In any electrical circuit where there are no stabilizing and protective circuits, an unwanted increase in current can occur. This can be the result of natural phenomena (lightning near a power line) or the result of a short circuit (short circuit) or inrush currents. To avoid all these cases, the correct solution is to install a restriction device in the network or local circuit.
What is a current limiter?
A device whose circuit is constructed in such a way that it prevents the possibility of an increase in the strength of electricity above the 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 event of a short circuit.
In high-voltage lines with voltages up to 35 kV, short circuit limitation is achieved by using electric reactors, in some cases, fuses made from fine-grained fillers. Also, circuits fed by high and low voltage are protected by circuits assembled on the base:
- thyristor switches;
- reactors of non-linear and linear type, shunted with quick-acting semiconductor switches;
- nonlinear biased reactors.
The principle of the limiter
The basic principle behind current limiting circuits is to extinguish excess current on an element that can convert its energy into another form, such as heat. This is clearly seen in the operation of the current limiter, where a thermistor or thyristor is used as a dissipating element.
Another method of protection, which is also often used, is to cut off the load from the line in which the surge of electricity has occurred. These types of switches can be automatic, with the ability to self-reset after the threat disappears, or require replacement of a reacting protective element, as is the case with a fuse.
The most advanced are electronic circuits of limiters that work on the principle of closing the channel for the passage of electricity when it increases. In this case, special pass-through elements are used (for example, transistors), which are controlled by sensors.
Modern combined systems combine the function of current limiters for certain overloads and a protective option with load shutdown at short circuit currents. Typically, such systems operate in high-voltage networks.
Current limiter circuit
On the exampleWith the simplest circuit of a current limiting device, you can understand how an "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.
Assignment of circuit components:
- VT1 - pass transistor;
- VT2 - pass transistor control signal amplifier;
- Rs – current level sensor (low resistance resistor);
- R – current limiting resistor.
The flow of current in the circuit of an acceptable value is accompanied by a voltage drop across Rs, the value of which, after amplification on VT2, maintains the pass transistor in a fully open state. As soon as the strength of electricity has 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 design of the device is the large losses (voltage drop up to 1.6 V) on the sensor and the pass-through element, which is undesirable for powering low-voltage devices.
An analogue of the circuit described above is a more perfect one, where a reduction in the voltage drop at the junction is achieved by replacing the pass element from a bipolar to a field-effect transistor with a low junction resistance. On the field, the losses are only 0.1 V.
Inrush Current Limiter
Equipment of this type is designed to protect inductive and capacitive loads (of various capacities) from surges atstartup. It is installed in automation systems. Most of all, asynchronous motors, transformers, LED lamps are subject to such current overloads. The consequence of using a load current limiter in this case is an increase in the service life and reliability of devices, unloading electrical networks.
The ROPT-20-1 device can serve as an example of a modern model of a single-phase current limiter. It is universal and contains both an inrush current limiter and a relay for voltage control. The circuit is controlled by a microprocessor, which automatically dampens the starting surge and can turn off the load if the voltage in the network rises above the permissible level.
The device is included in the break of the power and load lines, it works as follows:
- When voltage is applied, the microcontroller turns on, which checks for the presence of phase voltage and its value.
- If problems are not detected during one period, the load is connected, which is signaled by the green LED "Network".
- The 40 millisecond countdown occurs and the relay shunts the damping resistor.
- When the voltage deviates from the norm or it fails, the relay cuts off the load, which is signaled by the red "Emergency" LED.
- When the network parameters (current, voltage) are restored, the system returns to its original state.
Generator current limit
In car generators, it is important to control not only the amount of output voltage, but also the outputinto the load current. If exceeding the first one can lead to failure of lighting equipment, thin windings of devices, as well as recharging the battery, then the second one can damage the winding of the generator itself.
The output current increases the more, the more load is connected at the output of the generator (by reducing the total resistance). To prevent this, an electromagnetic current limiter is used. Its principle of operation is based on the inclusion of additional resistance in the circuit of the exciting winding of the generator in the event of an increase in electricity.
Short circuit current limit
To protect power plants and large factories from surge currents, switching-type current limiters (explosive action) are sometimes used. They consist of:
- disconnect device;
- fuse;
- chip block;
- transformer.
By controlling the amount of electricity, the logic circuit sends a signal to the detonator (after 80 microseconds) when a short circuit occurs. The latter blows up the bus inside the cartridge, and the current is redirected to the fuse.
Features of different current limiters
Each type of restriction device is designed for specific tasks and has certain properties:
- fuse - fast acting but needs to be replaced;
- reactors - effectively resist short-circuit currents, but have significant losses and voltage drops on them;
- electronic circuits and quick acting circuit breakers - have low losses but little protection against surge currents;
- electromagnetic relays - consist of moving contacts that wear out over time.
Therefore, choosing which circuit to apply in your home, you need to study the whole range of factors specific to a particular electrical circuit.
Conclusion
It must be remembered that access to electrical networks requires certain knowledge in electrics and work experience. Therefore, when installing such equipment, it is important to observe safety precautions. But it is best, of course, to entrust such work to a qualified specialist.
