40. A 120.0-V motor draws a current of 7.00 A when running at normal speed. The resistance of the armature wire is . (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?

 

For a motor we know that current follows

 

Solve for back emf

 

When the motor is first turned on and not begun to spin, the back emf is zero so

 

 

To limit the starting current we need a series resistor and then the starting current would be

 

Solve for Rseries

 

 

 

 

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This page last updated on June 25, 2018