Question

Please do not complete the problem, I provided the problem and the solution. I don’t understand what it means when The armature is equal to a constant. Do we disregard the armature current if it’s constant? Couldnt the constant be very large and therefore significant? Why do we dosregard it when it’s a constant?

4.50. A de series motor is connected to a constant-torque load, which may be considered to require a constant electromagnetic torque regardiess of motor speed. Neglect the voltage drops due to the armature and series-field resistances, the arm ture reaction, and the effects of saturation. (a) By what percent is the motor speed changed when the line voltage is reduced from 230 V to 200 V? (b) Repcat (a), assuming a shunt motor. (c) State briefly what effects saturation might have on the answers to (a) and (b) Ans. (a) 13%; (b) 0 2 o0-23o 230

Answer 1

In this question, we have considered torque as constant. Since torque is proportional to armature current squared in series this implies that armature current is constant. Also in the first part, we have neglected saturation and armature reaction. since in series field current is also proportional to armature current, thus even if saturation is considered the flux will remain practically constant in both cases which implies our answer would not change. Also, we are neglecting the effect of armature reaction. If armature reaction is considered then the effect of armature current magnitude should be considered. If the armature current is very large it may distort the field flux to a great extent and field flux reduces. But since armature current is assumed to be constant in both the cases the same amount of demagnetization is produced in both cases and it does not affect the answer.

In the second case, we are given a shunt motor. In shunt motor field current is proportional to applied voltage. If armature current is very large and the effect of armature reaction is considered then this large current will reduce the field flux. Since we apply different voltages, in both cases the net filed flux which will be equal to ( field flux - the effect of armature reaction) will be different in both cases and speed will not remain constant.

Thus I will conclude by saying that when armature current is constant it will effect solution will only if armature reaction is considered that too in case of shunt motor and with high values of current as armature reaction is observed for high values of current. In series, motor same amount of demagnetization will be produced in both the cases and hence there will be no effect on our solution.