Difference between AC Relay and DC Relay
A relay is an electrical control device, which is an electrical appliance that makes a predetermined step change of the controlled quantity in the electrical output circuit when the change of the input quantity meets the specified requirements. It has the interactive relationship between the control system and the controlled system. Usually used in automatic control circuits, a relay is actually an automatic switch that uses a small current to control the operation of a large current. Therefore, it plays the role of automatic adjustment, safety protection, and conversion circuit in the circuit.
The DC relay consists of a coil, an iron core and several groups of normally open and normally closed contacts. When the relay coil is connected to the DC power of the rated voltage, the coil generates a magnetic field, attracting the iron core to act, the normally open contact connected to the iron core is closed, and at the same time, the normally closed contact is disconnected. When the relay coil is de-energized, the coil loses the magnetic field, the attracted iron core returns to its original position under the action of the spring, the normally open contact connected to the iron core is disconnected, and at the same time, the normally closed contact is closed. The relay is to control the on/off of the coil to realize the on and off of the contact, so as to achieve the logical control of the equipment.
The working principle of the AC relay is basically the same as that of the DC electromagnetic relay. The AC relay works in the AC circuit. When the AC current passes through the coil, an alternating magnetic flux is generated in the iron core. Since the traction force (electromagnetic attraction) is the square of the magnetic flux φ is proportional, so when the current changes direction, the traction force does not change direction, always attracting the armature to the core in one direction.
The coil of the AC relay is short, and the wire diameter is relatively thick, mainly because the reactance of the coil is relatively large after the alternating current is applied, and the thick wire diameter can reduce the internal resistance and heat generation. When the magnetic field changes, an eddy current is formed when the short-circuit ring is formed, and an electromagnetic force opposite to the direction of the magnetic field change is formed, which lags the change of the magnetic field. , so that the electromagnet can be better attracted.
Difference between DC relay and AC relay
- Since the current passed into the AC relay is a changing AC current, the magnetic flux in the magnetic circuit also changes alternately (sinusoidal law instead of linear law). The suction force on the armature varies between 0 and the maximum value, so the suction force of the AC electromagnetic relay is pulsating, and the frequency of change is twice the AC frequency. This pulsating suction force will make the armature vibrate, so structurally Measures should be taken to eliminate chattering, which affects the life of the relay.
- When the AC power supply passes through the iron core, an alternating magnetic flux is generated, so that eddy current is generated in the iron core, and the magnetic field generated by the eddy current is opposite to the original magnetic flux, so that a part of the magnetic flux becomes a leakage magnetic flux and is lost, resulting in Magnetic loss, in order to reduce these losses, the iron core of the AC relay is generally made of silicon steel sheets to reduce the magnetic loss and eddy current loss, and the iron core of the AC relay is made of silicon steel sheets.
- The DC relay only has a back EMF when the power is turned on or off. In a stable state, the current through the coil is only determined by the resistance, while the AC electromagnetic relay, even in a stable situation, also has a back EMF. Therefore, the current of the AC relay is not mainly determined by the resistance, but by the inductive reactance of the coil, which means that the inductance of the coil must be considered when calculating the AC relay circuit. The coil current of the AC electromagnetic relay is determined by the reactance of the coil.
- The working principles of DC relays and AC relays are the same as the electromagnetic principle, but the power supply of DC relays must be DC, and the power supply of AC relays must be AC power. The DC resistance of the DC relay coil is very large, and the coil current is equal to the voltage divided by the DC resistance of the coil, so the coil wire is thin and has many turns.
- The number of turns of the AC relay coil is correspondingly small, because the limiting current in the AC circuit is mainly the coil inductive reactance except for the coil resistance. The size of the inductive reactance XL is proportional to the frequency of the alternating current. If the AC relay is connected to the DC circuit. Because the frequency of the direct current is equal to zero, the inductive reactance XL=0, and the internal resistance of the coil is very small, so the coil will heat up and burn out. On the contrary, when the DC relay is connected to the AC power supply, the coil cannot be pulled in due to the large internal resistance of the coil and the large inductive reactance, so it cannot be interchanged.
How to distinguish between AC relay and DC relay?
The working power of the AC relay is AC, and the working power of the DC relay is DC. The coil diameter of the AC relay is thicker and the number of turns is less, and the coil diameter of the DC relay has more turns than the thin wire diameter. The core of the AC relay has a short-circuit ring, and the DC does not. The cores of AC relays are mostly E-shaped, and the DC cores are cylindrical. The AC coil will generate heat due to eddy current and hysteresis loss in the iron core, so the coil has a skeleton to isolate the iron core from the coil, and the coil is made into a short and thick chunky type to dissipate heat from the coil and the iron core.
Most of the DC coils are thin and tall without a skeleton, so that the coil and the iron core are in direct contact with each other to facilitate heat dissipation. As far as the heating of the coil and the iron core is concerned, in the AC electromagnetic system, the iron core is a heating component, and there is a large gap between the coil and the coil, so as not to transmit heat to the coil, and the shape of the coil is short and thick, which is convenient for the iron core to dissipate heat; DC electromagnetic In the system, the coil is a heating component, there is no gap between it and the iron core, the iron core is used for heat dissipation, and the coil shape is slender, which is convenient for the coil itself to dissipate heat.
If the coil is connected to the AC circuit, can the contacts be connected to the DC circuit?
This situation is possible, and conversely, if the coil is connected to the DC circuit, it is also feasible that its contacts are connected to the AC circuit. In these two cases, the cables are generally different, and the case of the wire number is also different. However, this kind of loop is prone to generate some induced voltages, and it is prone to misjudgment during measurement and maintenance.
Should I choose AC or DC relay?
The scope of application of AC is relatively wide, but the use of DC relays is relatively less. The scope of application of DC relays generally has two situations: 1. It is used in the protection interlocking system. Even in the case of loss of power with the AC power supply, the protection circuit can be triggered. Of course, its contacts must also be DC systems; 2. It is used in large power occasions. DC relays can be used in places where there is a requirement for the electromagnetic force to be controlled, because the same is 220V, but the electromagnetic force generated by DC is much larger, which is more beneficial to the control of the main circuit. In automotive applications, we all use DC power to supply power, so DC relays are used.