Let the initial phase of
Note that is always in III and IV quadrant and is before by . When , is before by 0 or 180 degree.
Note that
The general solution is for some constants and and the eigen value of smaller absolute value
When capacitor does not exist, aka, is infinite,
Note that is before (therefore ) 90 degree.
Typical initial condition is that and are zero because no charge in the capacitor. If there is no capacitor, initial condition about can be omit, so instead, the initial condition is only
And:
Note that
Therefore,
Transformer example. , rating 3000 watt 220 V, then
Take the real parts,
Steady energy power:
If is near zero, i.e. the voltage starts applied at peak, it is safe to start the transformer. If is near or , i.e. the voltage starts applied at zero, then it will trigger huge current and fries the electronic parts. See for example of 240 VAC 50 Hz 0.02 R 0.03 L.
The solutions are:
- change v gradually so that is small aka "soft start". For example, to minimize the current less than 10A, change the voltage by steps of and wait for current stabilizing then repeat.
- Change gradually by some negative coefficient variable resistance. See The Simplest Soft Starter