Could someone give a (detailed) explanation (perhaps with formulas) of how the load (torque) affects the armature current of a DC motor (separately excited or shunt or series)?

As the load torque increases the speed of the motor decreases, and assuming the terminal voltage stays constant, the EMF will become lower and thus the armature current will increase.

But could someone give me the formula that shows the relation between load torque and armature current?

Is it something like:

$T_\text{developed} = T_\text{shaft} + T_\text{friction windage} + T_\text{load}$ ,

where $T_\text{developed} = K \, \text{flux}_\text{pole} \,I_\text{armature}$ ?

But then you have

$P_d = P_s + P_{f_w} + P_l$

where $P_d = K \,\text{flux}_\text{pole}\, I_\text{armature} \,\omega_m$

In the last formula you can't see that as the increase due to $P_\text{load }$ the armature current increases because maybe someone can say the speed $\omega_m$ increases.

And how come at no-load the armature current and thus the torque developed is zero? Is it really zero, or do I have to assume there is some current flowing?

Could also explain me the principle of torque developed. Because at no load, the shaft torque is equal to torque developed, right?

• Dec 6 '14 at 23:20