Overload relay working principle and features of thermal motor overload

Thermal overload relays play a very important for protection of motors and generators both. It should be very interesting to know how an Thermal overload works as all over load relays have same working principle.

Thermal motor overload protection relays feature bi-metal strips jointly with a trip mechanism in a casing made of insulating material. Whenever there is overload The motor current heats the bi-metal strips, making them bend and activating the trip mechanism after a particular interval which is based on the current- setting.

The release mechanism actuates an auxiliary switch that breaks the motor contactor's coil circuit as shown in circuit diagrams below. A changing position indicator signals the condition "tripped".

Above diagrams shows where overload is connected in motor circuit. You may see that in Control circuit overload relay comes first in line but in power circuit it comes at last in line.

Question arises why this is done??
This all done because to protect motor circuit as if any problem occurred in motor then instantly motor overload relay get tripped protecting other circuit from effects of motor faults.

Principle of operation of a three terminal delayed bimetal motor protection relay with temperature compensation

Fig. above shows the circuit of overload relay , various components are discussed below:-
A = heated bimetal strips
B = Trip slide
C = Trip lever
D = Contact lever
E = Reparation bimetal strip

The bimetal strips may be warmed directly or indirectly. In the first case, the current flows directly through the bimetal, in the second through an insulated heating.

The insulating material causes some delay of the heat flow so that the inertia of thermal relays that are heated is greater than with their right heated counterparts. Often both principles are joined.

For motor rated currents over approx. 100 A, the motor current is conducted via current transformers. The current transformer's secondary current subsequently heats the thermal overload relay.

This means that the dissipated power is reduced and, on the other, that the short-circuit withstand ability is raised.
The tripping current of bimetal relays can be set on a current scale - by displacement of the trip mechanism relative - so that the protection characteristic can be matched to the secure item in the key area of continuous responsibility.

The uncomplicated, economic layout can just approximate the transient thermal characteristic of the motor.

The thermal motor protection relay provides perfect protection for the motor, for beginning with following constant obligation. With frequent startups in intermittent operation the bimetal strips compared to the motor's significantly lower heating time constant leads to early tripping in which the thermal capacity of the motor is not used.

The cooling time constant of thermal relays is not longer than that of normal motors. This additionally leads to an increasing difference between the actual temperature and that simulated by the thermal relay in intermittent procedure.

Therefore, the protection of motors in irregular operation is not sufficient.


Temperature compensation

The principle of operation of thermal motor protection relays is centered on temperature rise.

Therefore the ambient temperature of the unit changes the tripping specifications. As the setup site and consequently the ambient temperature of the motor to be shielded usually is different from that of the protective device it is an industry standard the tripping characteris-tic of a bimetal relay is temperature-compensated, i.e. mostly independent of its ambient temperature.

Tripping tolerances for temperature -compensated overload relays
Figure above shows Tripping tolerances for temperature-compensated overload relays for motor protection under IEC 60947-4-1

I = Overload as a multiple of the set current
delta = Ambient temperature
Limit values-

That is attained with a compensation bimetal strip that makes the relative position of the trip mechanism independent.

Sensitivity to period failure

The tripping feature of three-pole motor protection relays applies subject to the state that all three bimetal strips are loaded with exactly the same current at exactly the same time.

If, when one pole conductor is interrupted, only two bimetal strips are not cool then the force needed to actuate the trip mechanism must be alone produced by both of these strips. This demands a higher current or results in a more tripping time (characteristic curve c in Figure).

Typical trip characteristics of a motor protection relay

Ie= Rated current set on the scale
T = Tripping time

From a cold state:
a = 3-pole load, symmetrical
b = 2-terminal load with differential release
c = 2-terminal load without differential release

From your warm state:
d = 3-pole load, symmetrical

As a way to also ensure the thermal overload protection of the motor in the instances of supply asymmetry and decline of a stage, high quality motor protection relays have mechanisms with phase failure sensitivity

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