When the motor is operating normally, the current through the heating element is the motor's rated current. The heating element heats up, causing the bimetallic strip to bend, bringing the push rod into contact with the herringbone lever, but preventing it from pushing. The normally closed contact of the thermal relay is closed, the AC contactor remains engaged, and the motor operates normally. If the motor experiences an overload, the current in the windings increases, and this increased current through the thermal relay element causes the bimetallic strip to heat up further, bending more deeply and pushing the herringbone lever. The herringbone lever then pushes the normally closed contact, causing it to open and disconnecting the AC contactor coil circuit. This releases the contactor, cutting off the power to the motor and stopping it, thus providing protection.
Thermal relays have an inverse-time operating characteristic, meaning that the shorter the operating time, the greater the overload current multiple; and the longer the operating time, the smaller the overload current multiple. After the thermal relay operates, the bimetallic strip cools down over a period of time, and can be reset by pressing the reset button.
Thermal relays, as overload protection components for electric motors, are widely used in production due to their small size, simple structure, and low cost. Thermal relays are mainly used for overload protection of asynchronous motors. Their working principle is that when an overload current passes through a heating element, it heats and bends a bimetallic strip, which in turn pushes an actuating mechanism to activate the contacts, thereby disconnecting the motor control circuit and stopping the motor, thus providing overload protection.
Part 1: Working Principle and Selection of Thermal Relays
Working principle
Thermal relays contain temperature-sensing elements such as bimetallic strips or thermistors. Under normal operating conditions, the heat generated by the current flowing through the motor causes the temperature-sensing element to heat up and deform or change its resistance. Once the motor is overloaded, the current increases, and the temperature-sensing element heats up rapidly. When the temperature reaches the set operating value, the drive contacts open, thereby cutting off the power supply to the motor and providing overload protection.
Selection principles
The following factors should be considered when selecting a thermal relay:
Rated current: The rated current of the thermal relay should be slightly greater than the rated current of the motor being protected, with a certain margin (generally 1.1 to 1.25 times).
Setting current: Adjust the setting current value of the thermal relay according to the actual operating conditions of the motor and the maximum allowable continuous operating current.
Type and function: Select a thermal relay with appropriate delay characteristics according to the application, such as a fast-acting type, a general type, or a thermal relay with phase loss protection function.
Part Two: Wiring Method for Thermal Relays
Basic wiring method
Thermal relays typically have two main terminals—the main circuit terminal and the control circuit terminal.
1. Main circuit wiring:
The thermal relay's "thermal element" is connected in series with the motor's three-phase power supply line, in the main circuit of the motor. This ensures that all the motor's operating current flows through the thermal element, allowing for real-time monitoring of the motor's load condition.
When using a motor with star starting and delta operation, it is necessary to ensure that the thermal relay can correctly sense current in both connection methods.
2. Control circuit wiring:
The normally closed (NC) contact of the thermal relay is connected to the coil circuit of the contactor. Once the thermal relay operates due to overload, its contact will open, thereby cutting off the power supply to the contactor coil, causing the main contacts of the contactor to release and stopping the power supply to the motor.
The specific wiring steps are as follows:
Connect the L1, L2, and L3 terminals of the thermal relay to the U, V, and W phase lines of the motor, respectively;
- The normally closed contact NO of the thermal relay is connected to one side of the contactor coil, and the other side is connected to the power supply;
- If there are auxiliary contacts for signal feedback or alarm systems, they can be connected to the corresponding lines.
Part Three: Application Examples and Precautions of Thermal Relays
In practical applications, thermal relays are not only used in direct starting systems for motors, but also in complex circuits such as star-delta starting and forward/reverse control to achieve comprehensive overload protection.
Precautions:
- Ensure good contact during wiring to avoid malfunctions or failure to operate due to increased contact resistance;
- Regularly inspect and maintain the thermal relays, and test their operating characteristics to ensure they meet requirements;
- The operating time and reset time of the thermal relay should be set appropriately according to the operating environment conditions and motor load characteristics;
- When installing the thermal relay, take care to avoid it being affected by external heat sources, which could cause it to malfunction.
In conclusion, a correct understanding and mastery of the wiring methods for thermal relays is crucial for ensuring the safe and reliable operation of motors. From selection and wiring to daily maintenance, each step requires careful attention to fully utilize the protective function of thermal relays and effectively extend the service life of motors and related equipment.
