What Is a Thermal Overload Relay?

Thermal overload relay is an overload relay that uses the principle of current thermal effect and uses electrical thermal bimetal as a sensitive component, also known as a thermal relay. The so-called electrical heat-sensitive bimetal is made by heating and rolling two alloys with large differences in linear expansion coefficients. When heated, the bimetal sheet is bent from a high-expansion layer (active layer) to a low-expansion layer (passive layer). When the current is too large (exceeding the setting value), the element will act due to "heat", so that the interlocking dynamic breaking contact will cut off the power of the controlled circuit and the protected equipment. ^[1]

The thermal element of the thermal overload relay is connected to the motor circuit directly or via a current transformer. When the motor is overloaded, the main bimetal is heated to the operating temperature, causing the relay to operate, breaking off the power circuit of the motor, and protecting the motor from overload and damage. The operating time of the thermal overload relay and the magnitude of the overload current change according to the inverse time relationship. Therefore, its thermal characteristics are easy to match with the thermal characteristics of the motor, and it also has simple features, low price, stable operating performance, and convenient use. Therefore, in the 1990s, most squirrel-cage rotor motors and partially wound rotor motors still used thermal overload relays as protection for overload, phase failure, and unbalanced current operation. The thermal overload relay can also be used as a control appliance for the heating state of other electrical equipment. ^[2]

Scope of application of thermal overload relay: Thermal overload relay is used in circuits with AC 50Hz, rated insulation voltage 660V, and current 0.1 630A. It is mainly used for overload and phase failure protection of three-phase AC motors. It can be combined with an adaptable AC contactor to form a starter, such as the outline of a thermal relay.

A current adjusting cam is used to adjust the setting current.

There is a temperature compensation device to ensure that the operating characteristics are basically unchanged within the ambient medium temperature range of -20C to 60C.

A reset adjustment knob is used to adjust the reset mode, and there are manual and automatic reset states.

There is a tensile spring flip jumping mechanism to ensure that the contact moves quickly and reliably.

There are differential phase-opening / three-phase unbalance protection devices.

The detection slider / switch position indicator simulates the tripping of the thermal relay and displays the operating status. Check and ensure that the auxiliary circuit wiring is correct through this simulation. When the marking on the slider is located at the "0" mark, a trip is displayed, and at the "l" mark, the job is displayed.

There is a disconnect button. When the disconnect button is pressed, the normally-closed contact opens the series contactor to open the circuit. The load is disconnected. After releasing the disconnect button, the load resumes working through the contactor.

Overload relay can only be used as a control

(1) Setting current: The current used as a reference for the operating characteristics of thermal overload relays can usually be adjusted.

(2) Setting current adjustment ratio: The ratio between the maximum value and the minimum value of the setting current of each thermal element. For example, the adjustment range of the setting current of 3UA5900-2D is 20 ~ 32A, and the setting current adjustment ratio is 1: 1.6.

(3) Setting current limit range: The range between the minimum setting current of the smallest thermal element and the maximum setting current of the largest thermal element of each type of thermal overload relay. For example, 3UA59, the smallest thermal element 3UA5900-OA, 0.1 0.16A, the largest thermal element 3UA5900-2P, 50 63A, then the setting current limit range of 3UA59 is 0.1 63A.

(4) Rated current of thermal element: the maximum setting current of each thermal element. For example, the 3UA5900-2D has a thermal element rated at 32A.

(5) Relay rated current: The maximum or median value of the setting current of the largest thermal element. For example, the setting current of the largest thermal element of 3UA59 is 50 ~ 63A, then the rated current of 3UA59 is 63A.

(6) Critical current: Under specified conditions, the thermal overload relay can operate with a current slightly larger than this value, and a current slightly smaller than this value cannot operate, that is, the minimum operating current and the maximum non-operation. The average value of the current.

(7) Operating characteristics: The relationship between the operating time and the multiple of the overload current. Change according to the inverse time limit relationship, that is, the larger the overload current multiple, the shorter the action time. The operating characteristic is also called time-current characteristic.

(8) Q in cold state operation: Under the specified conditions, the time from when the relay passes the overload current in the cold state to its operation.

(9) Thermal state operation time: Under the specified conditions, the relay first passes the pre-heating current (usually 1.0 or 1.05 times the setting current), after reaching thermal stability, and then passes the overload current to its operation time.

Obviously, under the same overload current multiple, the hot state operating time is shorter than the cold state operating time. Generally, the hot state operating time is about 25% of the cold state operating time.

(10) Temperature compensation: Eliminate or reduce the effect of ambient air temperature changes on the operating characteristics of the thermal overload relay.

(11) Main sheet (main bimetal sheet): A bimetal sheet that is bent by heat when subjected to an overload current, and causes the thermal overload relay to operate.

(12) Auxiliary sheet (compensated bimetal sheet): A bimetal sheet that senses the change in ambient air temperature to compensate for the effects of ambient air temperature changes on the performance of the main sheet. ^[2]