Application of Thermal Relay in Railway Signal Motor

Photocoupler

The reversible operation circuit of a railway signal motor is illustrated in the figure below. In this diagram, FR represents a thermal relay, which serves as an overload protection device. To enable the motor to rotate in both forward and reverse directions, two contactors, KM1 and KM2, are used to reverse the phase sequence of the three-phase power supply connected to the motor. However, it's crucial that KM1 and KM2 cannot be activated simultaneously, as this would result in a short circuit. Therefore, an effective interlock mechanism must be implemented in the circuit. Figure 4 shows a control circuit that allows for bidirectional motor operation with dual interlocking—mechanical interlocking using buttons and electrical interlocking via contactors. The interlocking mechanism works as follows: (1) Contactor Interlock: The coil circuit of KM1 is connected through the normally closed auxiliary contact of KM2, while the coil circuit of KM2 is connected through the normally closed contact of KM1. When the forward contactor KM1 is energized, its normally closed auxiliary contact opens, cutting off the power to KM2. This ensures that KM2 must be de-energized first before KM1 can be activated, preventing a dangerous short circuit. This setup is known as the interlock circuit. (2) Button Interlock: The forward and reverse operations are controlled by push buttons SB2 and SB3. Each button has a set of normally open and normally closed contacts that are connected to the respective contactor coils. For example, the normally open contact of SB2 is in series with KM2’s coil, while its normally closed contact is in series with KM1’s coil. Similarly, the normally open contact of SB3 is connected to KM1’s coil, and its normally closed contact is in series with KM2’s coil. As a result, pressing SB2 activates KM2 and deactivates KM1, while pressing SB3 activates KM1 and deactivates KM2. If both buttons are pressed at the same time, neither contactor will be energized, ensuring safe operation. Once the motor is running in one direction, you don’t need to press the stop button to change its direction. Instead, simply press the opposite start button to switch the motor’s rotation. This makes the system more efficient and user-friendly.