A thermal overload ralay is a device to protect an electrical device like a motor/generator from catching

fire/burning out due to excessive heat produced during operation. The relay has a bi-metallic contact that

operates with temperature. When the temperature of the system goes above a pre-set value, the contact goes

from closed position to open, and the power supply to the circuit is cut and the device is saved from damages.

So it keeps the system with in the safe operating temperatures. You may find thermal relays for different

temperature values, based on the operating range of your equipment.

Thermal Overload Relays Information

Function

 Thermal overload relays are protective devices. They are designed to cut power if the motor draws too much

current for an extended period of time. To accomplish this, thermal overload relays contain a normally closed

(NC) relay. When excessive current flows through the motor circuit, the relay opens due to increased motor

temperature, relay temperature, or sensed overload current, depending on the relay type. 

 

Thermal overload relays are similar to circuit breakers in construction and use, but most circuit breakers differ

in that they interrupt the circuit if overload occurs even for an instant.  Thermal overload relays are

conversely designed to measure a motor's heating profile; therefore, overload must occur for an extended

period before the circuit is interrupted.

 

Specifications

 The GlobalSpec SpecSearch database contains information about various thermal overload relay

specifications, including type, electrical rating, switch details, and features.

 Type Buyers may choose between several different kinds of relays, including bimetallic thermal, solid state, or

temperature control types.

 Bimetallic Thermal Relays

 As their name implies, bimetallic thermal relays use a bimetallic strip to mechanically open the contacts. 

Bimetallic strips consist of two conjoined pieces of metal which expand at different rates when exposed to

heat.  This difference forces the strip to bend when heated.  In a thermal relay, the strip is attached by a spring

to a contact.  When excess heat from overcurrent causes the strip to bend and pull the spring, the contacts are

pulled apart and the circuit is broken.  When the strip is cooled it then returns to its original shape.

 This video illustrates the use of bimetallic switch, with the bimetallic strip highlighted in the middle of the

video.  When the flame is applied to the switch, the strip bends and the switch opens.  Note that when the strip

cools, the strip returns to its original position and the switch closes. 

 

Solid State Thermal Relays Solid state relays are electronic devices that have no moving or mechanical parts. 

Instead, the relay calculates  the average motor temperature by monitoring its starting and running currents.  Solid state relays tend to be faster than electromechanical ones, and also feature

adjustable set points and trip times.  Because they are incapable of generating a spark, they can be used in

explosive environments. 

 Temperature Control Thermal Relays

 Temperature control relays directly sense a motor's temperature using a thermistor or resistance thermal

device (RTD) probe which is embedded in the motor winding.  When the nominal temperature of the probe is

reached, its resistance increases rapidly.  This increase is then detected by a threshold circuit, which opens the

relay contacts.   

 Melting Alloy Relays

 A melting alloy (or eutectic) overload relay consists of a heater coil, a eutectic alloy, and a mechanical

mechanism for breaking the circuit. Using the heater coil, the relay measures the motor's temperature by

monitoring the amount of current drawn.

 Electrical Rating

 Relay electrical specs include current range, trip information, phase, and control voltage.

  Trip Specifications

 

Tripping is used to describe the circuit interrupting action of overload relays and circuit breakers.  Thermal

overload relays may include several specifications about this action.

 Full load current range refers to a range of current values for a relay to be set to.  A motor's rating plate will

include a full load current rating for that particular motor.  In order for the thermal overload relay to trip, the

relay's full load current point must be set to match the value on the rating plate.

 Temperature trip range applies to relays which are designed to measure temperature instead of current,

such as solid state or temperature control relays. 

 Trip class refers to the maximum time, in seconds, that a relay can withstand 6 times its current rating before

tripping.  For example, a Class 10 relay can carry 600% of its rated current for 10 seconds until it trips.  Trip

class is an important specification because a motor's starting circuit spikes the current draw for short periods

of time each time a motor is started.  An overload relay must be able to handle these high starting currents

without tripping.  Trip class timing might be said to allow a relay to "distinguish" between normally high

starting currents and abnormally high overload currents. 

 

Poles

 

The term "pole" describes the number of separate circuits controlled by a switch.  The number of circuits

determines the number of switch contacts, which in turn determines the poles needed to make or break the

contacts.  Switches typically have between one and four poles.

  Control Voltage

 Control voltage is an important specification because the control circuit voltage is often different from a

motor's specified voltage.  This is known as "separate control."  The control voltage is usually less than the

motor voltage, and an overload relay should be selected according to this specification. 

Features

 Buyers may choose a relay featuring a number of special attributes.

A relay with automatic reset will return to its original

"closed" position after a specified period of time.  If the motor is still overloaded after the reset, the relay

will trip again.

Relays with ambient temperature compensation operate efficiently over a wide range of ambient

temperatures. 

Some relays feature various degrees of phase monitoring.  These products may check for phase loss,

reversal, or imbalance.  When any phase problems are detected, the relay would trip and cut power to the

motor.  Phase imbalance in particular can cause dangerous fluctuations in a motor's voltage or current

flow and lead to motor damage.

Underload detection refers to a relay's ability to detect a drop in current as a result of unloading.  This

may occur if, for example, a pump begins to run dry.  These relays are designed to detect these

differences and trip as they would if detecting overload.

Relays with visual indicators are products that feature light-emitting diodes (LEDs) or other status

indicators.

 

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TENUGHAT THERMAL POWER STATION

PO : TTPS, LALPANIA

DIST : BOKARO PIN 829 149

Phone :06544 225434 (Control Room) Fax225414,225404

From: EEE(C&I)

TTPS, Lalpania.

To, Site Incharge

M/s Ekhlaq Khan

TTPS, Lalpania.

Sub: Cancellation of gate pass of Md. Azimuddin Ansari, Helper of the firm from 01/05/2014.

Sir,

This is to intimate you that the gate pass Md. Azimuddin Ansari, Helper of the firm is hereby cancelled

from 01/05/2014 till further order. This may please be forwarded to the CISF, Plant main gate.

Thanking You.

EEE(C&I)