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 supply will be cut off. The specific process is: through the current transformer, the large current signal is converted into a small current, and converted into a voltage signal through a resistor, and then input into the LOGO! through its analog input terminal. After processing by 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 sends out 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 required, and an alarm signal will be issued again. This makes LOGO! selective. When the current is momentarily too large due to motor starting, LOGO! will not start again, reducing misoperation of the equipment. And after an alarm, if the current becomes too large again, it will alarm again within a short time, reducing losses caused by unresolved faults.
Problems during commissioning
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 6 seconds, giving it a selection function, so that under normal overcurrent conditions such as motor starting, no alarm signal is issued. And when setting the number of pulses, the actual value is 80, not 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 during 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 voltage regulator and ground and its voltage drop are often large. A capacitor C3 of less than 10μF is connected in parallel across R2. The DC high voltage generator can effectively suppress the ripple at the output terminal. When a short circuit occurs at the input or output terminal, the discharge of capacitor C3 will generate a surge voltage on R1, which may 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 terminal of the regulator can also work without a capacitor. Since the regulator operates under deep negative feedback of 1:1, when the load at the output terminal is capacitive at a certain value, the regulator may self-oscillate. Therefore, a 0.1μF capacitor C1 is connected at the input terminal of the regulator, and a 1000μF electrolytic capacitor C5 is connected at the output terminal to provide sufficient current supply, while preventing possible self-oscillation and reducing high-frequency noise and improving the transient response of the load. When a short circuit occurs at the input terminal, C5 discharges through the adjustment tube of the regulator. If C5 is large, the discharge surge current is large, and the voltage will discharge through the internal output transistor of the regulator, which may cause 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 terminal 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 the device has its normally closed contact connected in the main circuit. The analog input terminals 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 realized by using the 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 through the analog triggers B08 and B12. The settings of modules B08 and B12 are the same. The analog switch turn-on voltage is 6V, and the turn-off voltage is 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 6 seconds, module B02 outputs a high level. At this time, output Q1 is turned on, connecting 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 6 seconds, 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 has not gone out, or within the time period after the alarm light goes out (controlled by B09), there is still overcurrent, then within 2 seconds, 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 has not been eliminated. When Q2 becomes 0, B09 starts timing. If after a self-set time, Q2 still has no alarm output, then B09 outputs a low level, which is inverted to a high level after passing 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 has not been eliminated, the alarm device will still connect the power supply after a delay of 6 seconds. 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 2 seconds to alarm 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!
