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Working Principle Of Thermal Motor Protection Relay

Working Principle Of Thermal Motor Protection Relay

Principle of operationThermal motor protection relays contain three bimetal strips together with a trip mechanism in a housing made ofinsulating material. The bimetal strips are heated by the motor current, causing them to bend and activating the tripmechanism after a certain travel which depends on the current-setting of the relay.

The release mechanism actuates an auxiliary switch that breaks the coil circuit of the motor contactor (Figure 1). Aswitching position indicator signals the condition tripped.

A = Indirectly heated bimetal stripsB = Trip slideC = Trip leverD = Contact leverE = Compensation bimetal strip

The bimetal strips may be heated directly or indirectly. In the first case, the current flows directly through thebimetal, in the second through an insulated heating winding around the strip.

The insulation causes some delay of the heat-flow so that the inertia of indirectly heated thermal relays is greater athigher currents than with their directly heated counterparts. Often both principles are combined.

Figure 1 - Principle of operation of a three pole thermally delayed bimetal motorprotection relay with temperature compensation

Figure 2 - Tripping tolerances for temperature-compensated overload relays for motor rotection under

IEC 60947-4-1

For motor rated currents over approx. 100 A, the motor current is conducted via current transformers. The thermaloverload relay is then heated by the secondarycurrent of the current transformer.

This means on one hand, that the dissipatedpower is reduced and, on the other, that the short-circuit withstand capacity is increased.

The tripping current of bimetal relays can be set ona current scale by displacement of the tripmechanism relative to the bimetal strips so thatthe protection characteristic can be matched to theprotected object in the key area of continuous duty.

The simple, economical design can onlyapproximate the transient thermal characteristic of the motor.

For starting with subsequent continuous duty, the thermal motor protection relay provides perfect protection for themotor. With frequent start-ups in intermittent operation the significantly lower heating time constant of the bimetalstrips compared to the motor results in early tripping in which the thermal capacity of the motor is not utilized.

The cooling time constant of thermal relays is shorter than that of normal motors. This also contributes to anincreasing difference between the actual temperature of the motor and that simulated by the thermal relay inintermittent operation.

For these reasons, the protection of motors in intermittent operation is insufficient.

Temperature compensationThe principle of operation of thermal motor protection relays is based on temperature rise.

Therefore the ambient temperature of the device affects the tripping specifications. As the installation site andhence the ambient temperature of the motor to be protected usually is different from that of the protective device it isan industry standard that the tripping characteris-tic of a bimetal relay is temperature-compensated, i.e. largelyindependent of its ambient temperature (see Figure 2 below ).

I = Overload as a multiple of the set current = Ambient temperature- Limit values under IEC 60947-4-1

This is achieved with a compensation bimetal strip that makesthe relative position of the trip mechanism independent of thetemperature.

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Sensitivity to phase failureThe tripping characteristic of three-pole motor protection relaysapplies subject to the condition that all three bimetal strips are loaded with the same current at the same time.

Typical trip characteristics of a motor protection relay

If, when one pole conductor is interrupted, only two bimetal strips are heated then these two strips mustalone produce the force required to actuate the trip mechanism. This requires a higher current or results in alonger tripping time (characteristic curve c in Figure below).

Ie= Rated current set on the scalet = Tripping time

From a cold state:a = 3-pole load, symmetricalb = 2-pole load with differential releasec = 2-pole load without differential release

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

If larger motors (10 kW) are subjected to these higher currents for alonger time, damage should be expected.

In order to also ensure the thermal overload protection of the motor inthe cases of supply asymmetry and loss of a phase, high qualitymotor protection relays have mechanisms with phase failure sensitivity(differential release).

Resource: Low Voltage Switchgear and Controlgear Rockwell

Working Principle Of Thermal Motor Protection RelayPrinciple of operationTemperature compensationSensitivity to phase failure