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Below you can find a complete list of the most commonly used types of relays, divided into four sections, Measuring relays, Control relays, Contacts and Relay coil, and finally Electromagnets and Controls, along with a simple description of their function, form of work and use in industrial automation. If you wish, you can “click” on the symbol of each relay to go to our download page and download the CAD.dwg file of each type of relay to be able to use it directly in the designs and diagrams of your projects

### Relay symbol – measurement relays

###### Low power or sync relay
It is used to open or close an auxiliary or secondary circuit depending on whether the intensity or power of the circuit being controlled remains in the range of a predetermined value and does not fall below that value, they are known as synchrony relays since guarantee that there is synchrony to connect two sections of a circuit.
###### Low impedance relay
In this case, the adjustment of the relay is achieved by varying the number of turns of the coil so that the torque on the contact head stock and the opposite torque exerted by the spring are balanced at the moment that the ratio between the voltage and intensity applied to the relay is equal to the adjustment impedance
###### Reverse Current Relay
The Relay of current or reverse power is sensitive to the direction of current flow, and is used in systems in which several electrical generators are synchronized. This relay acts by analyzing the reverse power of the load that we want to control, so that when one of the generators placed in parallel with other generators, is not generating the system, but is consuming energy from the other generators the relay is inverted and sent a signal to the generator synchronization control panel so that the generator that is consuming system power is disconnected
###### Maximum and minimum current relay
These relays are used to control the current flowing through a circuit, so that the relay changes from idle state to excitation as long as the intensity that circulates through the circuit to be controlled is out of a predetermined range of minimum and maximum intensity . See example of Hall effect current sensor When the relay is energized in these circumstances, a contact is closed that can be used in another auxiliary circuit to activate an alarm signal. Thus, when the maximum intensity is being controlled, the relay remains excited until the current to be controlled is lower than the set value. In the case of minimum current control, the relay remains activated until the intensity of the current to be measured is greater than the threshold value, returning to a value within the adjusted range. The normal thing is that the relay is de-excited with a few milli-seconds of delay to prevent oscillations that do not want to control because they would give rise to false excitations. This delay can be varied and adjusted at will by acting on a potentiometer arranged for this purpose. The hysteresis can also be controlled by another potentiometer to keep it in ranges from 1 to 45% of the maximum and minimum intensity ranges set in the control.
###### Split Driver Detection Relay
This belongs to a type of relays used to detect in three-phase lines the differences between the positive and negative sequence currents delivered by the supply networks, so that breakages of conductors that are either suspended in the air or even in the air can be detected or even touch the ground. This way, when the fault is detected, the supply can be cut to avoid overloads that damage the network.
###### Lack of voltage relay
There are many models of relays that can detect the lack of voltage in a circuit by adding other controls, timers, regulators, etc. However, if we need to know only if a circuit exists a certain voltage level, for example 24 volts of direct current, we can install in this circuit a relay whose coil works at this nominal voltage so that if the voltage in this circuit is lost , the relay we have installed would be de-excited and its reverse contact would change state by spring action so that another auxiliary circuit would close between the normally closed contact and the inverter contact, causing an alarm or any other device that interests us to jump
###### Frequency relay
They are used to protect the circuits powered by generators so that in the event of an increase in the rotation speed of the generator motor shaft and therefore an increase in the frequency, the relay opens the circuit and prevents equipment failures receivers
###### Maximum voltage relay
They are installed to protect the “downstream of the relay” circuits from voltages higher than a value set in the setting of the relay itself, so that the relay opens the circuit and prevents breakdowns due to overvoltage in the loads. For example, in a domestic single-phase circuit operating at a nominal voltage of 220 volts ac, the relay can be regulated to disconnect the loads when it detects an input voltage in the circuit greater than 240 volts ac, from which breakdowns could occur in connected appliances.

### Relay Symbol – Electromagnets and Controls

###### Anchor mechanism relay

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