A novel voltage sampling type motor protector

　　In the use of three-phase AC motors, they are increasingly used in three-phase irreversible transmission equipment (such as water pumps, fans, air compressors, elevator motors, etc.). In order to enable the motor to be protected in time under phase failure, phase sequence, overvoltage, and undervoltage conditions, the corresponding protector should have the above protection functions to ensure the normal and safe operation of the motor and equipment. In voltage sampling protectors, the operating voltage of the motor is usually monitored through the protector to determine its voltage status (such as overvoltage, undervoltage, phase sequence, phase error, and the above fault indication is displayed on the protector panel), such as If there is a fault, the protector controls the control appliances (such as AC contactor) in the three-phase power supply circuit and cuts off the main circuit power supply, thereby effectively protecting the motor and transmission equipment.

　　Compared with the current sampling type, the biggest feature of the voltage sampling type protector is that it has nothing to do with the power of the protected electrical equipment, but only protects based on the quality of the power grid, so that the protected electrical equipment can avoid damage caused by poor power quality. Especially in three-phase irreversible equipment or feeder lines, after the phase sequence is determined, when the power supply line is connected to the wrong phase due to repair or change, the protector can identify the original phase sequence, thereby affecting the motor. and corresponding equipment for protection.

Protector protection characteristics

　　Usually the voltage sampling type protector mainly protects the motor phase loss and phase sequence. However, in actual operation, the working voltage of the motor is often under voltage or over voltage, but it is difficult to effectively protect it. According to the actual working conditions of the protected motor , the wide working range is the novelty of its protector; within this working range (AC three-phase 300V~460V), the protector realizes its undervoltage and overvoltage protection action characteristics (see Figure 1 and Figure 2 for the action characteristics). In addition, based on the above protection functions, the protector can also set the voltage values ​​of overvoltage protection (380V~460V adjustable) and undervoltage protection (300V~380V adjustable) according to needs, and its protection delay is also adjustable (Overvoltage protection: delay time is adjustable from 0.5S to 5S; undervoltage protection: delay time is adjustable from 1S to 10S), which expands the protection function and use space of the voltage sampling protector.

　　In the undervoltage and overvoltage operating characteristics of the protector (Figure 1), characteristic line 1 is the single-phase over- and under-voltage characteristic line, characteristic line 2 is the two-phase over and under-voltage characteristic line, and characteristic line 3 is the three-phase over and under-voltage characteristic line. .

Figure 1 Undervoltage and overvoltage operating characteristics of the protector

Motor protector

　　In the design, considering that the protector should have three protection functions of overvoltage, undervoltage and phase failure, a 3-input NAND gate and a voltage comparator are used to complete this function. If the above fault occurs, the input terminals ⑧, ②, and ① of the IC4B group (Figure 2) determine the wrong phase, overvoltage, and undervoltage faults respectively. At this time, the signal input is "0" level and is inverted by the IC4C group ( Use this set of output terminals ⑥ to drive the working state of V1) to cut off V1, change KA1 from the pull-in state to the release state, and control the contact conversion. In Figure 2, the contact of KA1 is in the released state (when V1 is cut off), and when the protector is in normal working state, KA1 is in the pull-in state (when V1 is amplified).

Figure 2 AS-11 motor protector schematic diagram

Wrong phase protection

　　The three-phase power supply is detected through terminals A, B, and C, and the three-phase power supply is added to the RC phase shift circuit respectively. If the phase sequence is certain, the _nw vector at both ends n and w is smaller. By rectifying and filtering the voltage , so that the L3 (out of phase) indicator cannot be driven, then the optocoupler IC2 (isolates the sampling signal from the operating voltage) is in a cut-off non-conducting state. As a result, the input terminals and terminals of IC4A are both in "0" state, and the output terminal ⑩ is "1". At this time, it mainly depends on the changes of the input terminals ② and ① of IC4B. If there is a wrong phase or phase loss fault, the _nw vector increases, and L3 and IC2 work through the rectification and filtering of this voltage. When the input terminal of IC4A is "1", the output terminal ⑩ is "0", which causes V1 to be cut off, KA1 to be released, and the protector to be in a protective working state.

Undervoltage working protection

　　When the ⑨ terminal (undervoltage) voltage of the IC1A group is the same as the ⑩ terminal (voltage divided by the IC4A terminal through R8 and R9 resistors), the ⑩ terminal voltage in IC1A is U⑩>U⑨, then the ⑧ terminal is "1", and the undervoltage indicator L1 works. AND comparison in IC1B (charge C5 through RP3 and R11, set the undervoltage protection time); when U

Overvoltage protection:

　　When the voltage of terminal ⑤ (overvoltage) of IC1C group is compared with terminal ⑥, U⑤＞U⑥, terminal ⑦ outputs "1", the overvoltage indicator L2 works, terminal ③ and terminal ② in IC1D (RP4, R17 to C9 Charging, overvoltage protection time setting) U③＜U②, then terminal ① is "0", then terminal ② in IC4B is "0", V1 is cut off, KA1 is released, and the protector is in the protection state.

Typical application circuit

　　A typical application circuit of the AS-11 motor protector is shown in Figure 3.

Figure 3 Application circuit example diagram

Conclusion

　　This voltage sampling motor protector has a wide protection voltage range in both overvoltage and undervoltage situations, and the voltage protection parameter value and protection time can be set, which provides users with great convenience in use. In addition, it protects the most common phase failure and phase sequence of the motor without being limited by the power of the motor equipment. This is the biggest feature of the voltage sampling protector and is now widely used.

References:

1. Qian Jinchuan, Jin Linsheng: A brief analysis of electronic motor protectors, Machine Tool Electrical Appliances, Issue 2004.2