How much current does the motor draw when running at itsoperating speed? How muc

Question

How much current does the motor draw when running at itsoperating speed? How much current does the motor draw whenrunning at half its final rotational speed? How much current does the motor draw whenstarting up? The starter motor in an automobile has a resistance of0.37 in its armature windings. The motor operates on 12 and has a back emf of 10 when running at normal operating speed. How much current does the motor draw when running at itsoperating speed? = How much current does the motor draw whenrunning at half its final rotational speed? = How much current does the motor draw whenstarting up? =

Explanation / Answer

   AppliedEMF   E   =   12   V    Resistance ofcoil   R   =   0.37          Effectivevoltage acossmotor   V   =   E   -   BackEMF    a.   V   =   12- 10             =   2   V       Ifull   =   V/ R                =   2/ 0.37                =   5.705   A    b.   Since back EMF changeslinearly with speed of motor, hence back EMF at half speed    =   Back emf at full speed /2    =   10 / 2      =   5  V    V   =   12   -   5          =   7   V    Current   Ihalf   =   7/ 0.37                            =   18.92   A       c.   Whenthe motor starts,   back emf is zero, hence    Istarting   =   E/ R                      =   12/ 0.37                          =   32.43   A          

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