“The main function of starter is protection!” Protection from Overload, Under-voltage, Over-voltage & phase loss conditions.
In DOL & Star-Delta starters; Thermal Overload relays are connected for the protection from overload & phase loss conditions. There are many companies which are manufacturing thermal overload relay In which the working mechanism remains the same But they all are having a different & unique design.
(NOTE- Next, We are using only Relay word instead to thermal overload Relay)
For understanding the working mechanism of relay; We just need to understand the single relay of any company. In this post, We are going to discuss L&T made MK-1 type overload relay which is much easier to understand. We will discuss it’s components, working mechanism & its pros-cons.
Thermal Overload relay Type MK1 by L&T
These relays are used in L&T made MK-1 DOL starters which is the most widely used DOL starters in industries for small rating induction motors.
According to motor sizes; This relay comes in different Ampere ranges :
0.6 to 1 Amps
1.5 to 2.5 Amps
2.5 to 4 Amps
4 to 6.5 Amps
6 to 10 Amps
9 to 14 Amps
At the top of relay; 3 input terminals are provided for 3phase connection. Whereas at the output side 4 terminals are provided in which 1 extra Terminal is used in hold Circuit.
In this relay 1NC & 1NO contacts are provided. ‘NC‘ contact is used As a stop button Whereas ‘NO‘ contact is used as a START button. In between both the contacts Adjustable current setting is been provided. A Red jumper wire is used to connect Both NO & NC while Blue wire is used in HOLD.
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Inside this relay, thermal strips are connected across each phase. Line current from all the three phases passes through these thermal strips. These thermal strips mechanically linked with NC push button in such a way that in overload or short circuit conditions these strips produces tripping command to NC switch.
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What’s inside Thermal Overload Relay
Relay Don’t break the load current directly. Instead, It is producing signal which is used to trip the control circuit. This relay don’t work independently, rather it works in along with a power Contactor.
[Contactor is electromechanical device which connects motor(output) with power supply(input) when it’s coil get energized.]
Tripping of these relay, actually break the path to energize the contactor’s coil which don’t allow contactor to operate.
In these relays, there are two types of tripping’s “Electromagnetic tripping” & “thermal tripping”.
- Electromagnetic tripping works in short circuit conditions. Sudden rise of current under short circuit creates higher magnetic field which produces tripping movement in the relay.
- Thermal tripping works in overload conditions. Due to the overload for particular interval of time, thermal bimetallic strips get heated up & get bend; which provides tripping movement in the relay.
In some cases, both the mechanism participates in tripping.
Next, Mk-1type thermal overload relay is been discussed :
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3 phase input is connected at the top of the relay.
See the image below In Which :
Point 1 is the point where input supply is connected then this supply passes to point 2(shown in yellow) After than supply passes to point 3 through a wrapped wire(Shown in Green).
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Actually, path 1 to 2 represents Thermal tripping which is basically a thermal Bimetallic strip upon which wire is wrapped with insulation paper under the wire. Path 2 to 3 represents Electromagnetic tripping which is basically an electromagnet. Point 3 is the output of relay.
This whole circuit is a power circuit where load current flows.
The thickness of wire & number of turns on Bimetallic strip depend upon the ampere rating of the relay. In an image below, We can see clearly that relay with ampere range 2.5 to 4 Amps contains thin wire & more number of turns While relay 9-14 Amps contains thick wire & less number of turns.
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When Higher current more than the setting current flows for particular interval of time then movement get produced either due to the Electromagnetic tripping or due to the Thermal tripping. In some cases, Both the tripping’s are responsible for tripping,
Actually, These Bimetallic thermal strips follows Inverse time characteristics which means more the fault current lesser will be the trip time. All the three Bimetallic strips are linked with one common plastic stick. Bend in any of the three strips produces the movement in plastic which is clearly explained in an image below.
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Before further explanation; We need to understand the mechanism of NO & NC Contacts, first !
NO & NC contacts
NO stands for “normally open”. NO contact stay open at its initial normal state. NO contact is fitted with torsion spring. At normal position; This torsion spring always pull the contacts to make the contacts open. Contacts closes only when it get pressed manually.
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NC stands for “normally closed”. NC contact stays closed at its normal initial state. NC contact is also fitted with torsion spring. This torsion spring always pull the contacts to make the contacts open But the element of NC contact is locked with trip system that make the NC contact closed at normal state. This NC contact get open when it get pressed manually or when trip system operates.(shown in an image below)
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We had observed the mechanism of NO&NC contacts which will help us to understand the working further. When thermal Bimetallic strips bend; it moves the connected stick with itself As explained above.
A Balancer is placed in between the stick which also moves along with the stick. This balancer further moves flat metal up which in turn moves the leaf metal spring upward which is locked with the NC contact mechanism. NC contact due to stored force by torsion spring get changes its contact open while leaf metal spring changes its position upward.
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Relay Reset
Relay get Reset only when Bimetallic strips cool down after than we need to press the NC contact.
Bimetallic strips needs time which further depend upon the fault current. More the current; More the time is needed to cool down the Bimetallic strips. At tripping time leaf spring try to normalize the balancer in Balanced state by pushing the flat metal back. When thermals get cool down; the balancer & leaf spring automatically reaches to its normal balanced state. But relay still not reset; This rest happen only after pressing the NC button.
Adjustable Current Setting
The trip system works when flat metal pushes the leaf spring up. After than leaf spring try to break the lock with NC as the push from flat metal increases.
In short, trip system works when flat metal pushes the leaf spring upward.
When we are adjusting current setting from minimum to maximum; we are actually increasing the gap between flat metal & leaf spring which is easily seen in an image Below:
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Working of overload relay with control circuit
How these relay trips the power circuit, we are going to explain in DOL starter MK1 by L&T. This MK-1 starter works when solenoid coil gets energized. Solenoid coil used in this starter is having a coil voltage of 415Volts(double phase).
As shown in control circuit below, one phase is directly connected to the right side of the coil & another phase is used in control circuit which is connected to the left side of the coil.
Control phase first passes through the NO contact of the relay than connected to NC contacts through red jumper wire. To complete the circuit, we need to press the NO contact of the relay. When Coil get energized starter starts working.
Since blue holding wire is connected in between both the NO&NC contacts, Starter get HOLD after pressing the NO contact only once. Starter will get OFF in three conditions:
- If we press the starter NC contact manually.
- In overload condition, tripping circuit trips the NC contact.
- Most obvious if power supply goes OFF.
For more explanation about the starter MK-1 you can go through DOL starter MK-1 by L&T
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Thermal overload relay manufactured by some other companies
There are many companies which manufacture thermal overload relay. The all are having different design but same working mechanism. Next we are going to show; overload relays from different manufacturers.
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Summary
- Starter employed for motors not only start the motor but provides protection from overload & phase loss conditions.
- It is thermal overload relay present in starter which provides protection from overload conditions.
- Overload relay don’t break the load current directly Instead it breaks the control circuit.
- Load current passes through Bimetallic strips inside the overload relay which continuously monitors load current. When load current exceed selected current value, thermal bimetallic strips bends & produces trip signal in NO & NC contacts available on overload relay.
- Contactor is the device which connects power supply(input) with motor(output) only when contactor’s coil get energizes.
- The control supply to energize the contactor passes through the NC contact of overload relay. In fault condition’s this NC contact breaks & interrupts supply to contactor
Hope ! Post is beneficial for you. Please do comments;
Comments and queries are always welcome. !
Dear Sanjeev ji, if the NO Contact (referred START) is not used and that jumper phase wire is directly connected to NC Contact (95 terminal) , also from NC-96 terminal one wire connected to contactor coil via a TIMER in order to automatically control the starter, IS THE OVERLOAD RELEY GETS BYPASSED FOR NOT USING THE NO(START) SWITCH?
NO !NC terminal 95-96 is used for tripping of the circuit
An excellent article, thanks for sharing your knowledge, been looking for this type of content on widely components in DOL starter, glad I found this one. Thanks
Thankyou for your valuable comment !