1 Introduction

In the switchgear portfolio, there are two types of products—powergear and controlgear. Powergear products are located at the upstream level and provide all the necessary protection features in addition to isolation. However, controlgear products are downstream devices used for switching the motors and other operating equipment. Along with that they can also sense fault and provide a tripping mechanism. They are not meant for isolation. The controlgear products can be widely categorized into two types—contactors and relays.

Contactors are devices with very high operating life meant for switching the motors on and off, even at very high frequencies. A relay is a fault-sensing unit, which provides a tripping command to the contactor in the event of any overload, short circuit, single phasing, locked rotor, or ground fault [1-3]. The project undertaken provides an in-depth study of the control gear product—“Relay.” The main aim of this project is “Yield Improvement of Overload Relay.”

The ideology of the project is to improve the existing overload relay (rating: 25A-37A) with design changes in existing mechanism which will improve the accuracy, reliability, and first testing yield of the product. Basically the existing product was verified by performing product tests and by finding various parameters regarding the mechanism and the quality of material used to manufacture the existing product [5-7]. As the power system, resources, and energy saving continue to progress, the demand of reducing their size and improvement of energy efficiency also increases. There is also a high demand of reducing the size and improvement of power consumption of thermal overload relays, which increases the competition between the industries.

Microcontrollers and electronic-based overload relays use comparators to compare the flowing current to a predetermined value, when the current value increases the predetermined value, the relay asserts an auxiliary output which opens the protected circuit, and thus the relay gets tripped protecting the circuit.

Relays are also installed in huge amount in railway signaling, since railway signal circuits must be highly protected electrically and there comes the need of installing relays. Railway signaling relays protect the system from false feeds, double switching contacts, and also used so that no false feed can cause a false signal [8]. MN-type thermal overload relays are specially manufactured for MNX-type contactors, and they work on the same principle as thermal overload relay. MN relays are also available in three frame sizes from 0.2A to 570A; they are directly mounted on MNX contactors and are modified for ambient temperature compensation.

2 Thermal Overload Relay (RTO Relay) and Its Classification

A relay can be defined as a switch; switches are generally used to close or open the circuit manually, and a relay is also a switch that uses electric signals to connect or disconnect two circuits instead of manual operation, which in turn connects or disconnects other circuits [9].

Thermal overload relay is an inverse time-delay overload relay which for its operation depends on a bimetal for producing differential thermal expansion due to the current flow thereby tripping a lever. Thermal overload relays are economic electromechanical protection devices for the main circuit. L&T’s thermal overload relays are manufactured mainly to complete the MO series of contactors.

3 Main Benefits

  1. a.

    Isolated alarm circuit contact.

  2. b.

    Ambient to temperature.

  3. c.

    Optimized match to MO contactors.

  4. d.

    Mounting kit is available separately.

4 Main Features

  1. a.

    Tripped/non-tripped indication from front.

  2. b.

    Sensitive to phase failure.

  3. c.

    Auto/manual functions at front.

  4. d.

    Easy access to START and STOP/ RESET buttons.

  5. e.

    Sealable transparent top cover.

  6. f.

    Easy to mount on MO contactors.

5 Principle of Operation

Above figure shows the circuit diagram of overload relay, various components are as follows:

A = Heated bimetal strips.

B = Trip slide.

C = Trip lever.

D = Contact lever.

E = Reparation bimetal strip.

The bimetal strips (heater) might be warmed legitimately or in a roundabout way. In the primary case, current directly flows through heater and in the second case current courses through a protection warming the heater. The protecting material causes some deferral of the heat stream so the inactivity of heat transfers that are heated is more noteworthy than with their privilege warmed partners.

The cooling time consistent of warm transfers is no longer than that of typical engines. This also prompts an expanding distinction between the real temperature and that reproduced by the warm hand-off in discontinuous technique.

6 Construction of Thermal Overload Relay

The construction of thermal overload relay is quite simple. Three bimetals are mounted on the housing of the relay. The three terminals of bimetal (heater element) act as an input for the relay. The three bimetal strips are assigned as R-pole terminal, Y-pole terminal, and B-pole terminal whose distance from the vertical wall of relay must be ideally 4 mm, 3 mm, and 3 mm, respectively. Two metals are attached on the bimetal strips, namely, metal A having low coefficient of expansion and metal B having high coefficient of expansion. Also one heating strip or wire is wound on every bimetallic strip. One end of the bimetal strips acts as an input and the other end acts as an output which goes to any contactor or motor.

On the bimetal strips a tripping lever is attached with a mechanism which is located exactly above the bimetal strips. This lever can move freely which is then attached to the tripping mechanism. This mechanism as a whole acts as a main tripping mechanism of thermal overload relay.

The front part of the relay has one START (green) and one STOP/RESET (red) button and along with that it has one knob (blue) for changing the range from maximum to minimum and also one knob which is for the setting of AUTO and MANUAL (yellow) mode. It also has one knob which can lock and unlock the STOP/RESET button.

7 Working of Thermal Overload Relay

When a relay is connected with a contactor of specified range and if the current is passed through it in the range of relay then the current will flow from relay to contactor without any disturbance but if an over current flows through the heating coil of relay, it heats up the bimetallic strips.

If the current flow in the circuit exceeds the specified range of relay, heat is generated in the coil, thus both the metals start to expand due to the heat generated by the coil. But metal B expands more than that of metal A. Thus, the bimetallic strip bends toward metal A because of this dissimilar expansion. This bending of bimetal will cause the tripping lever to displace from its position and it will apply force on the above tripping mechanism [11-13]. Thus, the tripping mechanism will trip the relay which will disconnect the circuit and the current will stop flowing. Thus, the contactor will be protected from getting damaged due to over current flow.

8 Contactor and Its Classification

A contactor is an exchanging device having just one situation very still and when worked, it is fit for making, conveying, and breaking electric flow under ordinary circuit conditions remembering working for over-burden conditions. Contactors are employed in control and power circuits, with load currents on the main contacts varying from a fraction of an ampere to hundreds of amperes, depending on the particular application.

Although the construction of a contactor is similar to that of a circuit breaker, its action is the exact opposite. A circuit breaker usually employs an electromagnet for its opening when a fault occurs, whereas a contactor employs an electromagnet for opening or closing a circuit under normal operating conditions. However, a contactor can withstand short-circuit conditions when used with a suitable S.C.P.D (short-circuit protective device).

9 Main Benefits

  1. a.

    Motor starters (direct online starter and star-delta starter).

  2. b.

    Furnace.

  3. c.

    Lighting.

  4. d.

    For switching of small resistive loads.

  5. e.

    For switching of solid devices.

  6. f.

    In domestic applications.

10 Principle of Operation

The electromagnetic contactors take a shot at Faraday's first law of electromagnetic enlistment. A current conveying conductor creates an attractive field around it. At whatever point the electromagnetic loop is invigorated, an electromagnetic field is delivered. This electromagnetic field pulls in the metallic bar (armature) toward the empty barrel-shaped magnet. In contactors with split electromagnets, the mobile half piece of the electromagnet is pulled in toward the fixed piece of the electromagnet.

11 Working of Contactor

In normal condition when the contactor is connected in an assembly, the bridge is balanced by two return springs keeping the bridge and the contacts at the original position. Initially, at rest, the moving and fixed magnets are apart from each other.

When coil current is switched OFF, the electromagnetic force is reduced to zero and compressed return springs come back to the original position bringing the bridge assembly to the normal position. Thus, the magnets and contacts are separated. This is OFF position of the contactor.

12 Tests Performed on Relays

  1. a.

    IS (Indian Standard) test.

  2. b.

    Temperature rise test.

  3. c.

    Deflection test.

13 Conclusion

While testing the sample of RTO-1 (25A-37A) relay for IS test and deflection test, it was observed that the relay was tripping under 5 min at 1.25In on maximum current setting in IS test, but at minimum current setting it was not tripping in 1.2In as well as in 1.3In.

For this problem, some changes were made in the sample as follows:

  1. a.

    The distance between the wall and pip was decreased.

  2. b.

    The distance between the wall and pip was increased.

  3. c.

    The gap between the lever and wall was reduced by sticking paper in the gap.

  4. d.

    The strip-type heater was replaced with the wire-type heater.

  5. e.

    The heaters were replaced according to their resistance value.

When the distance between the wall and pip was decreased, 1.05In current is passed through the relay.

When the heaters were replaced some parameters were checked and kept constant like the resistance of the heaters in both strip type and wire type were kept constant. The strip or wire which is wound on the bimetal is made up of copper and has its specific resistance which causes the heating of bimetal resulting is tripping of relay.

Thus, the conclusion was made that the relay with all the parameters remaining same except the relay with strip-type heater will not trip in the rating 25A to 35A unless the heaters are replaced with wire-type heaters.