When do dipoles occur in a molecule?
Dipoles occur when there is a relative charge on each side of a molecule induced by the bond electronegativities.
The dipole moment of an entire molecule is made up of two bond moments -vector quantities having both magnitude and direction. Thus, a measured dipole moment is equal to the vector sum of the bond moments that comprise it.
Bond moments are vector quantities, possessing both magnitude and direction. Therefore, it's possible for a molecule to have bond moments and yet to be non-polar, if the individual bond moments in the molecule are equal in magnitude but opposite in their direction, therefore cancelling each other out. Thus, the sum is 0 and there is no dipole moment. An example of a molecule with bond moments and that is non-polar is CO2. You could have a linear molecule with the same atom on either side of the central atom that cancel each other out.