A DC Motor Part A Compute the field current If Express your answer in amperes. A

Question

A DC Motor Part A Compute the field current If Express your answer in amperes. A shunt-wound DC motor with the field coils and rotor connected in parallel (see the figure) (Figure 1) operates from a 125 V DC power line. The resistance of the field windings, Rf, is 200 . The resistance of the rotor, R, is 4.00 ·When the motor is running, the rotor develops an emf E. The motor draws a current of 4.02 A from the line. Friction losses amount to 45.0 W Hints Submit My Answers Give Up Part B Compute the rotor current I Hints Figure 1 Submit My Answers Give Up Part C Compute the emf Hints dc Submit My Answers Give Up

Explanation / Answer

Part A -

Field current, If = V / Rf = 125 / 200 = 0.63 A

Part B -

Rotor Current, Ir = Itot - If = 4.02 - 0.63 = 3.39 A

Part C -

The emf is equal to the rotor current multiplied by the rotor resistance is the difference between the line voltage and the counter emf.

means -
emf = Vline - Irotor*Rrotor = 125 - 3.39*4.00 = 111.44 V

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