Q:

Three phase system questions

I have been trying to solve homework questions on three phase systems but I am got  stuck in two questions  while solving solving them.

Three similar impedances are connected in star across 2000V, 50Hz three phase
  supply. Power absorbed is 300KW. Current taken is 100A lagging. Find the values of
  circuit parameters in each phase.

While Solving this, I calculated : V_ph =2000/√3=1154.7V and the impedence to be: Z=Vph/100=11.54 ohms.
The power factor is:2000/3XVphXIph which comes out to be 0.28.
My question is how can i caluculate the power for each impedence.As far as I know ,I only know equation for total power

A three phase 400V motor has an output of 20HP with efficiency of 90% and at a power
  factor of 0.8. Determine the reading on each of the two wattmeters connected to
  measure the input. Also find the line current. 

So  input Power is 90% of 20 that is 18 Watts
So I presume W1+W2=18 watts
My book gives the formula: tan(θ)=√3(w2-w1/w2+w1) or tan(θ)=-√3(w2-w1/w2+w1) where w1 and w2 are wattmeter readings in the wattmeter. My question is how do i identify which equation to use.
Can anyone please help me with these doubts and also tell me what I am doing wrong.
EDIT:
input power is 100HP=74.57kW.

A:

Three similar impedances are connected in star across 2000 V, 50 Hz three phase supply. Power absorbed is 300 kW. Current taken is 100 A lagging. Find the values of circuit parameters in each phase.

I calculated : V_ph = 2000 / √3 = 1154.7V and the impedence to be: Z = Vph / 100 = 1.154 ohms.

Double-check that last bit.

The power factor is: 2000 /3 × Vph × Iph which comes out to be 0.28.

That's not the correct way to calculate PF. You need to calculate the per-phase power (kW) and per-phase apparent-power (kVA) and go from there.

My question is how can I calculate the power for each impedence. As far as I know ,I only know equation for total power.

As shown above.

A three phase 400 V motor has an output of 20 HP with efficiency of 90% and at a power factor of 0.8. Determine the reading on each of the two wattmeters connected to measure the input. Also find the line current.

So input Power is 90% of 20 that is 18 watts. 

Alarm bell 1: You reckon you can supply it with 18 W and get 20 W out!
Alarm bell 2: You think that 20 HP = 20 W. (It doesn't so you need to look that up.)

So I presume W1 + W2 = 18 watts.
My book gives the formula: tan(θ) = √3(w2 - w1 / w2 + w1) or tan(θ) = -√3(w2 - w1 / w2 + w1) where w1 and w2 are wattmeter readings in the wattmeter. My question is how do i identify which equation to use.

It depends on the relative polarity of the two meters. Since it's a motor you know the load is inductive and can base your result on that.

From the comments:

Why can't we use: P = 3 x V_ph x I_ph x p.f.?

You can but you wrote "The power factor is:2000/3XVphXIph" or \$ PF = \frac V {3V_{ph}I_{ph}} \$. It should be \$ PF = \frac P {3V_{ph}I_{ph}} \$ where \$ V_{ph} is the phase to star-point voltage (not phase to phase).
Yes, you can calculate from the total power.

Also I didn't get how u concluded that the load is inductive.

It's a motor.

