Problem 103-5    Source:  Problem prepared by the website author.

A shunt DC motor has the following characteristics: 50 HP / 380 V. It has compensation windings and armature resistance equal to 0.1 Ω. In the field circuit we have R_aj + R_F = 100 Ω. Under nominal voltage the no-load speed is 1,200 rpm and having an armature current equal to 5  A. At full load and at rated voltage, the line current is equal to 93.8  A assuming a 5% reduction in flux due to the armature reaction in relation to the no-load value. Find the engine speed when at full load.

Solution of the Problem 103-5   

Initially, we will calculate the voltage generated by the armature running at no load. Let's call the voltage generated by the armature E_A0, as the motor is operating at no load. Let's use the eq. 103-06, reproduced below.

Substituting for numerical values ​​and performing the calculation, we find:

Let's calculate the current in the field winding.

Then, at full load the armature current will be:

With these data we are able to calculate the voltage generated by the armature when at full load, using eq. 103-06 above, or:

To find the value of the motor speed at full load we will use the eq. 103-20, reproduced below.

Algebraically working this equation, we find: