The protection of asynchronous motors is a complex issue. In practical applications, appropriate protection devices and starting equipment should be selected according to the motor's capacity, type, control method, and power distribution equipment. The relationship between motor protection and control is often intertwined; protection involves control, and control involves protection. For example, when a motor is directly started, it often generates a starting current of 4-7 times its rated current. If controlled by a contactor or circuit breaker, the contacts must be able to withstand the connection and disconnection tests of the starting current. Even frequently operated contactors can experience accelerated contact wear, potentially damaging the equipment. For molded case circuit breakers, even infrequent operation makes it difficult to meet these requirements. Therefore, they are often used in series with a starter in the main circuit. In this case, the contactor in the starter handles the connection of the starting current, while other electrical components only handle the disconnection of the motor's overload current during normal operation. The protection function is handled by the corresponding protection device. Furthermore, contactless control methods, i.e., soft-start control systems, can also be used for motor control. The main circuit of the motor is connected and disconnected by thyristors. To avoid continuous losses in these components, some systems use vacuum contactors (connected in parallel with the thyristors) to carry the load of the main circuit during normal operation. This control can be programmable or non-programmable; local or remote; slow or fast start, etc. Furthermore, relying on electronic circuitry, it is easy to achieve various protection functions similar to electronic relays. Motor damage is mainly caused by overheating of the windings or deterioration of insulation performance, and overheating of the windings is often caused by excessive current flowing through the windings. Motor protection mainly includes two types: current detection and temperature detection. The following is a brief introduction to some products. Current detection type protection devices: Thermal relays utilize the load current flowing through a calibrated resistive element, causing the bimetallic heating element to bend, thus causing the relay contacts to operate before the motor windings burn out. Its operating characteristics are close to the permissible overload characteristics of the motor windings. Although the accuracy of the operating time of thermal relays is generally not high, they can provide effective overload protection for motors. With continuous improvement and refinement of structural design, in addition to temperature compensation, it also features phase loss protection and load imbalance protection functions. Examples include the T-series bimetallic thermal overload relays imported from ABB; the 3UA5 and 3UA6 series bimetallic thermal overload relays imported from Siemens; and the JR20 and JR36 thermal overload relays, with the JR36 being a secondary development product that can replace the obsolete JR16. Circuit breakers with thermal-magnetic tripping for motor protection are used for thermal overload protection. Their structure and operating principle are the same as thermal relays. When the bimetallic thermal element bends, some directly push against the tripping device, while others close the contacts, ultimately causing the circuit breaker to trip. The electromagnet has a higher setting value and only operates during short circuits. Its simple structure, small size, low price, operating characteristics conforming to current standards, and reliable protection have led to its continued widespread use, especially in small-capacity circuit breakers. For example, the M611 type motor protection circuit breaker imported from ABB, the domestically produced DW15 low-voltage universal circuit breaker (200-630A), and the S-series molded case circuit breakers (100, 200, 400A). Electronic overcurrent relays detect fault current signals through internal phase current transformers, process them through electronic circuitry, and then execute corresponding actions. The electronic circuitry is flexible and offers diverse operating functions, widely meeting the protection needs of various types of motors. Its features include: ① Multiple protection functions. There are three main types: overload protection, overload protection plus phase loss protection, and overload protection plus phase loss protection plus reverse phase protection. ② Selectable operating time (compliant with GB14048.4-93 standard). Standard type (10A class): 7.2In (In is the motor's rated current), 4-10s operation, used for standard motor overload protection; Fast-acting type (10A class): 7.2In, 2-10s operation, used for submersible motors or compressor motor overload protection. Slow-acting type (30 levels): Operates in 9-30 seconds at 7.2In, used for overload protection of motors with long starting times, such as blower motors. ③ Wide current setting range. The ratio of its maximum to minimum value is generally 3-4 times, or even larger (1.56 times for thermal relays), making it particularly suitable for applications where motor capacity frequently changes (e.g., mines). ④ Fault indication. The fault category is displayed by an LED, facilitating maintenance. Solid-state relays have evolved from simple electronic devices performing relay functions to microprocessor devices with various functions. Their cost and price vary depending on the function; the most complex relays are practically only used for larger, more expensive motors or critical applications. Its main monitoring, measurement, and protection functions include: ① maximum starting inrush current and time; ② thermal memory; ⑤ prolonged acceleration of large inertial loads; ④ phase loss or unbalanced phase current; ⑤ phase sequence; ⑥ undervoltage or overvoltage; ⑦ overcurrent (overload) operation; ⑧ stall rotor; ⑨ loss of load (shaft breakage, conveyor belt disconnection, or pump drowsiness causing a drop in operating current); ⑩ motor winding temperature and load bearing temperature; ⑩ overspeed or stall. Each of these pieces of information can be programmably input into a microprocessor, primarily with the necessary time limit added to ensure that the power supply is cut off before damage occurs during motor startup or operation. The fault type and cause can also be displayed using LEDs or digital displays, and data can be output to a computer. Circuit breakers for motor protection with electronic tripping operate on a similar principle to the aforementioned electronic overcurrent relays or solid-state relays. The main functions include: circuit parameter display (current, voltage, power, power factor, etc.), load monitoring (disconnecting or connecting loads according to regulations), multiple protection characteristics (exponential curve inverse time, I²t curve inverse time, definite time, or combinations thereof), fault alarm, testing function, self-diagnostic function, and communication function. Examples include the M-series low-voltage circuit breakers manufactured by Schneider Electric. Soft starters use thyristors in their main circuit. The protection device controlling their disconnection or connection is generally a fault detection module, used to detect abnormal faults before and after motor starting, such as phase loss, overheating, short circuit, leakage, and unbalanced load, and issue corresponding action commands. Its characteristics include a simple system structure, which can be implemented using a microcontroller, making it suitable for industrial control. Temperature detection type protection devices include bimetallic strip temperature relays, which are directly embedded in the motor windings. When the motor is overloaded and the winding temperature rises to near its limit, the bimetallic strip with a contact bends due to heat, causing the contact to open and cut off the circuit. An example is the JW2 temperature relay. A thermal protector is a thermal overload protection relay installed on the motor body. Unlike a temperature relay, it uses a cup-shaped bimetallic strip with two contacts connected in series in the motor circuit. Both the overload current flowing through it and the motor temperature cause it to heat up. When a certain temperature is reached, the bimetallic strip instantly trips, the contacts open, and the motor current is interrupted. It can be used for temperature, overload, and phase loss protection of small three-phase motors. Products include the sPB and DRB type thermal protectors. A detection coil temperature measurement relay has 1-2 detection coils embedded in each phase winding of the motor stator, and the winding temperature is monitored by an automatic balancing thermometer. A thermistor temperature relay is directly embedded in the motor winding. Once the specified temperature is exceeded, its resistance increases sharply by 10-1000 times. In use, it is equipped with electronic circuit detection, and then the relay is activated. Products include the JW9 series marine electronic temperature relays. Coordination between protection devices and asynchronous motors: To ensure the normal operation of asynchronous motors and their effective protection, the coordination between the asynchronous motor and the protection device must be considered. Especially when using small-capacity asynchronous motors in large-capacity power grids, the coordination of protection systems becomes even more crucial. The coordination between overload protection devices and motors is as follows: The operating time of the overload protection device should be slightly longer than the motor's starting time. The characteristics of the motor overload protection device must avoid the characteristics of the motor's starting current to ensure its normal operation; however, its operating time cannot be too long, and its characteristics can only provide overload protection within the motor's thermal characteristics. The instantaneous operating current of the overload protection device should be slightly larger than the motor's starting inrush current. If some protection devices have an instantaneous overload action function, their operating current should be larger than the peak value of the starting current to ensure normal motor starting. The operating time of the overload protection device should be slightly shorter than the conductor's thermal characteristics to function as backup protection for the power supply line. The coordination between overload protection devices and short-circuit protection devices is also important. Generally, overload protection devices do not have the ability to interrupt short-circuit currents. Once a short circuit occurs during operation, a short-circuit protection device (such as a circuit breaker or fuse) connected in series in the main circuit is needed to disconnect the circuit. If the fault current is small and falls within the overload range, the overload protection device should still disconnect the circuit. Therefore, there should be selectivity between the two actions. The characteristics of the short-circuit protection device are illustrated using a fuse as a representative example. The intersection current with the overload protection characteristic curve is Ij. Considering the dispersion of fuse characteristics, there are two intersection currents: Is and IB. In this case, overcurrents of Is and below should be disconnected by the overload protection device, while overcurrents of Ib and above, up to the allowable limit short-circuit current, should be disconnected by the short-circuit protection device to meet the selectivity requirement. Obviously, it is difficult to ensure selectivity within the Is-IB range. Therefore, this range should be as small as possible. According to the current IEC standard, the limit values ​​are Is = 0.75Ij and Ib = 1.25IJ. Currently, the rated making and breaking capacity of overload protection devices are assessed based on 0.75IJ, which is obviously too low. Judging from the trend of IEC standard revision, it is possible that the rated making and breaking capacity will be assessed in the future to improve its reliability. Therefore, the above coordination should consider both its selectivity and its rated making and breaking capacity. In conclusion, the protection of asynchronous motors is one of the key aspects of the reliable and normal operation of electrical installations and mechanical equipment. Directly detecting the temperature of the motor windings to protect against overheating caused by overload is a very effective protection method, but due to the need to directly embed the protection device into the motor windings, it is expensive and difficult to maintain, and is only used in some frequently operated applications. From an economic perspective, current-sensing relays are more advantageous, and heating relays remain a cost-effective, simple, and reliable form of motor protection (currently the most widely used in practice). For the protection of large-capacity motors with high operational performance requirements, comprehensive functions, or high costs, electronic or solid-state relays can be used. For general requirements, circuit breakers with thermal-magnetic tripping for motor protection are more practical. However, regardless of the protection device used, the coordination between the overload protection device and the motor, and between the overload protection device and the short-circuit protection device, must be considered.