Medium-frequency furnace capacitors are critical components of medium-frequency induction heating power supplies. They serve primarily to improve the power factor, reduce harmonic components, and stabilize the supply voltage. These capacitors typically utilize metallized polypropylene film or metallized polyester film technology, offering advantages such as compact size, light weight, high insulation resistance, low dielectric loss, resistance to high temperatures and humidity, and excellent self-healing properties.
I. Overview of Medium-Frequency Furnace Capacitors
Also known as electro-thermal capacitors, these units employ roughened polypropylene film impregnated with a liquid (PCB-free) as the composite dielectric and high-purity aluminum foil as the electrode plates. Terminals consist of porcelain insulators fitted with copper threaded rods and copper water-pipe connectors. The housing is a cubic aluminum alloy tank, often equipped with water-cooling pipes. They are primarily used in controllable or adjustable AC power systems with rated voltages up to 3.6 kV and frequencies up to 50 kHz. Their function is to improve the power factor in applications such as induction heating, medium-frequency melting, round steel through-heating, casting equipment, and similar processes; they serve as the primary source of capacitive reactive power within the induction heating system's resonant circuit. Product performance complies with standards JB7110–1993 (*Electro-thermal Capacitors*) and IEC 60110–1998 (*Power Capacitors for Induction Heating Installations*).
II. Functions of Medium-Frequency Furnace Capacitors:
1. Improving Power Factor: These capacitors compensate for reactive power, thereby improving the power factor, reducing harmonic components, and minimizing pollution to the power grid.
2. Stabilizing Supply Voltage: They smooth out the supply voltage and reduce fluctuations, ensuring the stable operation of the medium-frequency induction heating power supply.
3. Enhancing Heating Efficiency: By improving the power factor, these capacitors allow for the full utilization of active power, thereby increasing the efficiency of metal heating.
III. Operating Conditions for Medium-Frequency Furnace Capacitors
1. Altitude not exceeding 1000 meters; for indoor installation.
2. The installation site must be free from severe mechanical vibration, harmful gases and vapors, and conductive or explosive dust.
3. The inlet temperature of the cooling water must not exceed 30°C. For capacitors rated below 1000 kVar, the water flow rate must be at least 4 L/min; for those rated at 1000 kVar or above, it must be at least 6 L/min.
4. Long-term overvoltage (not exceeding 4 hours within a 24-hour period) must not exceed 1.1Un, and long-term overcurrent (including harmonic current) must not exceed 1.35In.
IV. Usage of Medium-Frequency Furnace Capacitors:
1. The rated voltage and current of the medium-frequency furnace capacitor must match the medium-frequency induction heating power supply; they must not be arbitrarily replaced or substituted.
2. Medium-frequency furnace capacitors should be installed in a well-ventilated environment with a suitable temperature, avoiding direct sunlight and exposure to high heat.
3. Medium-frequency furnace capacitors should undergo regular inspection and maintenance; any abnormalities detected should be addressed or the unit replaced promptly.
4. During operation, the capacitor should be monitored for signs such as bulging or oil leakage, as well as unusual noises or odors; if any abnormalities occur, the equipment should be shut down immediately for inspection.
V. Structure of Medium-Frequency Furnace Capacitors:
The RFM-type water-cooled all-film medium-frequency furnace capacitor consists primarily of a core assembly and a casing. The core assembly is composed of multiple element groups connected in parallel; a cooling water pipe is welded to one side of the core (the common electrode plate), except in air-cooled models. The casing is fabricated from bent and welded aluminum alloy plates and features brackets at both ends for lifting and installation; the cover is equipped with water inlet/outlet pipes and terminals supported by bushings. The interior of the casing is filled with a high-performance insulating liquid surrounding the core assembly.
VI. Model Designation for Medium-Frequency Furnace Capacitors:
Model designation and meaning (Example: RFM3 0.75 - 1000 - 1 S)
R Series code (Electric heating) F Liquid dielectric code (Diarylethane)
M Solid dielectric code (Polypropylene film) 3 Design serial number
0.75 Rated voltage (kV) 1000 Rated capacity (kVar)
1 Rated frequency (kHz) S Suffix code (Water-cooled)
VII. Fault Analysis for Medium-Frequency Furnace Capacitors:
1. Internal insulation failure: If a noise is heard inside the capacitor during operation, it indicates insulation breakdown; the capacitor should be replaced promptly.
2. External grounding: If the insulation between the capacitor casing and the ground is compromised during operation, the casing may ground out, leading to a casing breakdown. In this event, the insulation of the capacitor cabinet requires reconditioning.
3. Cooling water blockage: Interrupted cooling water flow causes internal overheating and insulation damage. Therefore, the cooling water flow rate must be monitored during operation; appropriate measures should be taken if any flow abnormalities occur.
4. Oil leakage due to manufacturing defects: Poor sealing leads to oil leakage; the unit should be replaced. 5. Excessive intermediate-frequency voltage: If the operating voltage exceeds the capacitor's rated voltage (common ratings include 750V and 1200V) during prolonged use, overvoltage breakdown may occur. In such cases, the intermediate-frequency voltage should be lowered, or the capacitor replaced with a model having a higher voltage rating.
In summary, capacitors are critical components of intermediate-frequency induction heating power supplies; their performance directly impacts the system's power factor, heating efficiency, and service life. Therefore, when selecting and using these capacitors, it is essential to source them from reputable suppliers and authorized channels, as well as to prioritize proper maintenance and management to ensure the stable operation and longevity of the power supply system.
