Taking the overcurrent control of a certain power supply line as an example. The line current is detected by the current transformer field tester. When the current is too large, an alarm signal will be issued and the power will be cut off. The specific process is: through the current transformer, the large current signal is converted into a small current, and through a resistor it is converted into a voltage signal, which is then input into the LOGO! through its analog input terminal. Through the processing of LOGO!, after a delay time T1, it controls the relay TM and the alarm signal light, cuts off the power supply of the circuit, and issues an alarm signal. It can also automatically reconnect the circuit after cutting off. If the circuit current is normal at this time, the alarm light goes out. Moreover, within a specified time after the overcurrent, if the DC high voltage overcurrent occurs again, there is no need to delay T1 again; only a delay time T2 is needed before another alarm signal is issued. This gives LOGO! selectivity. In cases such as motor startup causing instantaneous excessive current, LOGO! will not restart, reducing false operations of the equipment. And after one alarm, if the current becomes excessive again, it will alarm again within a shorter time, reducing losses caused by unresolved faults.
Problems during debugging
Using a current-voltage DC high voltage generator conversion device to convert the current signal into a voltage signal, and the output voltage can be adjusted to better meet actual requirements. In the frequency trigger module, the counting time period is set to 6S, giving it a selection function, so that under normal overcurrent conditions such as motor startup, no alarm signal is issued. And when setting the number of pulses, the actual value is 80 instead of 300. This avoids the situation where the alarm device cannot start when the power supply current is too small due to interference. At the same time, it reduces the situation where the delay time is too long when overcurrent occurs within the timing time of B02. Similarly, when setting the number of pulses for B03, it is only set to 70.
To obtain a higher output voltage value, the resistance R2 between the adjustment terminal of the LM317 regulator and ground and its voltage drop are often large. Connecting a capacitor C3 of less than 10μF in parallel across R2, the DC high voltage generator can effectively suppress the ripple at the output. When a short circuit occurs at the input or output, the discharge of capacitor C3 will generate a surge voltage on R1, which can endanger the reference voltage circuit of the regulator. Therefore, a diode D3 needs to be connected in parallel across R1 to protect the regulator.
The output of the regulator can work without a capacitor. Since the regulator works under deep negative feedback of 1:1, when the load at the output is capacitive at a certain value, the regulator may self-oscillate. Therefore, a capacitor C1 of 0.1μF is connected at the input of the regulator, and an electrolytic capacitor C5 of 1000μF is connected at the output to provide sufficient current supply, while also preventing possible self-oscillation, reducing high-frequency noise, and improving the transient response of the load. When a short circuit occurs at the input, C5 discharges through the adjustment transistor of the regulator. If C5 is large, the discharge surge current is large, and the voltage may discharge through the output transistor inside the regulator, potentially causing reverse breakdown of the emitter junction of the output transistor. For this reason, a diode D2 is connected in parallel across the regulator. When the input is short-circuited, C5 discharges through D2, protecting the regulator.
The LOGO! model used in this device is 24RCLB11, with a power supply of 24V DC, and the output uses 220V AC to drive relays and alarm lights. The relay TM of this device has its normally closed contact connected in the main circuit. The analog inputs of LOGO! are I1 and I2. The outputs are Q1 and Q2. The current signal output by the transformer enters the I1 and I2 terminals of LOGO! through the current-voltage conversion device (I/V).
Design concept of overcurrent protection
The program design of LOGO! is implemented by using function modules in LOGO!. The internal working principle of LOGO! is: the analog quantities input from the analog input terminals AI1 and AI2 are converted into digital quantities by the analog triggers B08 and B12. The settings of modules B08 and B12 are the same. The analog switch turns on at 6V and turns off at 1V. When the peak value of the input signal is greater than 6V, the module outputs a digital pulse signal with a frequency of 50Hz. After passing through B06, it is counted by the frequency trigger B02. When 280 pulses are input within 6S, module B02 outputs a high level. At this time, output Q1 is turned on, activating the TM trip controller to disconnect the circuit. At the same time, output Q2 is set to high level, outputting an alarm signal. At this time, the circuit has been cut off, and there is no analog signal input. After 6s, the frequency trigger outputs a low level, that is, Q1 has no signal output, and the circuit is connected. If there is no overcurrent in the circuit at this time, the alarm light goes out. If the alarm light is not extinguished, or if overcurrent still occurs within the time period after the alarm light goes out (controlled by B09), then within 2S, as long as B09 detects 70 current pulses, the alarm device will start again to prevent the alarm device from handling normally when the fault is not cleared. When Q2 becomes 0, B09 starts timing. If after the set time, Q2 still has no alarm output, then B09 outputs a low level, which is inverted to a high level through the NOT gate B10, resetting the RS flip-flop B05. At this time, the alarm device returns to its original state.
Considering that when the fault is not cleared, the alarm device will still connect the power supply after a delay of 6S. To avoid danger, B03 is added, and modules B04, B05, B09, and B10 are used to assist in generating delay signals. This makes the device more sensitive to overcurrent signals within a specified time after an overcurrent, that is, when overcurrent occurs, the overcurrent signal only needs to last for 2S before alarming again, to reduce danger.
The overcurrent protection of the DC high voltage generator is widely used in cable applications, especially in the protection of cables in the power sector!
