Problem 103-1    Source:   Issue 12 - Page 128 - HUGHES, Austin -    Book: Electric Motors and Drives - 3rd Edition - Ed. Elsevier - 2006.

Suppose a 250 V DC motor with an armature resistance equal to R_A = 1   Ω and it's spinning no load, drawing a current of 2   A at a rotation of 1800   rpm. Estimate the friction torque.

Solution of the Problem 103-1   

To solve this problem we must find the power generated by the engine to overcome the friction torque. The power on the motor shaft is given by eq. 103-17 reproduced below:

Therefore, it is necessary to find the value of E_A. For that, we will use the eq. 103-06, reproduced below:

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

On the other hand, we know the relationship between torque and power through eq. 103-16, reproduced below:

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: