Determining terminal voltage, field current,etc.

[Vladi]

Homework Statement

The emf induced in the armature of a shunt generator is 596 V. The armature resistance is 0.100 Ω. (a) Compute the terminal voltage when the armature current is 460 A. (b) The field resistance is 110 Ω. Determine the field current, and the current and power delivered to the external circuit.

Homework Equations

Armature Current= (Line voltage - Back emf)/Armature resistance
R=V/I
P=(V)(I)

The Attempt at a Solution

I felt good about my work until I checked the key. Where did I go wrong?

 

[cnh1995]

A DC generator's terminal voltage can not be greater than its generated emf (induced).
Draw the circuit diagram for shunt generator. Your armature current equation needs a couple of sign changes.

 

[Vladi]

A DC generator's terminal voltage can not be greater than its generated emf (induced).
Draw the circuit diagram for shunt generator. Your armature current equation needs a couple of sign changes.

I drew a circuit diagram and re-did the problem. I got the right answers, but I still have a few questions. Are the sign changes only
necessary in order to ensure that the terminal voltage was less than the generated emf? Why is the armature's current equal to the sum of the field's and external circuit's current? I wrote down that was true because I found solutions to a similar problem that stated this fact. Thank you for your time.

 

[cnh1995]

Are the sign changes only
necessary in order to ensure that the terminal voltage was less than the generated emf?

DC generator supplies power to the external circuit, hence, armature current flows out of the armature winding through its positive terminal.

Why is the armature's current equal to the sum of the field's and external circuit's current?

Because the field is current is supplied by the generator terminal voltage. Hence, as per KCL, the armature current splits into field current and external circuit current.

 

[Vladi]

DC generator supplies power to the external circuit, hence, armature current flows out of the armature winding through its positive terminal.

Because the field is current is supplied by the generator terminal voltage. Hence, as per KCL, the armature current splits into field current and external circuit current.

Thank you for all your help.