In high-power marine motors, issues often arise when the motor is not in operation or stored for extended periods. The humid environment inside the cabin can significantly reduce the insulation resistance of the motor, which may fall below the minimum required level for safe startup. If the propulsion motor’s insulation resistance does not meet this threshold, it cannot be started. To prevent this, internal drying must be carried out before the motor resumes operation to restore the insulation resistance to an acceptable level.
Therefore, during the design phase of high-power marine motors, it is essential to incorporate electric heaters. These heaters help prevent moisture and condensation from lowering the motor's insulation resistance, thereby enhancing the overall reliability of the motor during operation. When designing the electric heater, factors such as the motor’s weight, power capacity, internal airflow circulation, and structural layout must all be considered. The appropriate type and size of the electric heater are determined based on the specific requirements of the motor.
The power capacity of the electric heater is typically determined using empirical data. Based on factory experience, a table is used to establish the relationship between the motor’s weight and the required heater power. Once the power supply voltage is known, the heaters are connected in series or parallel depending on the motor’s structure, and the necessary resistance values for each heating element are calculated.
The electric heating element is then designed according to the required resistance value. The choice of electrothermal material depends on the maximum temperature that the hot part will reach. The allowable surface power density of the heating element is determined by referring to standard charts, taking into account the extreme temperatures of the selected alloy.
When designing the electric heater structure, it is crucial to ensure that key components, such as the ends, are properly sealed. As shown in the figure, the end of the heating element achieves insulation and sealing through two layers of protection. The metal tube of the electric heating element is usually made of ordinary steel or a material with better performance. If other alloys are used, the wall thickness should be at least twice the standard. For materials like copper or copper alloys, sufficient mechanical strength is required to withstand harsh working conditions.
The shape of the electric heating element varies depending on the internal structure of the marine motor. For curved elements, the bending radius must be at least twice the diameter of the pipe. During the manufacturing process, the ends of the rods must remain in the straight section of the tube, with a minimum distance from the bend point. The insulating filler between the metal tubes must be thick enough to ensure proper insulation between the current-carrying parts and the metal tubes.
The cross-sectional area of the lead bar should be at least twice that of the heating wire. The selection of the electric heating element’s shell material depends on the medium it will be exposed to, such as water, weak acid, weak alkali, or boiling solutions. Common materials include stainless steel, carbon steel, nickel-based alloys, and aluminum alloys. Each material has specific maximum temperature and surface load limits that must be adhered to.
Nickel-chromium alloy wires are often recommended for marine applications due to their excellent heat resistance, good processing performance, and cost-effectiveness. These wires maintain their structural integrity at high temperatures and provide reliable performance under challenging conditions.
Commonly used electrothermal materials include nickel-chromium and iron-aluminum alloys. New electric heater designs must comply with industry standards and undergo various tests, including appearance inspection, rod length measurement, cold-state temperature testing, dielectric strength testing, sealing performance evaluation, power deviation measurement, leakage current testing, and overload testing.
In addition to these standard tests, marine electric heaters must also pass vibration tests to account for the harsh environmental conditions they may encounter. This ensures the heater remains functional even under shock and vibration.
In conclusion, the electric heater system described here features a robust design, straightforward calculation methods, and long service life. It is well-suited for use in wet and hot marine environments, as well as in devices subjected to severe mechanical stress.
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