# Homework Help: Phasor Diagram

1. Jan 30, 2014

### DunceKirchhoff

http://i60.tinypic.com/nmntpj.jpg

How do you differ what angle for example Vab, Vbc, and Vca are?

I notice the reference angle is always 0 degrees but how can you tell for the other two?

2. Jan 30, 2014

### Staff: Mentor

Look up "Three phase power". The power supply is designed to deliver voltages with a specific angular relationship between the supply leads.

3. Jan 30, 2014

### DunceKirchhoff

Yes but just by looking at the diagram you must be able to tell which is which?

Using Vab as a reference, the reference is always 0 degrees.

So if Vxy is used as a reference in another question that will also be 0 degrees.

How do I find out the other two?

4. Jan 30, 2014

### Staff: Mentor

The angles used (0, 120, -120 degrees) are fixed, but their assignment to particular leads is arbitrary in theory. It looks as though they've made the assignment for you by providing the phasor diagram and formulas. You take them as given.

In practical systems, such as the power grids of countries or industrial generators, there are standards in place that define the wiring colors associated with the particular angles.

5. Jan 30, 2014

### DunceKirchhoff

The phase angles are given here because it is a worked example. Questions there after do not give you these angles, instead they say for example use Vab for reference.

6. Jan 30, 2014

### Staff: Mentor

Then feel free to assign the angles as you please to the otherwise undefined leads. Or take the worked example as a motivation to assign them in the same way. Usually the only important criteria is that the each of the required angles is represented.

7. Jan 30, 2014

### DunceKirchhoff

I cant see any indication on how they have chosen which are angles which. Its important to know as it obviously effects the overall answers.

8. Jan 30, 2014

### Staff: Mentor

Why don't you post one of the questions that's giving you problems so we can see the issue? We can only give general suggestions to general concerns... a specific example might shed some light.