PPT Slide
Section 13.9: Starting of DC Motors cont.
At the 7 A. load, n7A. = 1120 RPM, If = 230 V. / 75 W = 3.07 A. and Ia = I - If = 7A. - 3.07A. = 3.93 A. so the speed equation is:
n7A. = 1120 RPM = (230V. - 0.05W´3.93A.)/(3.07A.´k) = (Vt - RaIa)/kIf = E/kIf
Which yields k = 0.0668 (what are k’s units?). So at the 46A. load (with Rf = 75W) Ia = I - If = 46A. - 3.07A. = 42.93 A. and the speed equation yields:
n = (230V. - 0.05W´42.93A.)/(3.07A.´0.0668) = 1111 RPM