- #1

David112234

- 105

- 3

## Homework Statement

A shunt-wound DC motor with the field coils and rotor connected in parallel (see the figure) (Figure 1) operates from a 140 V DC power line. The resistance of the field windings, Rf, is 248 Ω . The resistance of the rotor, Rr, is 4.40 Ω . When the motor is running, the rotor develops an emf E. The motor draws a current of 4.24 A from the line. Friction losses amount to 41.0 W .

**A**

Compute the field current If.

**B**

Compute the rotor current Ir.

**C**

Compute the emf E.

**D**

Compute the rate of development of thermal energy in the field windings.

Express your answer in watts.

**E**

Compute the rate Pth,rotor of development of thermal energy in the rotor.

Express your answer in watts.

**F**

Compute the power input to the motor Pin.

**G**

Compute the efficiency of the motor.

## Homework Equations

I=v/R

3. The Attempt at a Solution

3. The Attempt at a Solution

For Part A it I

_{2}=140/248 = .565

For part B since the points A,B, and C have the same potential and the motor has a resistance you can use ohms law again.

I

_{3}= 140/4.40 = 31.81

but that is wrong, why?

Also I do not understand the EMF part, so when the motor becomes active it will develop its own EMF that will lower the batteries voltage. But wouldn't that then change the current trough the battery, and it would no longer be able to create that EMF?

So the motor is dependent on the current caused by the batteries voltage which is dependent on the motor? How does that work?