In a series circuit of resistors and inductors, the phenomenon of voltage and current being in phase is called series resonance (also known as frequency conversion resonance). Its characteristics are: the circuit is purely resistive, the power supply, voltage, and current are in phase, the reactance x is equal to 0, and the impedance z is equal to the resistance r. At this time, the impedance of the circuit is the smallest and the current is the largest. Series resonance, also known as voltage resonance, may generate high voltages on inductors and capacitors that are many times greater than the power supply voltage. In power engineering, due to the occurrence of overvoltage and high current caused by series resonance, which can damage electrical equipment, it is necessary to avoid series resonance.
In parallel circuits of resistors, capacitors, and inductors, the phenomenon of the circuit terminal voltage and total current being in phase is called parallel resonance. Its characteristics are: parallel resonance is a complete compensation, and the power supply does not need to provide reactive power, only the active power required by the resistor. The total impedance of the parallel resonance circuit is large, so the total current of the circuit becomes small. However, for each branch, its current may be much larger than the total current. Therefore, parallel resonance, also known as current resonance, does not produce resonance overvoltage that endangers equipment safety, but each branch will produce overcurrent.