Introduction
Relays are essential and crucial electronic control devices in power systems and automation control. There are so many varieties of relays, but two of these that are to be discussed here include voltage relays and current relays. Each type of relay has its respective functions and application scenarios. This paper will make a detailed comparison and analysis in terms of the definition, function, working principle, classification, application, and other aspects of voltage relays and current relays to help readers understand and choose appropriate types of relays.
Definition and Function
Voltage relay, as an electronic control device, achieves circuit control and protection in accordance with voltage variation monitoring in the circuit. Voltage relays are usually equipped with the control system (so-called input circuit) and the controlled system (so-called output circuit). Typically, they are employed as start elements for over-voltage protection or low voltage lockout in relay protection devices of generators, transformers, and transmission lines.
The main functions of the voltage relays include automatic regulation, safety protection, and conversion circuits. When the voltage in the circuit exceeds or falls below the set value, the voltage relay can rapidly cut off or connect the circuit so as to protect the device from being damaged. In addition, the voltage relays can achieve the purpose of automatic switching of the circuit according to the change in voltage within the circuit.
The current relay is another important electronic control device that controls and protects the circuits by monitoring the changes in current in the circuit. Current relays also have two systems of control and controlled and are very commonly used in the relay protection circuits for overload and short circuit of motors, transformers, and transmission lines.
The key functions of the current relay are to detect both magnitudes and variation of the current signal, and cut off the circuit in case of over or less current from the set value to prevent overload or overheating of equipment. Besides, current relays can also be employed to monitor the current load of additional equipment and consequently reach additional equipment operating in an optimized way or in energy-saving management thanks to automatic control.
Working principle
Voltage relay
The working principle of voltage relay is based on the change of voltage in the circuit to trigger the action of the relay. When the voltage in the circuit exceeds or falls below the set value, the electromagnetic system of the voltage relay will generate corresponding electromagnetic force, thereby changing the on/off state of the contacts. Specifically, when the voltage reaches the set value, the iron core will attract the tongue, the relay will act, the contacts will close or open, and the control circuit will be turned on and off.
Voltage relays can be divided into overvoltage relays and undervoltage relays based on input and characteristic quantities. The former acts when the voltage value increases and reaches the set value, while the latter acts when the voltage value decreases and reaches the set value.
Current relay
The working principle of current relay is based on the change of current in the circuit to trigger the action of the relay. When the current in the circuit exceeds or falls below the set value, the electromagnetic system of the current relay will generate corresponding electromagnetic force, thereby changing the on/off state of the contacts. Specifically, when the current reaches or exceeds the set value, the relay will activate, closing the dynamic contact and disconnecting the dynamic contact. When the current drops to 0.8 times the set value, the relay returns, the dynamic contact opens, and the dynamic contact closes.
Current relays can be divided into electromagnetic current relays and static current relays according to their structural types; According to the installation method, it can be divided into rail current relays and fixed current relays; According to the current action, it can be divided into overcurrent relays and undercurrent relays.
Classification and Characteristics
Classification
Voltage relays and current relays are alike in some ways regarding classification; however, they also have their differences. They appear as electromagnetic and static forms for both. According to the structural type, there is a protruding fixed structure, protruding plug-in structure, inlay plug-in structure, rail structure, etc. However, according to voltage and current action types, voltage relays can be divided into overvoltage relays and low-voltage (undervoltage) relays, while the current relays could be divided into overcurrent relays and under-current relays.
characteristic
Yet, voltage relays and current relays exhibit differences concerning some characteristic points. Voltage relays have their main applications within smaller-sized circuits for the purpose of controlling low-power loads, switching low-voltage signals, etc., while current relays are suitable for high current loads, motor control, and other high current applications. In addition, voltage relays generally adopt parallel connection and connect with the load in parallel in the circuit, while current relays adopt the series connection method in general and are connected with the load in series in the circuit.
Application scenarios
Voltage relay
Voltage relays are mainly used for situations where voltage changes are to be traced and controlled in circuits. Taking a power system as an example, within its relay protection devices for generators, transformers, and transmission lines, voltage relays can serve as starting elements for over-voltage protection or low-voltage lock-out functions. In addition, voltage relays may be used for low-power load control, low-voltage signal switching, etc.
Current relay
Presently, relays find widespread applications in the situations that demand observation and control of variations in current through circuits. For example, in the case of overload and short circuiting relay protection circuits for electric motors, transformers, and transmission lines, the current relays safeguard the situation by quickly interrupting the circuit to avoid overloading, heating up, or any other hazardous situation that may arise in the equipment. Moreover, current relays already in existence can be adopted to monitor the current load of additional equipment and thereby realize automatic control during optimized operation or energy management.
Summarize
In summary, there are significant differences between voltage relays and current relays in terms of definition, function, working principle, classification, and application scenarios. Voltage relays are mainly used to monitor and control voltage changes in circuits, and are suitable for smaller circuits and parallel connection methods; Current relays are mainly used to monitor and control current changes in circuits, and are suitable for high current loads and series connections. In practical applications, we need to choose the appropriate relay type based on specific needs and scenarios to ensure the stable operation and safety protection of the circuit.