# DC Motor inertia voltage drop

following a tutorial on the web I created in LTSpice a DC motor model, but I have a question about the results, the model is: and this is the spice netlist:

V_MOTOR N001 0 PULSE(0 10 0.05 1u 1u 1 10 1)
R_MOTOR N001 N002 {R_M}
H_TORQUE H_TORQUE_PROBE 0 V_SENSE_1 {Kt}
R_Friction H_TORQUE_PROBE V_SEEN_BY_L_INERTIA {R_F}
V_SENSE_1 N004 N003 0
V_SENSE_2 N005 0 0
H_BE N003 0 V_SENSE_2 {K_bemf}
L_MOTOR N002 N004 {L_M}
L_INERTIA V_SEEN_BY_L_INERTIA N005 {L_I}
.tran 2
.PARAM R_M 0.5
.PARAM L_M 1.5m
.PARAM L_I 0.25m
.PARAM Kt 0.05
.PARAM R_F 0.0001
.PARAM K_bemf 0.05
.backanno
.end


The results are: My question is: why V(V_SEEN_BY_L_INERTIA), representing inertia of the motor is not 0 at steady rate? As far as I know voltage across an inductor is given by L*d(i)/d(t) so as you can see by the green line when current is (almost) constant there should be no voltage drop across inductor! Obviously this is a result we expect considering that at steady rate the motor has an inertia, but I cannot understand how it is possible being no current change.

Thanks, Vincenzo

• Where is the tutorial? – Dave Tweed Feb 3 '15 at 15:47
• Looking at your netlist, you have one node labeled V_SEEN_BY_L2 and another node labeled V_SEEN_BY_L_INERTIA. It looks like these are supposed to be the same node. Although I can't explain why you get any voltage on the latter node at all. – Dave Tweed Feb 3 '15 at 15:52
• Sorry I've changed the name to reflect the image content, going to edit it. – Vincenzo Maggio Feb 3 '15 at 16:10
• – Vincenzo Maggio Feb 3 '15 at 16:11