Calculating Field Current and Resistance for a DC Motor

[Vagrant]

Homework Statement

A dc machine(6W, 120V,1500rpm) is separately excited and delivers rated current at rated terminal voltage.Field winding is connected to 120V supply. R_fw=100$$\Omega$$.
a) Determine the value of field current
b) Determine the value of R_fc required.

Homework Equations

V_f= I_fR_f

The Attempt at a Solution

In the question the value of V_f is given, while both I_f and R_f are variables. So I need another relation to find out both the values. Is there another relation that I'm missing?

 

[montoyas7940]

Unless I misunderstand the problem, you can get everything you need from the power equations P=IE and P=I^2*R.