Parallel resonance

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Parallel 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. 

The principle of parallel resonance

In an oscillating circuit where inductance, capacitance, and an external AC power sHVHIPOTce are connected in parallel, the inductance coil is usually represented by a series combination of resistance and inductance. The losses and leakage currents of capacitors are generally small and can be ignored under certain conditions. If the inductance and capacitance of the circuit are much larger than the resistance, that is, ω L (ω C)>>R, the natural frequency of the parallel circuit can be approximated as f=1/2 π LC. If Q, L, and C meet certain conditions to make the inductance and capacitance of the parallel circuit equal, BL=BC (BL=ω L, BC=1/ω C), so that the admittance B is equal to zero (B=BL – BC=0), then the current and voltage will be in phase (ω=0), which is called R, L, and C parallel resonance.

The conditions for the generation of 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. 

Characteristics of Parallel Resonant Circuit

1.If the external frequency is higher than the resonant frequency, the circuit impedance is capacitive, equivalent to a capacitor

2.If the external frequency is equal to the resonant frequency, the impedance of the circuit is purely resistive and has a maximum value, which is called a frequency selective circuit in practical applications 

3.If the external frequency is lower than the resonant frequency, the circuit becomes inductive, equivalent to an inductive coil 

So when the series or parallel resonant circuit is not tuned at the signal frequency point, the signal passing through it will produce a phase shift (i.e. phase distortion)

The hazards of parallel resonance

When parallel resonance occurs in power lines, the branch current often greatly exceeds the total current of the circuit, causing accidents such as fuse melting, switch tripping, or burning electrical equipment. But in radio engineering, it is often used to select signals and eliminate interference.