inductor resistor circuit

[lecto1234]

hi all
I am trying to work out a part of a question for some time now and not getting true, I need a little help here

The circuit comprises of an inductor of value 9H and is connected in series with a 60 ohm resistor to a 240V d.c source
I found the time constant which is L/R = 0.15, I also found the current after one time constant which worked out to be 2.527A, I found the time for the current to reach 2.5A which worked out to be 0.147sec . The steady state value of the circuit worked out to be 4A (Vs/R)
My problem is working out the (i)Time to develop maximum current.
I(1-e-(t/time constant))
used this formula but not getting the correct answer and (ii) the initial rate of change of current.
According to the book the answers should be (i)0.75 and (ii)26.67A/s

 

[MikeMl]

The first 0.5s to see the time constant:



The first 1ms to get the initial slope:

 

[Tony Stewart]

τ=L/R is the 60% time constant.

i) Steady state is regarded to be 5x τau.
ii) dI/dt = V/L or 240V/9H=26.6666...

In reality the iron core might begin to saturate before steady state and di/dt might reduce as much as 50% below 30% of steady state and leave the core magnetized with remenance.