Problem 103-10    Source:  Problem prepared by the website author.

In a DC motor in shunt configuration, the armature has a resistance of 0.28 Ω and a voltage drop on the 2 V brushes. A voltage of 130 V is applied across the armature terminals. Calculate the armature current when:

a) the rotor speed produces an induced voltage of 120 V;

b) when applying an additional load, the speed drops and the induced voltage reduces to 114 V;

c) in the condition of item a) the engine was running at 1,400 rpm. Calculate the new engine speed in the case of item b) .

Solution of the Problem 103-10   

Item a

As there is a voltage drop across the brushes, we will use eq. 103-08, repeated below.

So we can algebraically work this equation and isolate the variable I_A, that is:

Doing the math, we find:

Item b

With increasing load there is a reduction in the voltage induced in the armature. Then, the new armature current is given by:

Doing the math, we find:

Item c

To calculate the new motor speed, we will use the eq. 103-21, repeated below.

So, working this equation algebraically and isolating the variable n_A, we get:

Doing the math, we find: