Problem 7, Chp. 40 Camara Practice Probs.

[frecoder78]

I was wondering how the solutions manual arrive to Q/3 phases = VaIa = Va(Eg-Va)/Xs?

When I first looked at this problem I figure that the calculated rotor current would have been Va/Ra. Any help would be great appreciate!

chap_40.doc

 

[dzdave00]

Va/Ra would give you the armature current, not the rotor current. The rotor current is tougher to visualize. I do not like how Camara did the solution on this. It is unclear and lazy and the notation used to describe what value is being solved for is unclear. On top of that, the reference book is of no help as well.

Let's see if this makes more sense:

What you are trying to calculate is the current in the rotor using values given that are on a different part of the motor. The rotor current is eventually determined by the Field Voltage (Eg in the book) divided by the impedance of the field. You know the reactive power output of the motor. In the first equation of the solution, they break down Ia into a voltage drop across the synchronous reactance in order to calculate the field voltage using the synchronous motor equivalent circuit in the reference book. The next couple equations are just plugging in known values and solving for Eg.

The field resistance is the phase voltage divided by the no load (field) current = 254 V/5 A.

Now you have the two values needed to calculate the rotor current.

I left out some explanation on how the solution was done in the book, so if anything is still unclear, let us know. Hope this helps.

 

[frecoder78]

Va/Ra would give you the armature current, not the rotor current. The rotor current is tougher to visualize. I do not like how Camara did the solution on this. It is unclear and lazy and the notation used to describe what value is being solved for is unclear. On top of that, the reference book is of no help as well.
Let's see if this makes more sense:

What you are trying to calculate is the current in the rotor using values given that are on a different part of the motor. The rotor current is eventually determined by the Field Voltage (Eg in the book) divided by the impedance of the field. You know the reactive power output of the motor. In the first equation of the solution, they break down Ia into a voltage drop across the synchronous reactance in order to calculate the field voltage using the synchronous motor equivalent circuit in the reference book. The next couple equations are just plugging in known values and solving for Eg.

The field resistance is the phase voltage divided by the no load (field) current = 254 V/5 A.

Now you have the two values needed to calculate the rotor current.

I left out some explanation on how the solution was done in the book, so if anything is still unclear, let us know. Hope this helps.

Thanks dzdave00! I appreciate your help immensely. I have been having a little difficulty understanding DC and AC motors. My main concern is when I'm reading a problem and I can't visualize what the problem is asking for and how to find the unknowns. I'm just trying to do as many as possible until it sticks in my head. Once again I appreiate the help!