Working principle of transformer pdf

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The bimetal strips may be heated directly or indirectly. In the first case, the current flows directly through the bimetal, 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 at higher currents than with their directly heated counterparts. For motor rated currents over approx.

100 A, the motor current is conducted via current transformers. The thermal overload relay is then heated by the secondary current of the current transformer. This means on one hand, that the dissipated power is reduced and, on the other, that the short-circuit withstand capacity is increased. The simple, economical design can only approximate the transient thermal characteristic of the motor. For starting with subsequent continuous duty, the thermal motor protection relay provides perfect protection for the motor. With frequent start-ups in intermittent operation the significantly lower heating time constant of the bimetal strips 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 an increasing difference between the actual temperature of the motor and that simulated by the thermal relay in intermittent operation. For these reasons, the protection of motors in intermittent operation is insufficient. Therefore the ambient temperature of the device affects the tripping specifications. As the installation site and hence the ambient temperature of the motor to be protected usually is different from that of the protective device it is an industry standard that the tripping characteristic of a bimetal relay is temperature-compensated, i. This is achieved with a compensation bimetal strip that makes the relative position of the trip mechanism independent of the temperature.

If, when one pole conductor is interrupted, only two bimetal strips are heated then these two strips must alone produce the force required to actuate the trip mechanism. Hello i want to ask something that i m a bit confused. Or must the all three currents be above to trip the relay? THE THERMAL PROTECION, DOES NOT HAVE ANY COILS, RIGHT?