Final answer:
When the stator flux is reduced to 90% of its original value, the speed of the DC series motor increases. Using the formula N1/N2 = Φ2/Φ1, the new speed is calculated to be 833 RPM, making option (c) correct.
Step-by-step explanation:
The student is asking about the change in speed of a motor when the stator voltage is altered, which results in a change to the stator flux. Assuming the motor mentioned in the question closely follows the characteristics of a typical DC series motor, the speed of the motor is directly proportional to the back electromotive force (EMF) and inversely proportional to the flux. If the stator voltage is changed to reduce the stator flux to 90% of its original value and if everything else remains constant, the speed of the motor will increase as the flux has decreased.
To find the approximate new speed, you would use the formula that relates speed and flux: N1/N2 = Φ2/Φ1, where N1 and N2 are the initial and final speeds, and Φ1 and Φ2 are the initial and final flux values. Since the flux is reduced by 10%, Φ2/Φ1 is 0.9. If we set N1 to 750 RPM (the initial speed), we can solve for N2 by the formula N2 = 750 RPM / 0.9, which gives us 833 RPM. Therefore, with a reduction of stator flux to 90%, the new approximate speed is option (c) 833 RPM.