Problem 103-2    Source:   Problem 14 - Page 128 - HUGHES, Austin -    Book: Electric Motors and Drives - 3rd Edition - Ed. Elsevier - 2006.

Suppose, in the previous problem, the motor running at full load and consuming a current equal to 32  A. Calculate the motor speed at full load and the torque developed by the machine.

Solution of the Problem 103-2   

Initially we will calculate the voltage generated by the armature so that we can calculate the power developed by the machine. Let's use the eq. 103-06, reproduced below.

Using the numerical values provided by the problem and performing the calculation, we find:

With this data we can find the value of the new rotational speed of the machine. So, we must use the eq. 103-21, reproduced below.

Algebraically working this equation, we find:

Now we can employ the eq. 103-17 and calculate the power developed by the machine. Soon:

On the other hand, we know that the machine consumes 2  A to overcome the friction torque. So the net power that the machine makes available is given by:

As the machine speed was given in rpm, we must transform it to rad/s using eq. 103-12. Then:

Therefore, algebraically working the eq. 103-06, we get: