By analyzing the case of competition and risk issues in electrical control loops, it highlights the hidden dangers and potential consequences of such problems. This serves as a reminder to electrical engineers and practitioners to pay close attention to these issues, learn from past experiences, and continuously improve design principles to enhance the reliability, efficiency, and quality of electrical control systems.
In the context of electrical control loops, the presence of competition and risk can directly threaten the safety of both personnel and equipment, potentially leading to serious accidents with severe consequences. Therefore, it is essential to give these issues due consideration during the design and implementation stages. Competition and risk problems are particularly important in electronic circuit design because they directly affect whether the system operates reliably. Additionally, since electrical control systems involving strong currents are usually intuitive and easy to understand, issues related to competition and risk are often overlooked. These problems may arise due to poor design or incorrect wiring during installation or maintenance.
After the soft starter initiates the motor and switches to bypass mode, the control principle for free stop and delayed stop is illustrated in Figure 1. The motor starts through the energization of the intermediate relay KA1. Once the motor reaches the required speed, the intermediate relay KA2 closes, which energizes the bypass contactor KM, allowing the motor to run in bypass mode. When SB1 is pressed, the coils of KA2 and KM are de-energized, enabling the motor to stop freely. When SB2 is pressed, the time relay KT is energized, its normally open contact closes instantly and remains closed, until the time-delayed normally closed contact opens, at which point KA2 and KM lose power, and the motor stops. While the control logic appears sound, careful observation reveals potential issues in the delayed stop circuit.
If KA2 and KT are different types of components (e.g., one being an electromagnetic relay and the other an electronic relay), there may be a timing conflict during operation. When SB2 is pressed, KT is energized and latches on, but when the time-delayed normally closed contact of KT opens, the coil of KT is de-energized, causing the self-locking contact to open. At this moment, the time-delayed normally closed contact may close again, depending on the timing difference between the two relays. If KA2 has not yet released by that time, the motor will not stop properly. This inconsistency leads to unreliable performance, making it a classic example of a competition and risk issue in control circuits.
For instance, if KT is an electronic time relay like ST3PC-D, and KA2 is an electromagnetic intermediate relay like JZC3-22d, the electronic relay may act faster, while the electromagnetic relay may have residual magnetism, causing a delay in release. This can result in the KA2 not turning off properly, preventing the motor from stopping as intended. However, if both components are of the same type—such as replacing KT with JSK4-224d—it helps eliminate this problem and ensures more stable and reliable operation.
Figure 1 shows the principle of motor free stop and delayed stop control.
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