Principle of operation
Thermal motor protection relays contain three bimetal strips together with a trip mechanism in a housing made of insulating material. The bimetal strips are heated by the motor current, causing them to bend and activating the trip mechanism 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). A switching position indicator signals the condition “tripped”.
A = Indirectly heated bimetal strips
B = Trip slide
C = Trip lever
D = Contact lever
E = Compensation bimetal strip
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. Often both principles are combined.
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.
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.
For these reasons, the protection of motors in intermittent operation is insufficient.
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 characteristic of a bimetal relay is temperature-compensated, i.e. largely independent of its ambient temperature (see Figure 2 below).
I = Overload as a multiple of the set current
δ = Ambient temperature
– Limit values under IEC 60947-4-1
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 Figure below).
Ie= Rated current set on the scale
t = Tripping time
From a cold state:
a = 3-pole load, symmetrical
b = 2-pole load with differential release
c = 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 a longer time, damage should be expected.
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).
Resource // Low Voltage Switchgear and Controlgear – Rockwell