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This post was last edited by sdscj0122 on 2020-3-14 at 17:20. It presents nine configuration options for the main circuit of electric motors in a graphical format, allowing readers to understand the application characteristics of each option; it is intended as a reference for power supply technicians and instrument technicians. This article covers 9 schemes, with one diagram corresponding to each scheme. It can be observed that from Scheme 1 to Scheme 9, all motor circuits have plug symbols above the circuit breaker, and there are also plug symbols above the motors; hence, these 9 schemes represent the main motor circuits in low-voltage drawer-type switchgear. The configuration of the motor main circuit can be found at yunrun.com.cn/tech/2884.html. Schemes 1 to 4 are schemes for direct motor starting, but the components of the main circuit differ slightly: ① In Scheme 1, the circuit breaker is a miniature circuit breaker MCB with thermal-magnetic protection. The MCB is equipped with long-delay L protection against overload and instantaneous I protection against short circuits. The other two components in the main circuit are a current transformer and an AC contactor. Option 1 is generally used for the direct starting of low-power motors. http://yunrun.com.cn/upload/202001/19/202001191440560873.png ②The main circuit of Option 2 is equipped with a thermal relay, and the circuit breaker is of the single-magnet type; therefore, Option 2 can be used for the direct starting of motors with higher power ratings. http://yunrun.com.cn/upload/202001/19/202001191421479066.png ③ The thermal relay in Option 3 is located on the secondary side of the current transformer; therefore, this circuit can drive motors with higher power ratings. http://yunrun.com.cn/upload/202001/19/202001191429268621.png Note: The bimetallic strip in the thermal relay is bypassed using a time relay during motor startup, so that the thermal relay does not send out any protection signal at that time. ④Plan 4 uses an MCU to protect the motor, where the MCU (Motor Control Unit) refers to the comprehensive motor protection device. http://yunrun.com.cn/upload/202001/19/202001191409163079.png Schemes 5 to 7 are reversible starting schemes for motors, but the main circuit components differ slightly. ⑤Scheme 5 is similar to Scheme 1; it uses a MCB with thermal and magnetic protection, and is generally used for the reversible starting of low-power motors. http://yunrun.com.cn/upload/202001/19/202001191441184053.png ⑥Solution 6 is similar to Solution 2: a thermal relay is included in the circuit, while the circuit breaker is of the single-magnet type; therefore, Solution 6 can be used for the reversible starting of motors with higher power ratings. http://yunrun.com.cn/upload/202001/19/202001191417157184.png ⑦Plan 7 is similar to Plan 3; the thermal relay is located on the secondary side of the current transformer, so this circuit can drive motors with higher power. http://yunrun.com.cn/upload/202001/19/202001191451255819.png Schemes 8 and 9 are motor star-delta starting schemes. ⑧The circuit breaker in Option 8, like those in Options 2 and 6, is a single-magnet circuit breaker ； http://yunrun.com.cn/upload/202001/19/202001191407290517.png Option 9 is the approach that uses an MCU. http://yunrun.com.cn/upload/202001/19/202001191407587689.png Explanation of the main circuit schemes using miniature circuit breakers dedicated for motors. Schemes 1 and 5 are suitable for the main circuits of motors with a power rating of 0.08~11 kW that are started directly. The MCB in question is a product specification designed specifically for motor protection. Table 1 shows the configuration scheme using the ABB motor protection miniature circuit breaker MS25. Table 1 Configuration schemes for MS325: http://yunrun.com.cn/upload/202001/19/202001192234173627.png Explanation of the main circuit scheme using thermal relays. As mentioned earlier, it is known that thermal relays are used in the primary circuit in schemes 2, 3, and 6; in scheme 6, the thermal relay is installed in the secondary circuit. Thermal relays encounter the issue of motor overload starting during use. If the motor’s starting current exceeds 6 times the rated value, or if the starting time is more than 10 seconds, then the motor’s starting process is referred to as a heavy-load start. For the main circuit scheme of motors with heavy-starting loads, special measures need to be taken for the thermal relay. **Table 2 in section 5.7.3.2 of the standard GB 14048.4-2010 \"Low-voltage switchgear and control gear – Low-voltage magnetic contactors and motor starters\" provides specific definitions; for details, see section 2.6.2. The relevant information is summarized as follows: Grade, Tripping time/s – 10A: 2＜Tp≤10; 10; 4＜Tp≤10: 20; 6＜Tp≤20: 30; 9＜Tp≤30. Grades 20 and 30 in this list refer to thermal relays designed specifically for starting motors under heavy load conditions; these relays have a relatively long delay before tripping in order to accommodate the surge in current that occurs during motor startup. Let’s take a look at ABB’s motor starting configurations for light and heavy loads under a rated voltage of 400V and a short-circuit current of 50kA, as shown in Table 2: http://yunrun.com.cn/upload/202001/19/202001192316126993.png ① When the motor starts, it is used in conjunction with a bypass contactor of the same specification. The purpose of the bypass contactor is to bypass the current input and output (temperature sensing) ports of the thermal relay. As can be seen from Table 2, the main circuit for heavy-load starting of motors of 3 kW and below requires a bypass contactor connected in parallel to the primary circuit of the thermal relay, while the main circuit for heavy-load starting of motors ranging from 5.5 to 75 kW requires a thermal relay of type 30. For the main circuit of motors over 90 kW, whether for normal starting or heavy-load starting, the thermal relays are all of type 30; therefore, the main circuit configuration remains the same. Description of the main circuit scheme using motor star-delta starting. In the motor star-delta starting main circuit, there are circuit breakers, current transformers, star-connected contactors and delta-connected contactors, main contactors, and thermal relays, among others. Table 3 shows ABB’s star-delta starting motor circuits for 18.5~200kW. Table 3 Motor star-delta starting configuration at a rated voltage of 400V and a short-circuit current of 50kA. http://yunrun.com.cn/upload/202001/20/202001201350497628.png In the table, the combination of contactors and circuit breakers is of type 2. The coordination among components in the motor’s main circuit: Information regarding the coordination between the circuit breakers (fuses) and contactors in the motor’s main circuit can be found in the \"Type testing for coordination between contactors and low-voltage electrical devices for short-circuit protection\". Here is a brief review: 1. Types of coordination between contactors and thermal relays. When an overload or short-circuit fault occurs in the main circuit of a three-phase asynchronous motor, the contacts of the contactor may weld together. Therefore, standard GB 14048.1-2012 specifies two types of coordination: ① Coordination type 1: It allows the components inside the motor starter to be damaged during an overload or short-circuit fault, but normal operation can be restored after replacing those components. ②Combination type 2: It allows the contacts inside the motor starter to weld together only during overload or short-circuit faults, and normal operation can be restored after repair using simple tools. 2. Basic configuration of the motor control main circuit: circuit breaker + contactor + thermal relay. The most basic configuration for a motor’s main circuit is a combination of a circuit breaker, a contactor, and a thermal relay; the trip characteristic curves for this combination are shown in the figure below. The tripping characteristics of the combination are described as follows: ① When the motor starts, within the first 30 ms of startup, a peak starting inrush current I appears in the motor’s main circuit; this value is approximately (8~12)IN. The I rating of the circuit breaker must be greater than Ip, otherwise the circuit breaker may trip accidentally during motor startup. ②The starting current of the motor at 1 s is (4~8.4) In. ③Once the motor has started, it enters its rated operating condition. The thermal overload protection setting value of the thermal relay should be between (1.05~1.20)In, as shown in Figure 4-21, which depicts the tripping characteristics of the circuit breaker + contactor + thermal relay combination. The setting value of the thermal relay must ensure that it does not trip within two hours when the overload current is 1.05In, but it must trip within two hours when the overload current is 1.20In. ④When a motor experiences a short circuit, although the short-circuit current is limited by the cables, the I parameter of the circuit breaker must trigger a protective action against this short-circuit current. The setting current I3 for parameter I should be no less than Ip, that is, no less than 12In. Generally, the I parameter is set to (12~15)In. Most importantly, the maximum current rating of the thermal relay must be greater than I3; in other words, the thermal tolerance limit current of the thermal relay must lie to the right of the set value of the circuit breaker’s I parameter, to ensure that the circuit breaker can provide protection for the thermal relay. ⑤When the motor is overloaded, the thermal relay and the contactor will experience an overload current for an extended period of time; eventually, the thermal relay sends a protection signal that causes the contactor to trip, and an arc will form at the contacts of the contactor ； Similarly, when the motor short-circuits, the circuit breaker performs a short-circuit protection trip, but the short-circuit current also flows through the thermal relay and the contactor. If the thermal relay and contactor meet Type 1 requirements, they may be damaged during overload or short-circuit conditions; the user must carry out maintenance and replacement afterward ； If the thermal relay and contactor meet Type 2 requirements during overload or short-circuit conditions, it is not necessarily the case that they will be damaged; the user only needs to properly maintain the contacts of the contactor.