A shunt-wound DC motor with the field coils and rotor connected in parallel (see

Question

A shunt-wound DC motor with the field coils and rotor connected in parallel (see the figure) operates from a 135 V DC power line. The resistance of the field windings, R_f, is 250 Omega. The resistance of the rotor, R_r, is 6.00 Omega. When the motor is running, the rotor develops an emf EMF. The motor draws a current of 4.92 A from the line. Friction losses amount to 45.0 W.

part D) Compute the rate of development of thermal energy in the field windings.

part G) Compute the efficiency of the motor.

Explanation / Answer

A) The field current      
      If = 135V/250       
      If = 0.54 A B) Roter current      Ir = Itot - If      
         = 4.92 A - 0.54 A         
    Ir  = 4.38 A C)   V = +IrRr       = V - Ir Rr        = 135 V - (4.38A)(6 )     = 108.72 V D) Pf = I2fRf         = (0.54 A)2 (250)            Pf = 72.9 W E) Pr =I2r Rr         = ( 4.38A)2 (6 )    Pr = 115.1 W F) Power input = Voltage line xItotal                        = (135 V)(4.92 A)                  Pin = 664.2 W G) Efficiency =Poutput/Pinput                     = (664.2 W- 115.1 W)/664.2W                     =0.826

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