored. Normally, it operates at low speed and low water pressure (no water output) at set intervals; during firefighting, it operates at full speed and full load. Combining several motors with control logic can form a domestic water pump system or other specialized systems. Motors can be turned on sequentially as needed, or the number of motors can be gradually reduced to achieve optimal efficiency. Furthermore, according to customer requirements, multiple motors can be automatically switched during operation, ensuring that each motor operates within the same lifespan. 3.5 Application Scenarios of Soft Starters In principle, squirrel-cage induction motors are suitable for all applications where speed adjustment is not required during operation. The current application range is AC 380V (660V is also possible), with motor power ranging from a few kW to 800kW. Soft starters are particularly suitable for various pump or fan loads that require soft starting and soft stopping. Similarly, for variable load conditions, where the motor operates under light load for extended periods and only experiences short-term or momentary heavy loads, using a soft starter (without a bypass contactor) provides energy savings under light load conditions. Of course, in this case, using a frequency converter for starting results in even more significant energy savings. In principle, squirrel-cage induction motors are suitable for all applications where speed regulation is not required during operation. The current application range is AC 380V (660V is also possible), with motor power ranging from a few kW to 800kW. Soft starters are particularly suitable for various pump or fan loads that require soft starting and soft stopping. Similarly, for variable load conditions, where the motor operates under light load for extended periods and only experiences short-term or momentary heavy loads, using a soft starter (without a bypass contactor) provides energy savings under light load conditions. Of course, in this case, using a frequency converter for starting results in even more significant energy savings. 3.6 High-voltage liquid resistance reduced pressure starting The high-voltage liquid resistance reduced pressure starting method has been widely used in recent years, but it has also made new developments, and the performance of its products has been continuously improved. 3.6.1 QXQ-S high-voltage squirrel cage (including wound) motor liquid resistance speed regulation device Liquid (liquid resistance, water resistance) starting high-voltage squirrel cage motor technology has a strong competitive advantage compared with high-voltage frequency converters, switch transformer type soft starters and traditional high-voltage reactor soft starters. It is mainly reflected in the maturity of intelligent control technology, less maintenance and low price. Although the previous high-voltage squirrel cage motor starting equipment can achieve soft start and soft stop, the disadvantage is that it cannot solve the problems of frequent soft start/stop and energy-saving speed regulation during operation. This is mainly because problems such as withstand voltage, insulation, temperature rise and automatic control are difficult to solve. 3.6.2 Speed ​​regulation performance characteristics of QXQ-S series speed regulation device (1) It is mainly composed of liquid resistance system, cooling system, automatic control system, actuator and other parts. (2) Energy saving rate can reach 20% to 50%, can be operated on-site and remotely controlled, can communicate with computers, and is flexible in operation and installation. (3) There are three types of speed controllers: relay type, PLC type and single-chip microcomputer control type, each with its own characteristics, which can also meet the needs of customers in different industries. Compared with other speed control methods, it has excellent performance, simple operation, reliable operation and convenient maintenance. (4) It also has the soft start and soft stop functions of liquid resistance starter, which can directly and conveniently start motors, especially high-voltage squirrel cage motors. (5) It is widely used in the speed control of motors such as fans, pumps, mills and rolling mills in industries such as steel, petrochemical, metallurgy, power, building materials, water conservancy, grain processing and pharmaceuticals. Its electrical cost performance is high. In 2008, the starting of high-voltage motors will still be based on hydraulic coupling speed control devices used in conjunction with mechanical equipment, with conventional liquid resistance and frequency converter starting as the fundamental methods. The QXQ-S series high-voltage squirrel-cage (including wound-rotor) motor liquid resistance speed control devices will be gradually promoted. The protection of high-voltage motors will fully adopt comprehensive microcomputer protection systems, enabling the widespread application of new products. 4. Conclusion: In 2008, the electrical starting and protection methods of China's industrial enterprises will, based on summarizing the successful experiences prior to 2007, gradually promote the use of new technologies to further maintain the safety, reliability, and stability of electrical equipment starting and operation, continuously promote the growth of enterprise economic benefits, and achieve the sustainable, rapid, coordinated, and healthy development of enterprises.