DC Shunt Generator: Armature Current, Terminal Voltage & Efficiency

[TimWillz]

Homework Statement

A shunt generator has an armature resistance of 0.8Ω, a field winding resistance of 120Ω and a mechanical loss of 115W. The load resistance of 8.5Ω is connected to the output terminals of the generator. An external mover rotates the shaft of the generator at a speed of 1670rpm. If a back e.m.f. constant of the generator is 0.86Vs/rad calculate:

a) armature current
b) terminal voltage
c) efficiency

I really can't get going on this question, can anyone help me out?

Homework Equations

The Attempt at a Solution

 

[TimWillz]

Is there any way to find a voltage from the back e.m.f.?

 

[uart]

Is there any way to find a voltage from the back e.m.f.?

Yes there is. Can you draw the equivalent circuit? You have the internal EMF with the armature resistance in series, and then both the field resistance and load resistance in parallel to complete the circuit. The terminal voltage is the one across the parallel resistors.

 

[TimWillz]

Yes there is. Can you draw the equivalent circuit? You have the internal EMF with the armature resistance in series, and then both the field resistance and load resistance in parallel to complete the circuit. The terminal voltage is the one across the parallel resistors.

Hi, thanks for the reply. I have tried to draw the equivalent circuit and calculate the values but to no success, is there any way you could show me how you would do it?

Thanks

 

[TimWillz]

Ive got this as the circuit, I'm not sure if its correct though.

 

[uart]

Ive got this as the circuit, I'm not sure if its correct though.

Yes that looks right. Use the rotational EMF constant to find Ea and also combine the field resistance and the load resistance into a single equivalent resistance. Now you can use the voltage divider formula to find the terminal voltage.

 

[TimWillz]

Yes that looks right. Use the rotational EMF constant to find Ea and also combine the field resistance and the load resistance into a single equivalent resistance. Now you can use the voltage divider formula to find the terminal voltage.

Thanks! I've got Ea = 0.86*174.9 = 150.4v

Is it just (RA*RF*RL)/(RA+RF+RL) to find the total resistance?