A compensation winding in a
DC shunt motor is a
winding
An electromagnetic coil is an electrical conductor such as a wire in the shape of a coil (spiral or helix). Electromagnetic coils are used in electrical engineering, in applications where electric currents interact with magnetic fields, in de ...
in the
field
Field may refer to:
Expanses of open ground
* Field (agriculture), an area of land used for agricultural purposes
* Airfield, an aerodrome that lacks the infrastructure of an airport
* Battlefield
* Lawn, an area of mowed grass
* Meadow, a grass ...
pole face plate that carries
armature current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
to reduce
stator field distortion. Its purpose is to reduce
brush
A brush is a common tool with bristles, wire or other filaments. It generally consists of a handle or block to which filaments are affixed in either a parallel or perpendicular orientation, depending on the way the brush is to be gripped durin ...
arcing
An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The electric current, current through a normally Electrical conductance, nonconductive medium such as air produces a plasma (p ...
and erosion in DC motors that are operated with weak fields, variable heavy loads or reversing operation such as steel-mill motors. When
flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
from the armature current is about equal to the flux from the field current, the flux at the field pole plate is shifted. Under a fixed load, there is an optimal
commutation
Commute, commutation or commutative may refer to:
* Commuting, the process of travelling between a place of residence and a place of work
Mathematics
* Commutative property, a property of a mathematical operation whose result is insensitive to th ...
point for the brushes that minimizes arcing and erosion of the brushes. When the ratio of armature flux to field flux varies greatly or reverses, the optimum commutation point shifts as result of the varying flux at the pole face plate. The result is arcing of the brushes. By adding a compensating winding in the pole face plate that carries armature current in the opposite direction of current in the adjacent armature windings, the position of the flux at the pole face plate can be restored to the position it would have with zero armature current. The main drawback of a compensation winding is the expense.
Figure A. shows a cross-sectional view of a two pole DC shunt motor. Armature windings (A), field windings (F) and compensation windings (C) use the dot and cross convention where a circle with a dot is a wire carrying current out of the figure and a circle with a cross is a wire carrying current into the page. For each wire in the armature that is next to the field pole face plate there is a wire in the face plate carrying current in the opposite direction.
Figure B. shows the flux caused by the field winding alone.
Figure C. shows the flux caused by the armature winding alone.
Figure D. shows field flux and armature flux being about equal. The result is that the center of flux in the gap between the pole face plate and the armature has shifted. For a more detailed drawing, see Richardson.
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Figure E. shows compensation wires in the field pole face plate that are carrying current opposed to the current in the armature wire adjacent to the gap. The flux in the gap has been restored to the same condition as the case where there is no armature flux. Even though the armature wires are next to wires carrying current in the opposite direction, the wires of the armature still experience
magnetic force
In physics (specifically in electromagnetism) the Lorentz force (or electromagnetic force) is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge moving with a velocity in an elect ...
from interaction with the field flux.
References
Electric motors