Question 7.5: To counter the effects of armature reaction, a fourth turn i......
To counter the effects of armature reaction, a fourth turn is added to the series field winding of the dc generator of Examples 7.3 and 7.4, increasing its resistance to 0.007 Ω. Repeat the terminal-voltage calculation of Example 7.4.
As in Examples 7.3 and 7.4, I_s = I_a = 405 A. The main-field mmf can then be calculated as
\begin{aligned}\text{Gross mmf} &= I_f + \left( \frac{N_s}{N_f} \right) I_s = 4.7 + \left( \frac{4}{1000} \right) 405 \\ &= 6.3 \quad \text{equivalent shunt-field amperes}\end{aligned}
From the I_a = 400 curve of Fig. 7.14 with an equivalent shunt-field current of 6.3 A, one reads a generated voltage 269 V which corresponds to an emf at 1150 r/min of
E_a=\left( \frac{1150}{1200} \right) 269 = 258 V
The terminal voltage can now be calculated as
V_t = E_a – I_a (R_a + R_s) = 258 – 405(0.025 + 0.007) = 245 V
