DC relay
A relay is an electrical control device that causes a predetermined step change in the controlled variable in an electrical output circuit when the input quantity (excitation quantity) changes to meet specified requirements. It has an interactive relationship between the control system (also known as the input loop) and the controlled system (also known as the output loop). Usually used in automation control circuits, it is actually an "automatic switch" that uses low current to control high current operation. Therefore, it plays a role in automatic regulation, safety protection, and circuit conversion in the circuit.
This article mainly introduces the differences between DC relays and AC relays. Firstly, let's understand the structural characteristics of DC relays and AC relays, and how to distinguish between AC relays and DC relays
Structural characteristics of DC relay
The DC does not produce reactance when applied, so the diameter of the coil is relatively thin mainly for increasing the internal resistance to avoid the approximate short circuit phenomenon. Because the amount of heat produced during working is huge, the relay is made higher and longer to provide a good heat dissipation effect.
Operation principle of DC relay
A DC relay consists of a coil, an iron core, and several groups of normally open and normally closed contacts.
Once the relay coil is connected with the rated voltage DC, the coil will generate a magnetic field to attract the iron core to move. The normally open contact, which connects with the iron core opens, and the normally closed contact opens.
If the coil of a relay loses its power, the coil will immediately lose its magnetic field. Under the action of the spring, the attracted iron core goes back to its old position. The normally open contact connected to the iron core has been disconnected, and at the same time, the normally closed contact is closed.
Relays can achieve logical control of equipment only by controlling the on/off of the coils and realizing the connection and disconnection of the contacts.
AC relay
The operating principle of the AC electromagnetic relay is similar to that of the DC electromagnetic relay. AC electromagnetic relay applies in an AC circuit. When AC goes through the coil, an alternating magnetic flux is set up in the iron core. Since the traction force is proportional to the square of the magnetic flux, the traction force does not change its direction, which only attracts the armature towards the iron core in one direction when current changes direction.
However, an AC electromagnetic relay has its own characteristic features and structure since alternating current generates an alternating magnetic flux in the iron core.
Structure of communication relay
The coil of an AC relay is relatively short and the wire diameter is relatively thick, mainly because when AC power is applied to the coil, the reactance is large. A thicker wire diameter can reduce internal resistance and heat generation. In addition, when AC power passes through zero, the electromagnetic force of the coil will decrease, resulting in weak attraction and vibration.
Therefore, a short-circuit ring is added to the magnetic attraction surface. When the magnetic field changes, eddy currents are formed in the short-circuit ring, which in turn forms an electromagnetic force in the opposite direction of the magnetic field change, lagging behind the magnetic field change and allowing the electromagnet to be well attracted.
Characteristics: (Difference from DC Relay)
- 1. Due to the fact that the current passed through the AC electromagnetic relay is a variable alternating current, the magnetic flux in its magnetic circuit also changes alternately (following a sinusoidal pattern rather than a linear pattern). The suction force acting on the armature varies between 0 and the maximum value, so the suction force of the AC electromagnetic relay is pulsating, with a frequency of twice the AC frequency. This pulsating suction force will cause the armature to vibrate, so measures should be taken structurally to eliminate vibration and affect the life of the relay.
- 2. Due to the alternating magnetic flux generated by the AC power supply passing through the iron core, eddy currents are generated in the iron core. The magnetic field generated by the eddy currents is opposite in direction to the original magnetic flux, causing some of the magnetic flux to become leakage flux and be lost, resulting in magnetic losses. In order to reduce these losses, the iron core of AC electromagnetic relays is generally made of silicon steel sheets stacked to reduce magnetic and eddy current losses.
- 3. In addition, the back electromotive force only occurs when the DC electromagnetic relay is powered on or off; In steady state, the current through the coil is determined solely by the resistance. In communication electromagnetic relays, even in steady state, there is still a back electromotive force. Therefore, the resistance of AC relays mostly does not determine the current; In turn, it is decisively influenced by the inductance of the coil. This means that when calculating the circuit of an AC relay, the inductance of the coil should be included. In communication electromagnetic relays, the reactance (inductance) of the coil determines the coil current.
The difference between DC relay and AC relay
Principle of operation DC and AC relays work on electromagnetic principles, without any difference between them; the only major difference is that the power supply must be DC power for DC relays, whereas for AC relays, it must be AC power. The DC resistance of the DC relay coil is very high, with coil current equal to the voltage divided by DC resistance of the coil. Because of that the wire of the coil is thin and the turns are numerous.
The number of turns in the coil of an AC relay is relatively few since limitation of current in an AC circuit is mainly by inductance of the coil in addition to the resistance of the coil. The magnitude of inductance XL is proportional to the frequency of the AC power. Since the frequency of the DC power is zero, XL=0, if an AC relay be in a DC circuit, and internal resistance of the coil is very small as well, so heating of the coil and its burning out takes place. By contrast, AC source cannot be switched with a DC relay and vice versa because of high internal resistance and inductance that can make it difficult for the coil to close the.
How to distinguish between AC relays and DC relays
- Many customers are confused about whether to choose DC relays or AC relays for their products. Customers often ask: My relay input is DC and the output is AC; Or the input terminal I want is AC and the output terminal is DC; Or direct current to direct current, alternating current to alternating current, is that okay. Faced with the question of whether to use AC relays or DC relays, it is necessary to popularize what AC relays are, what DC relays are, and how to use them.
- Put simply: AC relays are referred to as AC relays, while DC relays are known as DC relays. Working Ability: AC relays work with AC power, while DC relays work with DC power. An AC relay has a thicker coil diameter with fewer turns, while a DC relay coil is thinner in diameter but has more turns. Among the differences, the iron core of the AC relay contains a short-circuit ring while the DC relay does not. The iron core of AC relays is majorly E-shaped, whereas the iron core of DC relays is cylindrical. Eddy current and hysteresis loss in the iron core create the heat at the AC coil. Therefore, the iron core is separated from the coil with a skeleton and the coil is designed as in a short-and-thick shape for its short and bulkiness for heat dissipating between the coil and iron core.
- DC coils are often of the boneless, tall and thin slender type, which allows direct contact between the coil and the iron core for easy heat dissipation. In terms of the heating situation of the coil and iron core, in the AC electromagnetic system, the iron core is the heating component with a large gap between it and the coil, which does not transfer heat to the coil. Moreover, the coil shape is short and thick, which facilitates the heat dissipation of the iron core; In a DC electromagnetic system, the coil is a heating component with no gap between it and the iron core. It uses the iron core to dissipate heat, and the coil shape is slender, making it easy for the coil to dissipate heat.
FAQ
Question: If the coil is connected to an AC circuit, can the contacts be connected to a DC circuit?
This situation is possible, on the contrary, if the coil is connected to a DC circuit, its contacts can also be connected to an AC circuit. These two situations usually have different cable labels, and there are also differences in the selection of wire numbers. However, this type of circuit is prone to generating some induced voltages, which may lead to erroneous judgments during measurement and maintenance processes.
Question: How should I choose between AC or DC relays?
The application range of AC relays is wide, but the use of DC relays is relatively limited. DC relays generally have two applications: 1. They are used to protect interlocking systems, and even in the event of a power outage in the factory's AC power supply, they can trigger protective circuits. Of course, their contacts must also be in the DC system; In high-power applications, DC relays can be used in places that require electromagnetic force for control. This is because, despite being 220V, the electromagnetic force generated by DC is much greater, which is more advantageous for controlling the main circuit. In automotive applications, we all use DC power supply, so we use DC relays.
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