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1, Overview of Characteristics of Series Resonance and Parallel Resonance
The variable frequency series resonance test device, also known as series resonance, is composed of a variable frequency power supply, excitation transformer, reactor, and capacitive voltage divider. The capacitor and reactor of the test sample form a series resonant connection; The voltage divider is connected in parallel to the test sample to measure the resonant voltage on the test sample and provide an overvoltage protection signal; The frequency modulation power output is coupled to the series resonant circuit through an excitation transformer, providing excitation power for series resonance.
In a circuit where inductors and capacitors are connected in parallel, when the size of the capacitor precisely causes the voltage and current in the circuit to be in phase, that is, when all the power supply energy is consumed by the resistor and becomes a resistor circuit, it is called parallel resonance. Parallel resonance is a complete compensation, where the power supply does not need to provide reactive power, only the active power required by the resistor.
During resonance, the total current of the circuit is minimized, while the current of the branches is often greater than the total current of the circuit. Therefore, parallel resonance is also known as current resonance. When parallel resonance occurs, a large current flows through the inductive and capacitive components, which can cause accidents such as circuit fuses melting or burning electrical equipment; But in radio engineering, it is often used to select signals and eliminate interference.
2, Circuit characteristics of series resonance and parallel resonance
Circuit characteristics of series resonance
1. Minimum total impedance value: Z=R+j (wl-1/wc)=R;
2. When the power supply voltage is constant, the current is at its maximum; I=I0=U/|Z|=U /R;
3. The circuit is resistive, and the voltage of the capacitor or inductor may be higher than the power supply voltage.
Characteristics of Parallel Resonance Circuit
1. When the voltage is constant, the current is minimized during resonance;
2. Maximum total impedance;
3. The circuit is resistive, and the branch current may be greater than the total current.
Through the analysis of circuit resonance, we can grasp the characteristics of resonant circuits. In practical reproduction, we should use their strengths and avoid their weaknesses.
3, Product characteristics of series resonance and parallel resonance
Main characteristics of series resonant products
1. The required power capacity is greatly reduced
The series resonance test device uses a resonant reactor and the capacitance of the test object to generate resonance, thereby obtaining the required high voltage and high current. In the entire system, the power supply only needs to provide the active consumption part of the system. Therefore, the required power supply for the test is only 1/Q times the test capacity (Q is the quality factor).
2. The weight and volume of the equipment have been greatly reduced
In a series resonant power supply, not only is the bulky high-power voltage regulating device and ordinary high-power power frequency test transformer eliminated, but the resonant excitation power supply only requires 1/Q of the test capacity, greatly reducing the system weight and volume, usually 1/5 to 1/10 of ordinary test devices.
3. Improve the output voltage waveform
A resonant power supply is a resonant filtering circuit that can improve the waveform distortion of the output voltage, obtain a good sine wave, and effectively prevent the false breakdown of the test sample caused by harmonic peaks.
4. Prevent large short-circuit currents from burning the fault point
In the resonant state, when the weak point of the insulation of the test sample is broken down, the circuit immediately becomes detuned (capacitance changes, not meeting the resonance condition), and the circuit current rapidly decreases to 1/Q of the normal test current. However, when using parallel resonance or traditional test transformers for AC withstand voltage testing, the breakdown current immediately increases by tens of times. Compared with the two, the short-circuit current is hundreds of times different from the breakdown current. So, series resonance can effectively identify insulation weaknesses without the concern of large short-circuit currents burning the fault point.
5. There will be no overvoltage recovery
When the test sample experiences breakdown and flashover, due to the loss of resonance conditions, the high voltage immediately disappears, the arc is immediately extinguished, the protective circuit of the device is activated, and the output is cut off.
Main characteristics of parallel resonance products
Parallel resonance tuning and boosting are very stable, and tuning can be performed at low voltages. The tuning process depends on the magnitude of the current on the high voltage side of the boosting transformer. When the current is minimized, it is the resonance point, and then the voltage is boosted to the required voltage. For automatic tuning, it is also easy to control and can avoid voltage oscillations during the tuning process of series lines.