Help with Vector sum Questions. Please

[andy1]

I'm an electrical apprentice looking for alittle help with my schooling. Question is Two voltages of 90 V each are out of phase by 120 degree's, what is their Vector Sum. thanks

 

[bluesmoke...oops]

provided i remember correctly

vector A: 90 at 0 degrees (assumption on angle)

vector B: 90 at 120 degrees (120 degree separation)

Rx = Ax+Bx

Ry = Ay+By

magnitude: sqrt((Rx^2)+(Ry^2))

angle: inversetangent(Ry/Rx)

 

[rrc14]

The following may help;

Ax = r cos Φa
Bx = r sin Φa
&
Ay = r cos Φb
By = r sin Φb

 

[bluesmoke...oops]

oh yeah, my bad. so used to just doing those i forgot to write them down. :grin:

 

[andy1]

thanks for the Help, i guess the math is not that easy for me. Anyway you could lay it out more.

 

[chris kennedy]

I'm an electrical apprentice looking for alittle help with my schooling.

Welcome to the Forum and good luck with your studies.

Question is Two voltages of 90 V each are out of phase by 120 degree's, what is their Vector Sum. thanks

I predict 435 post on this.

 

[rattus]

Be careful now:

Be careful now:

By inspection, the magnitude of the sum is 90V, that is the third leg of a delta config.

However, if we want the line to line voltage of a wye config, we take the difference of the two phasors. The magnitude is 1.73x90V = 156v

Of course, one may use complex math to obtain the sum or difference if need be.

This sounds suspiciously like a two phase problem with the angles and voltages swapped. Could we really mean 120V @ 0 and 120V @ -90?

 

[tallgirl]

This sounds suspiciously like a two phase problem with the angles and voltages swapped. Could we really mean 120V @ 0 and 120V @ -90?

Unlikely -- three phase has a 120 degree voltage separation. My guess is the person wrote the question with 90V to keep someone from using their knowledge that the answer for L-L voltage of a system where L-N was 120 is 208.

Even more unlikely because two phase power is dead-dead-dead.

 

[LEO2854]

I'm an electrical apprentice looking for alittle help with my schooling. Question is Two voltages of 90 V each are out of phase by 120 degree's, what is their Vector Sum. thanks

When we refer to two phase electrical we are referring to two phases that are 90 degrees out of phase to each other. These will read the same to ground but this will not be doubled when reading phase to phase as in the residential panel.

Let’s say that the polyphase circuit (two phase) is 120 volts to ground for each phase then the voltage between the two phases would be about 170 volts. Welcome to the forum.

Check out this link
http://en.wikipedia.org/wiki/Two_phase

 

[rattus]

Not quite:

Not quite:

Even more unlikely because two phase power is dead-dead-dead.

I hear that there are pockets of 2-phase here and there, but no matter, I would think it still should be taught for the history.

 

[rattus]

Not quite:

Not quite:

Even more unlikely because two phase power is dead-dead-dead.

I hear that there are pockets of 2-phase here and there, but no matter, I would think it still should be taught for the history.

 

[steve66]

thanks for the Help, i guess the math is not that easy for me. Anyway you could lay it out more.

To add vectors, start by drawing the two vectors so that the tail of the second vector is at the head of the first vector. You have to do this without changing the length or direction of the vectors. Just move the second vector so its tail is at the head of the first vector.

Now the vector sum is the vector drawn from the tail of the first vector to the head of the second vector. If you drawn it right, all three vectors will make a triangle.

Now that you have your vector sum drawn, you could measure its length and angle to get your answer. But that's not very accurate. You want to use trig. to mathmetically find the result.

 

[mull982]

To add vectors, start by drawing the two vectors so that the tail of the second vector is at the head of the first vector. You have to do this without changing the length or direction of the vectors. Just move the second vector so its tail is at the head of the first vector.

Now the vector sum is the vector drawn from the tail of the first vector to the head of the second vector. If you drawn it right, all three vectors will make a triangle.

Now that you have your vector sum drawn, you could measure its length and angle to get your answer. But that's not very accurate. You want to use trig. to mathmetically find the result.

The vector sum is calculated as rattus explains above and comes out to a value of 90V.

However althought the OP did not say if you wanted to find the voltage difference between these two voltages you would need to subtract the two vectors in which case you would come up with about 156V.

 

[rrc14]

There are numerous site on the Internet that will explain how to perform vector addition. There are even calculators that will do this math directly. Anyways, hope this helps your understanding.

What is the vector sum of the following two voltages?
V1 = 90 ∠0
V2 = 90 ∠120
Convert voltages to rectangular form for ease of summation:
V1x= r cos ∠ = 90 cos(0) = 90
V1y= r sin ∠ = 90 sin(0) = 0

V2x= r cos ∠ = 90 cos(120) = -45
V2y= r sin ∠ = 90 sin(120) = 77.9

therefore:
V1 = 90 ∠0 = 90x + 0y
V2 = 90 ∠120 = -45x + 77.9y

Add the x?s and y?s :
V1+V2 = (90-45)x + (0+77.9) = 45x + 77.9y
Convert back to polar form:
R = √ (x^2 + y^2) = √ (45^2 + 77.9^2) = 89.96
Angle = inverse tan (y / x) = inverse tan ( 77.9/45) = 59.98
And,
V1 + V2 = 90 ∠ 60