Electronic – Inductor Voltage and Current After Switches Open and Close

currentinductorswitchestimevoltage

I was given this problem where switch s1 and s2 are opening and closing at different times.

I have already solved for the voltages and currents, but now I need to plot vL and iL at each time. My question is, when I plot the voltage and current, will the values jump instantly? For example, the current at 1 second is 40A and at 3 seconds it is 20A. Should my graph immediately change from 40A to 20A or will there be a linear decrease?

Here is the schematic and the graph showing when each switch opens and closes.

enter image description here

enter image description here

Best Answer

You might recall this formula: -

$$\boxed{V = L\cdot\dfrac{di}{dt}}$$

At t = 0, you say that the inductor is carrying 10 amps. At that moment S1 closes and the current will ramp upwards linearly at a rate of: -

$$\dfrac{di}{dt} = \dfrac{V}{L} = 30\text{ amps per second}$$

So, after 1 second the current will be 40 amps (due to the initial current of 10 amps). The current rise is linear (as with any perfect inductor connected to an ideal voltage source).

When the switches change (at t = 1 second) to apply -1 volt to the inductor, the current will ramp down at 10 amps per second so, after a further 2 seconds, the current will linearly fall from 40 amps (at 1 second) to 20 amps at 3 seconds.

Should my graph immediately change from 40A to 20A or will there be a linear decrease?

As with all perfect inductors and ideal voltages, the current never changes instantly but changes as a ramp.