Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 145)

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 tripping current of bimetal relays can be set on a current scale – by displacement of the trip
mechanism relative to the bimetal strips – so that the protection characteristic can be matched
to the protected object in the key area of continuous duty.
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 (
Fig. 4.1-3 Section 4.1.2.1 ). For these
reasons, the protection of motors in intermittent operation is insufficient.
Temperature compensation
The 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 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 characteris-
tic of a bimetal relay is temperature-compensated, i.e. largely independent of its ambient
temperature (
Fig. 4.1-5 ). This is achieved with a compensation bimetal strip that makes the
relative position of the trip mechanism independent of the temperature.
Sensitivity to phase failure
The tripping characteristic of three-pole motor protection relays applies subject to the condition
that all three bimetal strips are loaded with the same current at the same time. 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. This requires a higher current or
results in a longer tripping time (characteristic curve c in
Fig. 4.2-13 ).
If larger motors ( ≥ 10 kW) are subjected to these higher currents for a longer time, damage
should be expected (see Section 4.1.2.4.3 ). In order to also ensure the thermal overload
protection of the motor in the cases of supply asymmetry and loss of a phase, high quality motor
protection relays have mechanisms with phase failure sensitivity (differential release).
LVSAM-WP001A-EN-P - April 2009 4-37