Question

Which of the following is the correct equation for DC series motor ________

(a) [ Vt / kφ]- [ Rse +Ra / kφ] Ia

(b) [ Vt / kφ]- [ Ra/ kφ2] Ia

(c) [ Vt / kφ^2]- [Rse +Ra/ kφ^2] Ia

(d) [ Vt / kφ^2] + [ Ra/ kφ^2] Ia

I have been asked this question during an interview.

The question is from Operating Characteristics of DC Series Motor in division Characteristics of DC Motors of DC Machines

Answer 1

Right choice is (a) [ Vt / kφ]- [ Rse +Ra / kφ] Ia

To explain I would say: Speed-torque characteristic equation is obtained by back emf = (Vt -IaRa-IaRse) and back emf= kφn. Thus, by combining these equations and by assuming all constants equal to k we get, the final equation shown above.