Hi,
I am stuck (again!) I am trying to calculate the total impedance in complex number form.


Z=V
____
I




= V+j0
_____
a - jb
Now all i have is the voltage/power factor(lagging)/current/kw rating.

Where do the a & b come from? If a= R and b=Xc i dont have these values.

Could someone please point me in the right direction to calculate this. Thanks

Edited: 06 November 2009 at 03:05 PM by sparky2000

Sorry, I could not read your formulas above well. Are you trying to say:
z = V / I = V+j0 / a - jb ????

Xc = 1 / jwC for capacitance where "w" is 2 pi F
So that would mean "a - jb" = a - jXc = a - 1/wC. Because the j cancels out.

Does this help?

Hi eswni,
I am fine at calcualting Xc 1/ 2.pi f c & Z=R+j(Xl-Xc)
but i have no values for a & b to put in the formula. Would a=R and jb=Xc. If so how can i calculate these. Thanks

Try to look past the one formula you have and see the other ways to calculate things.

http://www.allaboutcircuits.com/vol_2/chpt_11/3.html
http://www.allaboutcircuits.com/vol_2/chpt_11/2.html

Note how you get PF and thus how you can get the phase angle from it.

Note how you can use true power (kW) and PF to get Apparent power.

Note the power triangle.

Note the other ways to calculate Z and R.

And there are many other relationships and methods.

Regards.

Thanks again westonpa, im sure i will be back soon for some more advice!

Hi could someone please confirm if im on the right tracks for this please
true power 2240w
400v
pf 36.86 deg lagging

Total amps =2240/ (400x0.8) = 7 amps

Total impedance
opposite =adjacent
hypotenuse
opposite ??? ohms
adjacent 400volts
hyptenuse 7 amps

7 x sin36.86 =4.199 ohms ??

Sin (ang) = Opp / Hyp
Cos (ang) = Adj / Hyp
Tan (ang) = Opp / Adj

Hyp^2 = Opp^2 + Adj^2

If you say pick 'I' as your reference that goes along your x axis.

If you then plot your voltage triangle then:

Vl is y axis in upward direction.

VR is along x axis......in phase with current.

Vs is Hyp and obviously as per CIVIL (look up on Google if you do not understand it) it leads your ref (I) by the phase angle. From there you can use the triangle stuff to work out VR and VL.

But you could use V as a ref and plot IL, IS, IR.

Or you could plot an impedance triagle of Z, R, XL (in your case).

Or you could plot a power triangle of TP, AP, RP.

And do note that if your circuit is series the same current flows through it all.....it just leads or lags the voltage supply by the phase angle. This is why it is best to use it as a reference in series circuit, because it is COMMON, i.e., same current through inductor and resistor.

Regards.

Edited: 08 November 2009 at 05:41 PM by westonpa

Hi, im ok with TP AP RP + Z R XL and their positions in a triangle,
but am unsure for IL IS IR.. & Vi Vr Vs and their meanings?? Are they the same as working out the true/ reactive/ apparent but for V & I.

Edit; im ok on Vi Vr Vs positions in triangle

Thanks.

Forget IL, IS and IR unless you have a parallel circuit.

VL = voltage across inductance (L).
VR = voltage across resistance (R). Note Voltage and Current are in phase with resistance. No phase angle!
VS = voltage supply. So for example VS^2 = VR^2 + VL^2.
Or if you have VS and a phase angle and you plot a triangle you can get VL or VR from transposing the relevant Cos (ang) = Adj/Hyp and using the calculator.....or Sin (ang) = Opp/Hyp.....etc.

You can generate many different triangles but you need to keep things consistent.

Regards.

PS: Remember:

Rectangular coordinates are generally given as z = a + jb where for example which in effect means z = R + jXL....in ohms. But equally you could have VS = VR + jVL.

Polar form includes the phase angle.

See http://www.teacherschoice.com....angular_conversion.htm

Edited: 08 November 2009 at 08:22 PM by westonpa

I have series circuits to calculate + parallel.
Could you explain il Is Ir.

Thanks for your input to this.

IL = current through inductor.
IR = current through resistor.
IS = current supply.

In a paralled cct inductor may be in one arm and resistor in the other and thus you use V as your ref because it is COMMON across the branch but obviously the I though L and R can be different. However again I and V in phase with R hence IR on x axis with ref V. Reference is only really used to decide what/how to plot, i.e., you need an x line to lead and lag from and keep things consistent.

Regards.