In any electrical circuit where there are no stabilizing and protective circuits, an undesirable increase in current may occur. This happens due to natural phenomena (lightning discharge near a power line) or as a result of a short circuit (short circuit) or inrush currents. In order to avoid all these cases, the correct solution would be to install restrictions on the network or local circuit of the device.
What is a current limiter?
The device, the circuit of which is constructed in such a way that prevents the possibility of an increase in the strength of electricity above specified or permissible amplitude limits, is called a current limiter. The presence of network protection with a current limiter installed in it makes it possible to reduce the requirements for the latter in terms of dynamic and thermal stability in the case of shorting.
In high-voltage lines with voltages up to 35 kV, short-circuit limitation is achieved through the use of electric reactors, in some cases fuses, created on the basis of fine-grained fillers. Also, circuits powered by high and low voltage protect the circuits assembled on the basis of:
- thyristor switches;
- non-linear and linear reactors, with by-pass semiconductor bypass switches;
- non-linear reactors with magnetization.
The principle of operation of the limiter
The main principle laid down in the current limiting circuit is to extinguish the excess current on such an element that can convert its energy into another form, for example, thermal. This is clearly seen in the work of the current limiter, where a thermistor or thyristor is used as a scattering element.
Another protection method, which is also often used, is to cut off the load from the line in which the surge of electricity occurred. Switches of this kind can be automatic, with the possibility of self-healing after the disappearance of a threat, or requiring the replacement of a reactive protective element, as is the case with a fuse.
The most perfect are electronic circuits of limiters, working on the principle of closing the channel for the passage of electricity with its increase. In this case, special passage elements (for example, transistors) are used, which are controlled by sensors.
Modern combined systems combine the function of current limiters for certain overloads and a protective option with switching off the load at short-circuit currents. Typically, such systems operate in high voltage networks.
Current limiter circuit
Using an example of a simple device circuit for current limitation, we can understand how the “electronic fuse” works. The circuit is assembled on two bipolar transistors and allows you to adjust the strength of electricity in low-voltage power supplies.
The purpose of the circuit components:
- VT1 is a feed-through transistor;
- VT2 - amplifier control signal of the feed-through transistor;
- Rs - current level sensor (low resistance resistor);
- R is a current limiting resistor.
The flow of a permissible current in the circuit is accompanied by a voltage drop on Rs, the value of which after amplification on VT2 maintains the pass-through transistor in a fully open state. As soon as the power of electricity exceeded the threshold limit, the transition of the transistor VT1 begins to hide behind in proportion to the increase in electricity. A distinctive feature of this device is the large losses (voltage drop to 1.6 V) at the sensor and the feed-through element, which is undesirable for powering low-voltage devices.
An analogue of the scheme described above is more perfect, where reduction of the voltage drop across the junction is achieved by replacing the bushing element with a field effect transistor with a small junction resistance. On the field, losses are only 0.1 V.
Inrush current limiter
This type of equipment is designed to protect inductive and capacitive loads (of various capacities) from jumps at startup. It is installed in automation systems. Most of all such current overloads are subject to asynchronous motors, transformers, LED lamps. A consequence of the use of a load current limiter in this case is an increase in the service life and reliability of devices, unloading of electrical networks.
An example of a modern model of a single-phase current limiter is the ROPT-20-1 device. It is universal and simultaneously contains an inrush current limiter and a relay for voltage control. The circuit is controlled by a microprocessor, which automatically extinguishes the starting throw and can disconnect the load if the voltage in the network rises above the permissible level.
The device is included in the disconnection of the power line and load, it works as follows:
- When voltage is applied, the microcontroller turns on, which checks for the presence of phase voltage and its value.
- If malfunctions are not detected during one period, the load is connected, which is indicated by the green LED “Network”.
- A 40 millisecond countdown occurs, and the relay shunts the quenching resistor.
- If the voltage deviates from the norm or it disappears, the relay cuts off the load, which is indicated by the red LED “Alarm”.
- When restoring the network parameters (current, voltage), the system returns to its original state.
Generator current limitation
In automobile generators, it is important to control not only the magnitude of the output voltage, but also the current delivered to the load. If exceeding the first one can lead to the failure of lighting equipment, thin windings of devices, as well as overcharging the battery, then the second one will damage the winding of the generator itself.
The output current increases the more, the more load is connected to the output of the generator (by reducing the total resistance). To prevent this, an electromagnetic current limiter is used. The principle of its operation is based on the inclusion in the circuit of the exciting winding of the generator of additional resistance in case of an increase in electricity.
Short-circuit current limitation
To protect power plants and large factories from shock currents, switching current limiters (explosive action) are sometimes used. They consist of:
- disconnecting device;
- fuse;
- chip block;
- transformer.
By monitoring the amount of electricity, the logic circuit gives a signal to the detonator (after 80 microseconds) when a short circuit occurs. The latter explodes the bus inside the cartridge, and the current is redirected to the fuse.
Features of different current limiters
Each type of restriction device is developed for specific tasks and has certain properties:
- fuse - has a speed, but requires replacement;
- reactors - effectively resist short-circuit currents, but have significant losses and voltage drops on them;
- electronic circuits and high-speed circuit breakers - have low losses, but weakly protect against shock currents;
- electromagnetic relays - consist of moving contacts that wear out over time.
Therefore, choosing which scheme to apply at home, it is necessary to study the whole range of factors characteristic of a particular electrical circuit.
Conclusion
It must be remembered that access to electrical networks requires certain knowledge of the electrician and work experience. Therefore, when installing such equipment, it is important to observe safety precautions. But the best thing, of course, is to entrust such work to a qualified specialist.