# AC theory - Inductance

#### Steve1992

Joined Apr 7, 2006
100
Q. Inductor coil takes a current of 2A from a 12V dc supply.
When connected to 240V, 50Hz supply the current is 20A

Resistance ?
Impedance ?
Inductive Reactance?
Indutance of coil?

Ive been using the techniques shown in this sites tutorial.

R = 12V / 2A
Z = 240V / 20A

Since Z = R + XL

Rearranging:

XL = Z - R
= (12ohms at -90deg. (polar notation)) - (6ohms at 0deg.)

= 13.4ohms

Where am I going wrong?

Thanks

#### hgmjr

Joined Jan 28, 2005
9,029
10.39 ohms is the correct answer for the inductive reactance (XL).

It is not entirely clear how you arrived at your 13.4 ohms answer. Can you show a few more details of you calculation that lead to the 13.4 ohms? We should be able to spot where you went off the rails then.

hgmjr

#### Steve1992

Joined Apr 7, 2006
100
I was using polar notation and a calculator.

Can you show me, step-by-step, how you would do it with this method?

Thanks

#### hgmjr

Joined Jan 28, 2005
9,029
Steve1992 said:
XL = Z - R
= (12ohms at -90deg. (polar notation)) - (6ohms at 0deg.)
One flaw that I see is in the value you have substituted for Z in the above expression.

You indicate that the value for Z is purely inductive. This can not be true as you have already calculated that the load is made up of an inductor in series with a resistor. The only thing you can know from the problem statement is that the magnitude of the current is 20 Amps since it contains no information about the phase angle between the current and the applied voltage. Since you have only the magnitude ofd the current then you are in a good position to calculate the magnitude of the impedance Z. That is what you did to get the value 12.

I suggest that you take a look at the computing XL by using the method for finding the magnituide of a complex number.

hgmjr