Question

A 120.0-V motor draws a current of 7.00 A when running at normal speed. The resistance of the armature wire is 0.720 Ω. (a) Determine the back emf generated by the motor. (b) What is the current at the instant when the motor is just turned on and has not begun to rotate? (c) What series resistance must be added to limit the starting current to 15.0 A?

Answer 1

back emf = 114.96 V

current = 166.7 A

series resistance R is 7.28Ω

Step-by-step explanation:

given data

voltage = 120 V

current = 7 A

resistance = 0.720 Ω

to find out

back emf , current at the instant and What series resistance

solution

we apply formula to calculate back emf that is

back emf = V - IR

here V = 120 and I = 7 and R = 0.720

put all these value

back emf = 120 - (7) ( 0.720)

back emf = 114.96 V

and

we know emf = 0 when motor is on and no rotation

so

back emf = V - IR

0 = V -IR

so I = V/R

put the value

I = 120 / 0.720

current = 166.7 A

and

series resistance R is

R = 120 / 15 - 0.720

series resistance R 7.28Ω