Question #93756

DC series motor has 3.5 ohm resistance in starter, no load current of 0.3 amps and a full load current of 2 amps rotating at 1800 RPM find speed at full load?

Expert's answer

The speed of the motor is directly proportional to its back EMF:

Here "V_m" - supply voltage on the terminals of the motor.

For both cases (no-load and full-load) we have:

"n_1=\\frac{V_m-I_1r}{c\\Phi_1},\\\\\n\\space\\\\\nn_2=\\frac{V_m-I_2r}{c\\Phi_2}."

Divide one by another:

"\\frac{n_2}{n_1}=\\frac{\\Phi_1(V_m-I_2r)}{\\Phi_2(V_m-I_1r)},"

"n_2=n_1\\frac{\\Phi_1(V_m-2\\cdot3.5)}{\\Phi_2(V_m-0.3\\cdot3.5)}=1800\\frac{\\Phi_1(V_m-7)}{\\Phi_2(V_m-1.05)}."

In this problem we assumed that "3.5 ohm resistance in starter" is the total resistance of stator and armature.

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