A series DC motor is powered by a 270 V supply and has a speed of 2000 rpm. The armature resistance is 0.75 Ω, and the field resistance is 0.25 Ω. The motor takes 75 A from the supply. The friction loss of the motor is PFW = 422 W. Assume the motor is magnetically linear.
a) Determine the efficiency of the motor.
b) A resistor, Rpf = 0.40 Ω, is placed parallel over the field winding. The developed torque is kept constant. Calculate the new rotor speed.
I calculated efficiency by calculating the PD (E*I where E=270-(0.25+0.75)*75=195 PD=14625W). Then divide that minus PFW by PD (14625-422/14625=97.1%). I think this is a bit high but I can't see a problem with my working so I think it's correct. Please correct me if I'm wrong.
For b I obtained TD (PD/omega=14625/(2000pi/30)=69.829). Then I did E/(I*omega) (195/(75*200pi/3)=0.012414=KL). Then omega_new=270/(KL*TD)^0.5-(0.75+2/13)/KL=217.185 (1036.98rpm). I think I am wrong though because this speed is lower than the original but there should be more current because one resistor has another in parallel over it. Thus there is less resistance and more current so more speed. Could someone tell me where I am wrong?
EDIT: the 2/13 comes from 0.4 and 0.25 in parallel with each other.
a) Determine the efficiency of the motor.
b) A resistor, Rpf = 0.40 Ω, is placed parallel over the field winding. The developed torque is kept constant. Calculate the new rotor speed.
I calculated efficiency by calculating the PD (E*I where E=270-(0.25+0.75)*75=195 PD=14625W). Then divide that minus PFW by PD (14625-422/14625=97.1%). I think this is a bit high but I can't see a problem with my working so I think it's correct. Please correct me if I'm wrong.
For b I obtained TD (PD/omega=14625/(2000pi/30)=69.829). Then I did E/(I*omega) (195/(75*200pi/3)=0.012414=KL). Then omega_new=270/(KL*TD)^0.5-(0.75+2/13)/KL=217.185 (1036.98rpm). I think I am wrong though because this speed is lower than the original but there should be more current because one resistor has another in parallel over it. Thus there is less resistance and more current so more speed. Could someone tell me where I am wrong?
EDIT: the 2/13 comes from 0.4 and 0.25 in parallel with each other.
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