COMMUTATION:
In
larger machines the commutation process would involve too much sparking, which
causes brush wear, noxious gases (ozone) that promote corrosion, etc. In these
cases it is common to use separate commutation interpoles. These are separate,
usually narrow or seemingly vestigal pole pieces which carry armature current.
They are arranged in such a way that the flux from the interpole drives current
in the commutated coil in the proper direction. Remember that the coil being
commutated is located physically between the active poles and the interpole is
therefore in the right spot to influence commutation. The interpole is wound
with armature current (it is in series with the main brushes). It is easy to
see that the interpole must have a flux density proportional to the current to
be commutated. Since the speed with which the coil must be commutated is
proportional to rotational velocity and so is the voltage induced by the
interpole, if the right 7 number of turns are put around the interpole,
commutation can be made to be quite accurate.
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