# DC motor problem

1. Jan 13, 2014

### ATRIX

People of physics forum
i have come here to ask for help.
i am trying to model a armature controlled dc motor with no load but and opposite force in the form of frictional damping

the expression i have raised and am assuming it to be right is

ωout/Vin = k/(L(s)+R)(I(s)+C)+ kt.k

where K,kt are constants
L inductance
I inertia

R resistance armature circuit
C damping

the reference voltage is 25 volts. i really need i think a partial fraction expansion of this to allow me to do the inverse laplace transform

any body out there can you help me please?

thanks people.

2. Jan 13, 2014

### rude man

What does "kt.k" mean? Is it kt multiplied by k which you can write as (kt)(k)?

Ω(s)/V(s) = k/[(sL+R)(sI+C)+ (kt)(k)] ?

The denominator is a second-order response. I would put in numbers to determine if this is an over- or under-damped system , then use Laplace tables. If you use partial fraction expansion that is OK but you'd better know how to handle the complex fractions.

Last edited: Jan 13, 2014
3. Jan 13, 2014

### ATRIX

Hey rude man, thanks for getting back

Yes you are correct k and Kt and yes they can be expressed in the form you have shown. They relate to the properties and generated emf. I think but am not sure that they are possible the time constant for the circuit the "Tau" value any ideas?

4. Jan 13, 2014

### rude man

k*Kt would have to have dimension = T (time). But if your denominator is consistent, k*Kt has the dimensions RIT-1 = RMLT-1.

Anyway, if you believe your transfer function is correct, why are you speculating about a time constant? Do you have numbers for K and Kt including their units?