Working Principle of an Relay

 The element that controls one or more groups of switches (opens or closes the switches) in order to operate (switch) a receiver (load) operacccccc
Relays must be used in some air and liquid control systems where electronic circuits cannot enter (high temperature, humidity or liquid environments, etc.). The problems encountered in relay applications can be summarized as follows. Since it works mechanically, it malfunctions a lot. Since the contacts are constantly sticking together and opening, the electrical shocks that occur over time cause the contacts to oxidize and lose conduction, or even the contacts remain stuck closed and not open when it is required. The Principal cause is always overheating due to it remaining energized for a long time. If is connected a non-essential device, the problem can be solved, but where are connected critical devices, in this case, will be a big problem. In addition, the sound made by the contacts when they are pulled and released is not very pleasant.

The coils of relays, which are generally fed with voltage between 5V-48V, draw current between 5mA-150mA. Its contacts can withstand current values between 0.5A-70A. A wide coil supply voltage range provides convenience in application circuits. In this way, it is possible to drive it with both logic and analog circuits. When controlling high-current circuits, the heating that may be caused by the current passing through the contacts requires the use of contactors instead of relays in these circuits.

Figure 1 -Standard relay philosophy

1.  Relay Structure and Operation: The structure and operation of relays vary depending on their type. While describing relay types, the operation and structure of each type will be mentioned. However, in general, it can be said that for all relays, the contacts change places with an effect (current, voltage, heat, magnetic field, etc.) and a circuit is controlled by taking advantage of this change.
2. Relay Types: Relays can be diversified as magnetic relays, tongue contact, thermal relays and overcurrent protection relays. Among the relay types, the appropriate one should be selected according to the circuit in which it is used. For example, if switching is to be done by magnetic effect, tongue contacts can be used, and overcurrent protection relays can be used to limit high current. We will examine the relay types in more detail below.

 Magnetic Relays

The current passing through a conductive wire covered with an insulating surface wrapped around an iron rod creates a magnetic field and attracts nearby metals towards itself. Magnetic relays are manufactured by taking advantage of this feature of the current.

Figure 2 - inside magnetic relay

Magnetic relays consist of three basic parts. These are relay coil, relay contacts and pallet. The coil consists of a large number of conductive wires wrapped in a plastic sheath placed on an iron core (shown as 4) manufactured in the shape of a cylindrical or rectangular prism (shown as 3). The pallet (shown as 5) is a piece of soft iron that moves by being affected by the magnetic field of the coil. The reason why soft iron was chosen as the core is that it easily gains a magnetic field and at the same time loses this feature easily. At the point where the pallet contacts the contacts, there is an insulating material called fiber (shown as 7). When there is no magnetic field, it is held in tension by a spring. Contacts (shown as 1 and 2) are good conductive materials that allow current to flow when they come into contact with each other. Apart from these, a relay also has an insulating ground material (shown as 6) that prevents unintentional contact of the contacts.
The coil and core work like an electromagnet. When voltage is applied to the ends of the coil, it creates a magnetic field and pulls the pallet towards itself (the pallet is free in the yellow ring and the pallet is pulled towards the core in the green ring). As a result of this movement of the pallet, normally closed contacts (shown with red circles in the left part of the figure) are opened. If a load is connected to the ends of these contacts, their operation will stop. Again, the normally open contacts (shown with red circles in the right part of the figure) will close and the load connected to these contact terminals will operate. Also pay attention to the change in the current path indicated by G (current input) and Ç (current output) in the figure.
Magnetic relays can be classified according to the number and type of contacts on them. There is a single switch on the single-contact and single-position relay, and there is no other switch that works against this switch. It is only used for switching on and off a load. Single-contact dual-position relays, on the other hand, start one load and stop the other as a result of the switch on them changing position. Similarly, multi-contact relays can be obtained by increasing the number of parallel operating contacts in the relay. The relay whose internal structure is depicted in Figure 1.3 is a two-contact and dual-position relay.

 Language Contact Roles

The relay made by placing metal contacts that are easily affected by the magnetic field inside an air-emptied glass tube is called tongue contact, and if the number of contacts is large, it is called tongue contact relay. Reed contacts have many uses such as liquid level control, underwater devices, remote control switches and warning lights of automobiles. In relays with tongue contacts, it is sufficient to approach a magnet or pass current through the coil wound on the glass sheath to close the contact. Tongue contacts are magnetic field controlled switches. The opening and closing of the switch depends on the change in the magnetic field.

Figure 3 - Relay Contact Deals

Thermal Relays

It is a type of relay that opens or closes its contacts according to the temperature change in the environment. Thermal relays are frequently used to stop motors that overheat during operation or to prevent further heating at the appropriate temperature level in heat-producing devices such as irons and hair dryers.
Thanks to a special metal composition called bi-metal in thermal relays, it cuts off the current going to the load when the temperature of the environment exceeds the determined temperature value. Bi-metal consists of two metals that expand differently under heat. While one of the metals expands rapidly and excessively, the other expands very little and bi-metal. It converts heat energy into motion energy by causing the metal composition to bend in one direction. This kinetic energy is used to close the contacts. Thermal relays can have many different structures. While there is a chrome-nickel heater inside the thermal relays used in the motor control circuits to provide heating, ironing etc. Thermal relays used in devices use the heat inside the device and contain only bi-metal and contacts.

Figure 4 - Structure of thermal relay

 Working principle of thermal relay

When the motor operates normally, the thermal element of the thermal relay will not generate enough heat to make the protection function act, and its normally closed contact will keep closed state; when the motor is overloaded, the thermal element of the thermal relay will generate enough heat to make the protection function act, and its normally closed contact will be disconnected to make the motor lose power through the control circuit, so as to protect the motor. After troubleshooting, the thermal relay should be reset before the motor can be restarted.
Thermal relay generally has two reset forms: manual reset and automatic reset. The conversion of the two reset forms can be completed by adjusting the reset screw. When the thermal relay is delivered from the factory, the manufacturer usually sets it to the automatic reset state. In use, whether the thermal relay is set to manual reset state or automatic reset state depends on the specific situation of the control circuit. In general, the principle that even if the thermal relay resets automatically after the protection action of thermal relay is followed, the protected motor shall not restart automatically, otherwise, the thermal relay shall be set to manual reset state. This is to prevent the motor from repeatedly starting and damaging the equipment when the fault is not eliminated. For example, for the control circuit of manual start and manual stop controlled by button, the thermal relay can be set to automatic reset mode; for the automatic start circuit controlled by automatic element, the thermal relay shall be set to manual reset mode

Figure 5 - Thermal Relay

Overcurrent Protection Relays

Fuses placed at the beginning of the line in a load circuit protect the line, not the load, due to their operating characteristics. Various relays are used to protect the loads before failure. The overcurrent relay is used to prevent damage caused by overcurrents to the motor windings. Overcurrent protection relays are designed to protect motors and systems against overcurrent.
Overcurrent relays, which are connected to only one conductor in single-phase alternating current circuits and to all three phase conductors in three-phase circuits, control the normally closed contact connected in series to the circuit in the control circuit. Current adjustment in overcurrent relays is made with the adjustment screw on the relay. Current adjustment is made within certain limits according to the rated currents of the motors.

Electromechanical 50 (instantaneous overcurrent) relays are models of simplicity, consisting of nothing more than a coil, armature, and contact assembly (a “relay” in the general electrical/electronic sense of the word). Spring tension holds the trip contacts open, but if the magnetic field developed by the CT secondary current becomes strong enough to overcome the spring’s tension, the contacts close, commanding the circuit breaker to trip:

Figure 6 - Relay Overcurrent Diagram

The protective relay circuit in the above diagram is for one phase of the three-phase power system only. In practice, three different protective relay circuits (three CTs, and three 50 relays with their trip contacts wired in parallel) would be connected together to the circuit breaker’s trip coil, so that the breaker will trip if any of the 50 relays detect an instantaneous overcurrent condition. The monitoring of all three line currents is necessary because power line faults are usually unbalanced: one line will see a much greater share of the fault current than the other lines. A single 50 relay sensing current on a single line would not provide adequate instantaneous overcurrent protection for all three lines.
The amount of CT secondary current necessary to activate the 50 relay is called the pickup current. Its value may be varied by adjusting a movable magnetic pole inside the core of the relay. Calibration of an instantaneous overcurrent (50) relay consists simply of verifying that the unit “picks up” within a reasonably short amount of time if ever the current magnitude exceeds the prescribed pickup value.
Electromechanical 51 (time overcurrent) relays are more complicated in design, using a rotating metal “induction disk” to physically time the overcurrent event, and trip the circuit breaker only if the overcurrent condition persists long enough. A photograph of a General Electric time-overcurrent induction-disk relay appears here

Figure 7 - Relay Overcurrent

The round disk you see in the photograph receives a torque from an electromagnet coil assembly acting like the stator coils of an induction motor: alternating current passing through these coils cause alternating magnetic fields to develop through the rear section of the disk, inducing currents in the aluminum disk, generating a “motor” torque on the disk to rotate it clockwise (as seen from the vantage point of the camera in the above photo). A spiral spring applies a counter-clockwise restraining torque to the disk’s shaft. The pickup value for the induction disk (i.e. the minimum amount of CT current necessary to overcome the spring’s torque and begin to rotate the disk) is established by the spring tension and the stator coil field strength. If the CT current exceeds the pickup value for a long enough time, the disk rotates until it closes a normally-open contact to send 125 VDC power to the circuit breaker’s trip coil.
A silver-colored permanent magnet assembly at the front of the disk provides a consistent “drag” force opposing disk rotation. As the aluminum disk rotates through the permanent magnet’s field, eddy currents induced in the disk set up their own magnetic poles to oppose the disk’s motion (Lenz’s Law). The effect is akin to having the disk rotate through a viscous liquid, and it is this dynamic retarding force that provides a repeatable, inverse time delay.
A set of three photographs show the motion of a peg mounted on the induction disk as it approaches the stationary trip contact. From top to bottom we see the disk in the resting position, partially rotated, and fully rotated:

Figure 8 - Relay Overcurrent

Figure 9 - Relay Overcurrent

Figure10- Relay Overcurrent

The mechanical force actuating the time-overcurrent contact is not nearly as strong as the force actuating the instantaneous overcurrent contact. The peg may only lightly touch the stationary contact when it reaches its final position, failing to provide a secure and lasting electrical contact when needed. For this reason, a seal-in relay actuated by current in the 125 VDC trip circuit is provided to maintain firm electrical contact closure in parallel with the rotating peg contact. This “seal-in” contact ensures a reliable circuit breaker trip even if the peg momentarily brushes or bounces against the stationary contact. The parallel seal-in contact also helps reduce arcing at the peg’s contact by carrying most of the trip coil current.
A simplified diagram of an induction disk time-overcurrent relay is shown in the following diagram, for one phase of the three-phase power system only. In practice, three different protective relay circuits (three CTs, and three 51 relays with their trip contacts wired in parallel) would be connected together to the circuit breaker’s trip coil, so that the breaker will trip if any of the 51 relays detect a timed overcurrent condition

Figure11- Relay Overcurrent Diagram

The seal-in unit is shown as an electromechanical relay connected with its contact in parallel with the induction disk contact, but with its actuating coil connected in series to sense the current in the 125 VDC trip circuit. Once the induction disk contact closes to initiate current in the DC trip circuit, even momentarily, the seal-in coil will energize which closes the seal-in contact and ensures the continuation of DC trip current to the circuit breaker’s trip coil. The relay’s seal-in function will subsequently maintain the trip command until some external contact opens to break the trip circuit, usually an auxiliary contact within the circuit breaker itself.

Calibrating Overcurrent Devices

Calibration of a time overcurrent (51) relay consists first of verifying that the unit “picks up” (begins to time) if ever the current magnitude exceeds the prescribed pickup value. In electromagnetic relays such as the General Electric model showcased here, this setting may be coarsely adjusted by connecting a movable wire to one of several taps on a transformer coil inside the relay, varying the ratio of CT current sent to the induction disk stator coils. Each tap is labeled with the number of whole amperes (AC) delivered by the secondary winding of the CT required for relay pick-up (e.g. a tap value of “5” means that approximately 5 amps of CT secondary current is required for induction disk pickup). A fine adjustment is provided in the form of a variable resistor in series with the stator coils.
A photograph of the tap wire setting (coarse pickup adjustment) and resistor (fine pickup adjustment) are shown here. The tap in this first photograph happens to be set at the 4 amp position:

Figure12- Relay Overcurrent Calibration

Proper setting of the pickup tap value is determined by the maximum continuous current rating of the system being protected and the ratio of the current transformer (CT) used to sense that current.
After the proper pickup value has been set, the time value is established by rotating a small wheel called the time dial located above the induction disk. This wheel functions as an adjustable stop for the induction disk’s motion, positioning the disk closer to or farther away from the trip contact in its resting condition:

Figure13- Relay Overcurrent Calibration

The amount of disk rotation necessary to close the trip contact may be set by adjusting the position of this time dial: a low number on the time dial (e.g. 1) means the disk need only rotate a small amount to close the contact; a high number on the time dial (e.g. 10) sets the resting position farther away from contact, so that the disk must rotate farther to trip. These time dial values are linear multipliers: a time dial setting of 10, for example, exhibits twice the time to trip than a setting of 5, for any given overload condition.
Calibration of the time-overcurrent protective function must be performed at multiple values of current exceeding the pickup value, in order to ensure the relay trips within the right amount of time for those current values. Like process instruments which are often calibrated at five points along their measurement range, time-overcurrent relays must also be checked at multiple points along their prescribed “curve” in order to ensure the relay is performing the way it should.

Time Overcurrent Relay Curves

Time overcurrent relays exhibit different “curves” relating trip time to multiples of pickup current. All 51 relays are inverse in that the amount of time to trip varies inversely with overcurrent magnitude: the greater the sensed current, the less time to trip. However, the function of trip time versus overcurrent magnitude is a curve, and several different curve shapes are available for United States applications:

• Moderately inverse
• Inverse
• Very inverse
• Extremely inverse
• Short-time inverse

Time curves standardized by the Swiss standards agency IEC (International Electrotechnical Commission) include:

• Standard inverse
• Very inverse
• Extremely inverse
• Long-time inverse
• Short-time inverse

The purpose for having different curves in time-overcurrent relays is related to a concept called coordination, where the 51 relay is just one of multiple overcurrent protection devices in a power system. Other overcurrent protection devices include fuses and additional 51 relays at different locations along the same line. Ideally, only the device closest to the fault will trip, allowing power to be maintained at all “upstream” locations. This means we want overcurrent protection devices at the remote end(s) of a power system to be more sensitive and to trip faster than devices closer to the source, where a trip would mean an interruption of power to a greater number of loads.
Legacy electromechanical time-overcurrent (51) relays implemented these different inverse curve functions by using induction disks with different “cam” shapes. Modern microprocessor-based 51 relays contain multiple curve functions as mathematical formulae stored within read-only memory (ROM), and as such may be programmed to implement any curve desired. It is an amusing anachronism that even in digital 51 relays containing no electromagnets or induction disks, you will find parameters labeled “pickup” and “time dial” in honor of legacy electromechanical relay behavior.

The trip time formulae programmed within a Schweitzer Engineering Laboratories model SEL-551 overcurrent relay for inverse, very inverse, and extremely inverse time functions are given here:

Where,
t= Trip time (seconds)
T= Time Dial setting (typically 0.5 to 15)

Multiples of pickup current (e.g. if I_pickup= 4.5 amps, a 9.0 amp signal would be M=2)

REVIEW

• Instantaneous overcurrent, ANSI/IEEE code 50, is used most often in resistive systems where current spikes are undesirable and should cause the circuit branch to open.
• Time overcurrent, ANSI/IEEE code 51, is employed in inductive systems where current is expected to rise for a short duration after voltage changes before returning to a normal level.
• Time values for so-called ‘breaker curves’ are based on a series of graphical ‘curve’ values with current spikes leveling out as time increases.