question_answer 68)

  Draw a labelled circuit diagram of a simple electric motor and explain its working. In what way these simple electric motors are different from commercial motors?

Answer:

                  The circuit diagram of a simple electric motor is shown below Working (i)Let coil ABCD be in horizontal position, when the key is switched on, the current flows indirection DCBA and then leaves through the brushes ,via ring (ii) Applying Fleming's left-hand rule, no force acts on arm 6C and AD as they are parallel to magnetic field, arm A8 experiences a force in downward direction and arm CD experiences an equal force in upward direction. A torque acts on the coil and it rotates in anti-clockwise direction, (iii) While rotating, the coil reaches the vertical position, brushes loose contact with the rings and current stops flowing, But the coil does not stop due to inertia of motion, (iv) When the coil passes the vertical position, the rings automatically change their positions and come in contact with opposite brushes. (v) This reverses the direction of current through the coil but the direction of current on right-hand side remains same. (vi) So, the force on right hand side is always upward and a force on left-hand side is always in downward direction. Thus, the coil continues to anti-clockwise direction. A commercial electric motor is one which uses the following (i) An electromagnet in place of permanent magnet. (ii) Large number of turns conducting wire in current carrying coil. (iii) A soft iron core on which the coil is wound. The combination of soft iron core and coil is an armature. It enhances the power of motor. Thus, commercial electric motors do not use permanent magnet to rotate the armature because permanent magnets are weak and do not produce strong magnetic field in the region.