summary
Jiang Wandong, a researcher at Jiangsu State Grid Automation Technology Co., Ltd., published an article in the December 2018 issue of the journal *Electrical Technology*, introducing a low-voltage motor protection and anti-voltage fluctuation device. This device mainly consists of a low-voltage motor protection module and an AC contactor anti-voltage fluctuation module. This article describes the composition and working principle of the low-voltage motor protection module and the AC contactor anti-voltage fluctuation module, as well as the device's structure and main functional logic, and analyzes its applications and main technical specifications.
Low-voltage motors are important mechanical drive components in industrial systems. With the development of industrial modernization and the continuous improvement of process control, the requirements for their protection reliability and uninterrupted operation are becoming increasingly stringent. However, short-term voltage drops (commonly known as "voltage fluctuations") caused by lightning strikes, short circuits, reclosing, and the starting or fault clearing of equipment in the same section of the power grid occur frequently.
Low-voltage motor control circuits often use AC contactors. When the system experiences a voltage drop, the AC contactor may trip. Therefore, in applications requiring long-term continuous operation, anti-voltage-drop measures for the contactor must be considered. Currently, anti-voltage-drop measures for contactors mainly employ the undervoltage restart function of low-voltage motor protection devices and the use of independent anti-voltage-drop holding modules.
Anti-power-drop holding modules are advantageous when used in conjunction with power supply fast switching devices to enable rapid system power restoration. However, when the system experiences a power drop, using conventional low-voltage motors and anti-power-drop modules presents two problems: ① The reduction in bus voltage can cause the power supply module of the low-voltage motor protection device to malfunction, resulting in a loss of motor protection; ② While the anti-power-drop holding module can prevent the contactor from releasing during a power drop, the reduction in bus voltage causes a decrease in motor output torque. Under constant load conditions, this decrease in output torque will lead to an increase in motor current, and the protection device will then fail due to the power drop.
This article introduces a low-voltage motor protection and anti-voltage fluctuation device. This device can both realize the anti-voltage fluctuation function of AC contactors and avoid the problem of low-voltage motor protection function failing during voltage fluctuations.
1. Composition and working principle of the anti-power fluctuation module
Figure 1 shows the block diagram of the AC contactor anti-voltage fluctuation module. The module mainly consists of four parts: ① supercapacitor module energy storage and power conversion section; ② variable DC voltage output and AC/DC switching section; ③ current and voltage acquisition and switching interlocking section; ④ display and data communication functional section.
Terminals KL and KN in the module are for AC contactor control voltage input. The input voltage is processed by the module and output to terminals XL and XN. Terminals PL and PN are the module's power input terminals. Two RJ11 interfaces are internally connected to an RS485 bus; externally, they can be connected to a motor protection module and another anti-power-slip module using a 4-core wire. Address DIP switches are used to set the ID number of the anti-power-slip module to distinguish different modules. Terminals DO1 and DO2 are for the normal operation of the power-slip signal alarm relay.
Figure 1 Block diagram of the anti-power fluctuation module
Open node output. Two 8-segment LED displays are used to display the power-off setting time and module event codes.
The supercapacitor module's energy storage and power conversion section contains one AC/DC power module and three DC/DC power modules. POW1 in Figure 1 is a wide-input AC/DC power module with an input voltage range of AC85–265V and an output of DC15V, with a maximum output of 1000mA. When the system power supply is normal, the AC/DC module charges the supercapacitor module through the current-limiting resistor R; when the voltage drops below AC85V, diode D2 conducts while diode D1 is cut off, ensuring that the supercapacitor bank only supplies power to the downstream load.
POW2 and POW3 are non-isolated DC/DC power modules. POW2 has an input voltage of DC 9-36V and an output voltage of DC 96V with a maximum output of 500mA. POW3 has an input voltage of DC 60-96V and an output voltage of DC 220V with a maximum output of 10mA. POW3 has an output current limiting function, and its main function is to store energy for the high-voltage capacitor E2. POW4 is an isolated DC/DC power module with an input of DC 9-36V and an output of DC 5V with a maximum output of 1200mA, providing power to the microprocessor and control circuits.
The variable DC voltage output and AC/DC switching section consists of a half-bridge chopper circuit, an AC/DC switching circuit, and a switching protection circuit. The half-bridge chopper circuit is shown in Figure 2. The microprocessor controls PWM1 to output a voltage signal with a continuously adjustable duty cycle. This voltage signal is connected to the drive circuit after optocoupler isolation. The upper bridge arm Q1 and the lower bridge arm Q2 of the half-bridge chopper circuit conduct alternately, forming an adjustable DC voltage VDC for external output.
Figure 2 Block diagram of half-bridge chopper circuit
The AC/DC switching circuit is shown in Figure 3. The normally closed contact of relay DLJ and electronic switch S1 are connected in parallel and then in series to the KL circuit. Electronic switch S2 is connected in series with diode D and then in parallel to the KL circuit. During initial power-on, the anti-power-slip module cuts off electronic switches S1 and S2, closes the normally closed contact of DLJ, and directly connects the KL, KN, and KL*, KN* circuits. 20ms after the contactor closes, the microprocessor controls electronic switch S1 to first open the normally closed contact of DLJ and then open it. During system power slippage, electronic switch S1 is controlled to first cut off S2 and then open it, cutting off the AC circuit while simultaneously outputting VDC voltage to KL* and KN*.
Figure 3 Block diagram of AC/DC switching circuit
The current sensor CT converts the current signal of the contactor control circuit into a voltage signal AIac. After processing, the AIac signal is compared with the switching latch value Vset. If AIac is greater than the latch value, the latch will latch the AC/DC switching circuit, and the control logic will keep the normally closed contact DLJ closed, while S1 and S2 will be cut off. If AIac is less than the latch value and is manually confirmed, the program outputs a reset signal RES to release the switching lock. The anti-power-dip module's microprocessor is a 16-bit microcontroller with a DSP engine from Microchip Technology Inc., model dsPIC33FJ128GP306, with 128KB of on-chip Flash and 16KB of on-chip RAM.
2. Composition and Working Principle of Motor Protection Module
The low-voltage motor protection module is shown in Figure 4. The module consists of four parts: ① power processing and energy storage; ② main circuit current and voltage acquisition; ③ digital input/output; ④ communication interface.
The power processing and energy storage section includes an AC/DC module, a DC/DC module, and a supercapacitor bank. The supercapacitor capacity of the low-voltage protection module is smaller than that of the anti-voltage fluctuation module. The anti-voltage fluctuation module's supercapacitor capacity must account for the energy released to the contactor during a voltage fluctuation, while the protection module only needs to account for the energy stored in the supercapacitor to ensure the module continues to operate for a period of time (power interruption time) when a voltage fluctuation occurs and the input voltage drops below AC85V, causing the AC/DC power supply to fail.
The protection module uses a VV connection to acquire the bus voltage. After analog-to-digital conversion via the microprocessor's ADC interface, it calculates the amplitude and phase of the line voltages UAB and UBC. The motor main circuit current is typically converted into a small voltage signal by an external current converter before being input to the protection module. The module acquires the motor's three-phase currents Ia, Ib, and Ic, and the leakage current IL.
The protection module is designed with 4 input and 4 output signals. The input signals acquire external contact signals and output internal 24V. The output signals consist of 1 normally closed contact and 3 normally open contacts. The normally closed contact is the contactor trip output, and the 3 normally open contacts can be selected via software macro settings.
The protection module uses an STM32F103 microprocessor. This module uses three serial ports of the microprocessor. UART1 communicates with the anti-power-slip module to read the anti-power-slip action information and set the anti-power-slip parameters. UART2 communicates with the display module to realize human-machine interaction. UART3 communicates with the monitoring system to realize the device's remote control functions.
3. Device Composition and Main Functional Logic
The device configuration is shown in Figure 5. It consists of a low-voltage motor protection module and one or more contactor anti-slip modules. The protection module and anti-slip modules utilize a modular rail mounting structure. When the dimensions of the combined protection and anti-slip modules cannot be accommodated in the low-voltage drawer cabinet, the two modules can be installed independently in different locations. The two modules only need to be interconnected via a 4-core RJ11 connector. Multiple anti-slip modules can also be cascaded using RJ11 connectors.
The display module of the device is installed on the panel of the drawer cabinet. The display module and the protection module are connected via an 8-core RJ45 interface network cable. The display module enables the human-machine interaction function of the device, displaying information and parameters of the protection module and the anti-voltage fluctuation module on the LCD screen. The protection and anti-voltage fluctuation parameters can be modified and set by pressing the buttons.
Figure 5. Block diagram of the device
The flowcharts for the anti-voltage fluctuation control logic and the protection control logic are shown in Figure 6. The two logic control functions operate independently in their respective modules, achieving both hardware and functional logic independence. When either the motor protection module or the anti-voltage fluctuation module fails, they do not affect each other, thus improving the reliability of the device.
4. Typical Applications and Main Technical Specifications of the Device
A typical application of the device in a low-voltage motor control circuit is shown in Figure 7. X1 is an anti-voltage fluctuation module, which solves the problem of voltage fluctuation in AC contactors. For motor circuits controlled by two or more contactors (such as motors requiring forward and reverse control), an additional anti-voltage fluctuation module can be added. X2 provides protection for the low-voltage motor. The protection action outputs of the low-voltage motor protection module include contactor trip output and incoming circuit breaker trip output (since the relay output can be set through macro definitions, the circuit breaker trip output is not explicitly shown in the typical diagram).
For motor short-circuit faults and ground faults, the contactor's breaking current limit prevents it from disconnecting the fault, so the protection module trips the incoming circuit breaker. For motor voltage faults and other types of faults, the protection module trips the motor control contactor.
Figure 7 shows a typical application of the device in a low-voltage motor control circuit.
The technical parameters and specifications of the device are shown in Table 1 and Table 2, respectively.
Table 1 Technical parameters and specifications of the anti-power fluctuation module
Table 2 Technical Specifications of Protection Module
in conclusion
This paper introduces a research and development device for low-voltage motor protection and anti-voltage fluctuation. The device employs a modular design for both the low-voltage motor protection and anti-voltage fluctuation, ensuring complete hardware and software independence for the two modules. The protection module utilizes a supercapacitor for energy storage, solving the problem of motor protection failure during voltage fluctuations.
The modules can be installed either together or independently, and only require a 4-core RJ11 interface wire for cascading. For multi-contactor control circuits, only the corresponding anti-slip power module needs to be added. This achieves flexibility in device application configuration while ensuring reliability, and has certain value for widespread application.
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