Steve Bailey
Lifetime Supporting Member + Moderator
The discussion in this thread about inrush current in AC vs DC circuits got me thinking.
I wasn't satified with Vic's assertion that there is no inrush in a DC inductive circuit. In the section of my college physics book where they discuss induction and derive the same equations that Vic cited, there is an assumption that there is no iron in the vicinity of the inductor. Furthermore, the same equations apply to AC and DC circuits, the difference being that in the AC circuit the current is not a constant, but a sinusoidal function.
I'm hoping someone can post citations to references that explain the source of inrush currents.
Here's my take on it, unsubstantiated by any math.
First, there is a mechanical component to inrush currents. In the inductive devices we all deal with (typically starters and solenoid valves), you're moving an armature within the coil. That armature has to be accelerated. The energy required to accelerate the armature comes from the electrical current flowing in the circuit. Thus, the current in the circuit while the armature is accelerating will be higher than the current when the armature is at a constant (including zero) velocity.
Second, we know that in an AC inductive circuit, the current waveform lags the voltage waveform by a phase angle proportional to the inductance. Prior to closing the switch that completes an inductive circuit, the voltage and current can be anywhere from zero to 180 degrees out of phase. Could the inrush current have something to do with getting the two waveforms into a steady-state phase relationship?
We also know that we can reduce inrush by putting an RC network in parallel with the coil. In a capacitive circuit, the voltage lags the current, just the opposite to an inductive circuit. It would seem that the RC network in parallel to the coil, brings the circuit closer to a purely resistive circuit, in which the current and voltage are in phase with each other. This seems to confirm my thinking that inrush is related to the establishment of the phase relationship between current and voltage.
Idle speculation for a Sunday afternoon. Does anyone care to comment, poke holes in my reasoning, or point me to the applicable math?
I wasn't satified with Vic's assertion that there is no inrush in a DC inductive circuit. In the section of my college physics book where they discuss induction and derive the same equations that Vic cited, there is an assumption that there is no iron in the vicinity of the inductor. Furthermore, the same equations apply to AC and DC circuits, the difference being that in the AC circuit the current is not a constant, but a sinusoidal function.
I'm hoping someone can post citations to references that explain the source of inrush currents.
Here's my take on it, unsubstantiated by any math.
First, there is a mechanical component to inrush currents. In the inductive devices we all deal with (typically starters and solenoid valves), you're moving an armature within the coil. That armature has to be accelerated. The energy required to accelerate the armature comes from the electrical current flowing in the circuit. Thus, the current in the circuit while the armature is accelerating will be higher than the current when the armature is at a constant (including zero) velocity.
Second, we know that in an AC inductive circuit, the current waveform lags the voltage waveform by a phase angle proportional to the inductance. Prior to closing the switch that completes an inductive circuit, the voltage and current can be anywhere from zero to 180 degrees out of phase. Could the inrush current have something to do with getting the two waveforms into a steady-state phase relationship?
We also know that we can reduce inrush by putting an RC network in parallel with the coil. In a capacitive circuit, the voltage lags the current, just the opposite to an inductive circuit. It would seem that the RC network in parallel to the coil, brings the circuit closer to a purely resistive circuit, in which the current and voltage are in phase with each other. This seems to confirm my thinking that inrush is related to the establishment of the phase relationship between current and voltage.
Idle speculation for a Sunday afternoon. Does anyone care to comment, poke holes in my reasoning, or point me to the applicable math?