A noload test on a DC motor has returned the following resul

A no-load test on a DC motor has returned the following results: Motor Speed: 1500 rpm Applied voltage: 180 V Field current: 0.6 A (held constant for all subsequent tests) Calculate the motor machine constant K_m. The DC machine is now operated as a generator with an output current of 10A. The speed is maintained at exactly 1750 rpm, and the DC generated voltage is measured as 180V. Calculate the motor armature resistance.

Solution

1) given speed N=1500rpm

voltageV= 180V

Field current I_sh=0.6 A

but we know that V=Km *N* I_sh

     Km= V/N* I_sh

   Km =180/(1500*0.6)

Km ₌0.2

2)

Given that I_a=10A

N=1750 rpm

V= 180V

but E_g=V-I_aR_a

E_{g=K W}

W=2*3.14*N/60=2*3.14*1750/60=183.166 V

let us take k=0.7

E_g=0.7*183.166=128.216 V

E_g=V-IaRa

128.216=180-10R_a

R_a=5.178 ohm