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.
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