Question 7.2: Consider again the separately-excited dc machine of Example ......
Consider again the separately-excited dc machine of Example 7.1 with the field-current maintained constant at the value that would produce a terminal voltage of 125 V at a speed of 3000 r/min. The machine is observed to be operating as a motor with a terminal voltage of 123 V and with a terminal power of 21.9 kW. Calculate the speed of the motor.
The terminal current can be found from the terminal voltage and power as
I_a=\frac{\text{Input power}}{V_t}=\frac{21.9 × 10^3}{123} = 178 A
Thus the generated voltage is
E_a = V_t – I_aR_a = 119.4 V
From Eq. 7.8, the rotational speed can be found as
e_a = \left( \frac{n}{n_0} \right) e_{a0} \quad \quad \quad (7.8)
n=n_0\left( \frac{E_a}{E_{a0}}\right) = 3000 \left( \frac{119.4}{125} \right) =2866 r/min