A compensation winding in a
DC shunt motor is a
winding in the
field pole face plate that carries
armature current 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 during u ...
arcing
An electric arc (or arc discharge) is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma, which may produce visible light. An ar ...
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 in physics. For transport phe ...
from the armature current is about equal to the flux from the
field current
A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a electric motor, motor or electrical generator, generator. It consists of a coil of wire through ...
, the flux at the field pole plate is shifted. Under a fixed load, there is an optimal
commutation 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
Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, m ...
from interaction with the field flux.
References
Electric motors