The overload protection linkage design for an electric heater's rotary switch first requires establishing a precise mapping between gear positions and circuit parameters to provide a foundation for anomaly detection. Each gear position of the rotary switch corresponds to a specific heating power. The internal mechanical structure, through design features such as positioning slots and stop bumps, ensures that the contacts precisely switch to the corresponding circuit path during gear rotation. For example, a low gear position corresponds to a low-current circuit, while a high gear position corresponds to a high-current circuit. This prevents mechanical misalignment, which can cause anomalies such as "low gear position display but actual connection to a high-current circuit." Furthermore, a gear position detection element is integrated within the switch to capture the physical position signal of the gear position in real time and compare it with the current and voltage detection signals in the circuit. If a mismatch is detected (e.g., the gear position display is low but the current exceeds the corresponding threshold), the switch identifies the gear position as abnormal, triggering the preconditions for the protection linkage.
To address the common gear position anomaly scenario of contact sticking, the protection linkage design incorporates temperature sensing and fusing mechanisms within the contacts. The long-term on-off contact state of an electric heater's rotary switch can cause the contacts to stick due to arc erosion. This can prevent the contacts from opening after switching, causing the circuit to continue operating at high power and leading to overload. Therefore, temperature-sensitive components such as bimetallic strips or thermistors are placed near the contacts. When contact sticking causes circuit overload and elevated temperatures, the bimetallic strip deforms due to heat, pushing a mechanical linkage and forcibly disconnecting the main circuit. The thermistor's resistance changes dramatically with rising temperature, triggering the control unit to cut off power. These two complementary approaches prevent a single component failure from leading to a lack of protection. Furthermore, the contact material is made of a high-temperature, ablation-resistant alloy to reduce the risk of sticking and increase response time for the protection linkage.
The circuit-level overload protection component must be deeply linked to the rotary switch, rather than operating independently. The output of the rotary switch is connected in series with a thermal relay or overload protector. The tripping thresholds of these components are pre-set based on the rated current of each gear. For example, for high gear, the overload protector's tripping current is set to 1.2 times the rated current of that gear, while for low gear, it's set to a proportional value. When the circuit current exceeds the threshold due to a gear abnormality in the rotary switch, the overload protector quickly detects the current anomaly and disconnects the circuit directly through internal electromagnetic or thermal mechanisms, without relying on an external control unit. Simultaneously, the protector sends a fault signal to the rotary switch, causing it to lock the current gear, preventing the user from repeatedly switching gears and exacerbating the problem. This creates a closed-loop "detect-disconnect-lock" mechanism.
Mechanical torque monitoring and emergency disconnection design mitigate the risk of overload caused by gear jams. Some electric heater rotary switches have integrated torque sensors on their shafts. If the gears fail to shift properly due to internal component wear, foreign objects, or other factors, the user's torque on the rotary switch may exceed the normal range. The torque sensor detects this abnormal signal and, through a feedback mechanism, prevents further forceful rotation (e.g., by increasing rotational damping). Furthermore, it triggers an emergency power-off switch, shutting off the heater's main power supply. This design not only prevents mechanical damage from excessive force but also prevents overload caused by a gear stuck in a fixed power state (especially in high gears), thereby filling a gap in purely circuit-based protection for mechanical anomalies.
Gear position abnormality signal feedback and user alerts are crucial supplements to the protection linkage, ensuring that users are promptly notified of the fault and can address it. When the rotary switch detects a gear abnormality and triggers the protective cutoff circuit, the switch panel alerts the user with flashing indicators and a buzzer, clearly indicating a "gear abnormality" rather than a normal power outage. This prevents users from mistakenly believing the device is powered off and repeatedly attempting to power it on. At the same time, some intelligent switches transmit abnormality signals to the electric heater's main control unit, which then records the fault type (such as contact sticking or misaligned gears). This facilitates quick problem location during subsequent repairs, reducing maintenance time and preventing users from forcing the device into operation without correcting the fault, potentially creating secondary safety risks.
To address "hidden gear anomalies" caused by voltage fluctuations, the protection linkage design must incorporate voltage compensation and current correction logic. When the grid voltage suddenly rises, even if the rotary switch is in the normal gear, the actual circuit current may exceed the rated value, resulting in a "voltage-induced overload" that can be mistakenly identified as a gear anomaly. Therefore, the rotary switch control unit monitors the grid voltage in real time and dynamically adjusts the current threshold based on voltage fluctuations. For example, when the voltage rises, the current protection threshold for the corresponding gear is appropriately lowered to ensure that the actual operating current remains within a safe range despite voltage fluctuations. If the voltage fluctuation is excessive and exceeds the correction capability, the control unit directly triggers protection and disconnects the circuit, avoiding gear-current mismatch problems indirectly caused by voltage anomalies. This ensures that the protection linkage covers a wider range of abnormal scenarios.
The reset mechanism design of the protection linkage must take into account both safety and convenience to avoid accidental resets when the fault has not been eliminated. When the overload protection is triggered, the rotary switch will not automatically restore power supply, and the user must manually reset it. During the reset process, the switch will first perform a self-check to reconfirm whether the gear position and circuit parameters match. If the fault has been eliminated (such as contact adhesion removal and foreign objects removed), power can be restored only after the self-check passes. If the fault still exists, the self-check will prevent the reset and continue to issue warnings, forcing the user to resolve the fault first. The reset button of some switches is designed to be hidden in a hidden location or needs to be pressed for a specific length of time to further prevent misoperation and ensure that the equipment can only be put back into use after safety is confirmed, so that the overload protection linkage forms a complete safety chain from "abnormal identification-disconnection-warning-reset self-check".
