Expert: Dave Nyce - 4/30/2002

Question
The armature of a 220-V dc motor draws 15 A when operating at full load and has a resistance of 2.1 Homes.  Determin (a) the counter emf produced by the armature; (b0 the power developed by the armature.



This question is killing me.  I am not doing well in this class, I am a civil Eng. student but I have to take this class.  

I have the power = 472.5W
I got this from  P=I^2 R

Please let me know if I am correct and how I find the emf.

Thanks a ton!!
Mike

Answer
220 volts / 15 amps is 14.6 ohms. You subtract the armature resistance from this, because this does develop motor power, to get 12,567 ohms. Then 15 amps x 12.567 ohms = a back EMF of 188.5 volts.

Use the 12.567 ohms when calculating power.

Hope this helps!

Dave