A permanent magnet DC Motor has following ratings:

180V, 1.8A, 0.25hp (~180W), 6Kg-cm (0.588Nm), 3000RPM.

Let us see how many motor parameters can we find using above values.

Electrical Power Output is almost equal to mechanical power of the motor neglecting few losses. Pi (elec) = Po (elec) + mech loss + elec loss I^2.R

and, Po (elec) = Po (mech)

hence wT = EI

wT = 2.pi.N.T/60 = 184.7W (this is rated power of motor)

to find E, we calculate E= P/I = 184.7/1.8 = 102.63V

now E= Phi.Z.N.P./ (60.A) = Kw where K= (Phi.Z.P)/(2.pi.A)

hence K= E/w = 102.63/314.15 = 0.33

Now we can verify torque using both the equations:

T= EI/w = 102.63x1.8 / 314.15 = 0.588Nm

T = (V.K - K^2.w) / R = 180x0.33 - 0.33^2x314.15 / R

to find out resistance we use V=IR+E, R=(V-E)/I = 42.98 Ohms.

now T = 25.18/42.98 =0.586 Nm.

to this we can say the determined value of R is precise, T is precise, the value of K is correct.

But the contradiction is that

1. a motor ideally should NOT have such a high value of internal resistance.

2. Does this mean that there is another resistance in series with armature?

3. if yes, then why Internal resistance of a motor is not provided in the ratings/ specification from the manufacturer?

4. Motor constant, Km, is another parameter dependent on Internal resistance. (Km = K/sqrt(R)) this also is not specified by manufacturers. why?