Electronic – DC Motor inertia voltage drop

dc motorltspicemodelingsimulationspice

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:

LTSpiceModel

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:

LTSpiceGraph

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

Best Answer

I've understood the problem: at what it seems Spice, if no argument is given, set a series resistance of 1 mOhm with the inductor, if a value of 0 is set for inductors series resistence the voltage across the inductor drops to 0 as desired.

Source: http://ltwiki.org/LTspiceHelp/LTspiceHelp/L_Inductor.htm

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