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The working principle of an intermediate-frequency series resonant induction furnace

When the loop frequency f equals 1 / (2π√LC), the loop resonates, and the voltage across the test sample is Q times the output voltage at the high-voltage terminal of the excitation transformer. Here, Q represents the system’s quality factor—the voltage resonance multiplier—which typically ranges from tens to 100 or even higher. First, adjust the output frequency of the variable-frequency power supply to induce series resonance in the circuit. Then, under the condition of loop resonance, further adjust the output voltage of the variable-frequency power supply so that the voltage across the device under test reaches the specified test value. Due to the circuit’s resonance, the relatively small output voltage from the variable-frequency power supply will generate a significantly higher test voltage across the test capacitor CX.

Two medium-frequency series-resonant furnaces are powered by a single medium-frequency power supply, which can allocate its power arbitrarily between the two furnace bodies.

Composition of the Intermediate-Frequency Series Resonant Induction Furnace

Composition: It consists of a variable-frequency power supply, an excitation transformer, a high-voltage reactor, and a capacitor voltage divider.

n Variable-frequency power supply: Converts 220V/380V, 50Hz power into adjustable frequency and continuously variable voltage, while integrating operation, protection, control, and monitoring functions.

n Excitation transformer: It boosts the output voltage of the variable-frequency power supply while isolating high-voltage and low-voltage circuits.

High-voltage reactor: Series resonance with the capacitance test sample

n-voltage divider: Measures the high and low voltages on the voltage regulator.

A two-frequency intermediate-frequency series-resonant furnace has the following characteristics:

n High power factor operating characteristics

Since the system’s rectifier remains continuously enabled throughout the entire operating process, the system’s power factor is consistently at its highest level (≥0.96). A parallel-type medium-frequency power supply cannot achieve this performance.

The constant power output operation of the n-power supply can effectively shorten heating time and save energy.

When the system is operating, the output power of the intermediate-frequency power supply is adjusted by regulating the firing angle frequency of the inverter. This allows the system to maintain the output power at a given maximum level at all times through closed-loop control of voltage and current. As a result, the intermediate-frequency power supply can achieve constant power output, effectively enhancing the heating rate.

Excellent startup performance

Since power regulation is achieved by adjusting the firing angle frequency of the inverter, there are no starting issues, and 100% startup can be realized under all circumstances.