Some guidance please!

[hammond]

Hi there, im new and am coming to the end of my 2nd year. Have got the 204 left and 205.

The 205 im not so worried about, but on the 204 with the 3-phase and single phas circuits, once I have tested there are some questions that need answering, one beening....

Calculate the load power in the circuit.

So I think the formula is P=Square root of 3 X (VL X IL X Cosine)

But im confused on the whole formula and question

Any help?

 

[zupos40]

Hi there, im new and am coming to the end of my 2nd year. Have got the 204 left and 205.

The 205 im not so worried about, but on the 204 with the 3-phase and single phas circuits, once I have tested there are some questions that need answering, one beening....

Calculate the load power in the circuit.

So I think the formula is P=Square root of 3 X (VL X IL X Cosine)

But im confused on the whole formula and question

Any help?

What do you want to know, the load in question that you will hace to calculate will be single phase and you will be asked to measure it current and voltage and calculate it power rating P = VxI.

It is unlikely that you be calculating 3 phase power, it more Level 3.

Cosine or power factor (pf) would have to given to you unless the load is resistive then Cosine or pf = 1,
then Vl = 400V or measured
Il = measured using clamp ammeter

 

[hammond]

It was a 3 phase system and had to measure the following.

L1-N = 67.2
L2-N = 67.1
L3-N = 67.8

L1-L2 = 114.9V
L2-L3 = 115.5V
L3-L1 = 115.8V

We measured the current at different times when we added a bulb into the circuit

100W - 0.88A
200W - 1.83A
260W - 2.39A
230W - 2.94A

I believe the college restrict the mains and dont allow testing on 400v circuit.

And the question

" Calculate the load power (take the measured voltage for L1 and L2 and the measured current in the circuit)

 

[zupos40]

It was a 3 phase system and had to measure the following.

L1-N = 67.2
L2-N = 67.1
L3-N = 67.8

L1-L2 = 114.9V
L2-L3 = 115.5V
L3-L1 = 115.8V

We measured the current at different times when we added a bulb into the circuit

100W - 0.88A
200W - 1.83A
260W - 2.39A
230W - 2.94A

I believe the college restrict the mains and dont allow testing on 400v circuit.

And the question

" Calculate the load power (take the measured voltage for L1 and L2 and the measured current in the circuit)

Did you instal 100W in each phase, then 200W, then 260W, then 230W was this inductive ot typing mistake.

to calculate phase power = phase voltage x phase current then multiple by 3 or if balanced total power = square root x 3 x 115.9 x 0.88 repeat for each wattage.

 

[hammond]

Sorry, not to worry about that one have sorted it I think!

But another one that I am looking at which is in a single phase circuit.

It asks
"If the total lamp and choke voltage were added together and directly compared with the supply voltage, why would this be incorrect?"

Now is it because of the choke? Even though it helps reduce voltage drop, its output voltage dips? Or is there something else?

 

[zupos40]

Sorry, not to worry about that one have sorted it I think!

But another one that I am looking at which is in a single phase circuit.

It asks
"If the total lamp and choke voltage were added together and directly compared with the supply voltage, why would this be incorrect?"

Now is it because of the choke? Even though it helps reduce voltage drop, its output voltage dips? Or is there something else?

No the lamp voltagedrop is most resistive, where as choke is nearly pure inductive so the only way you can calculate them is via phasor diagram.

Try drawing the lamp voltage to scale along the horizontal, then draw the choke voltage vertically then draw a line between the end of each and this should equate to the supply voltage.

Sorry choke does not reduce the voltdrop it limits the current and produced sufficient voltage for lamp to strike.

 

[hammond]

Ok dokie

So why would it be incorrect then to add them together and directly compare them to the supply voltage?

 

[zupos40]

Because the voltages are not in phase ( 1 is resistive the other inductive look at you circuit theory notes) you could try square root V(lamp squared + V choke squared) to calculate Vsupply and set how far out you are.

 

[hammond]

Thats great! Thanks very much for your help!