A D.C. motor operates at 240 V and has a resistance of 0.5 Ω. When the motor is running at normal speed, the armature current is 15A. Find the back emf in the armature. (Ans: 232.5 V). 



Data Given:

Voltage = V = 240 V

Resistance = R = 0.5 Ω

Current = `\I` = 15 A



To Find:


Back emf  = Ɛ = ?


Solution:

Using relation 

 V = Ɛ + IR

or

Ɛ = V - IR


Putting the corresponding values 

Ɛ = 240 V - 15 A ｘ0.5 Ω

Ɛ = 240 V - 7.5 A Ω

Ɛ  = 232.5 V ----------------Ans.



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