Compensation winding
A winding in the pole face plate that reduces brush arcing.
A compensation winding is a set of wires embedded in the face plate of a field pole in a DC shunt motor. These wires carry the armature current and are used to counteract distortion of the stator’s magnetic field. The goal is to minimize brush arcing and wear, especially in motors that run with weak fields, handle varying heavy loads, or operate in reverse—common in applications like steel-mill motors.
When the magnetic flux produced by the armature current is roughly equal to the flux from the field current, the flux at the field pole face plate becomes displaced. Under a constant load, there is a specific brush position that gives the best commutation, reducing arcing and brush erosion. However, if the ratio of armature flux to field flux changes significantly or reverses direction, that optimal commutation point shifts because the flux at the pole face plate is altered. This leads to brush arcing.
To fix this, the compensation winding is placed in the pole face plate and carries armature current in the opposite direction to the current in the adjacent armature windings. This restores the flux at the pole face plate to the position it would have if no armature current were flowing. The main downside of adding a compensation winding is its cost.
Figure A shows a cross-section of a two-pole DC shunt motor. Armature windings (A), field windings (F), and compensation windings (C) are indicated using the dot-and-cross convention: a circle with a dot means current flowing out of the page, and a circle with a cross means current flowing into the page. For every armature wire next to the field pole face plate, there is a wire in the face plate carrying current in the opposite direction.
Figure B illustrates the flux produced by the field winding alone. Figure C shows the flux from the armature winding alone. Figure D depicts the situation where field flux and armature flux are roughly equal, causing the center of flux in the gap between the pole face plate and the armature to shift. (For a more detailed drawing, see Richardson.) Figure E shows the compensation wires in the field pole face plate carrying current opposite to the current in the adjacent armature wire. This restores the flux in the gap to the same condition as when there is no armature flux.
- Purpose
- Reduce brush arcing and erosion
- Application examples
- Steel-mill motors
- Operating conditions
- Weak fields, variable heavy loads, reversing operation
- Main drawback
- Expense
Lore & Background
When flux 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 point for the brushes that minimizes arcing and erosion. When the ratio of armature flux to field flux varies greatly or reverses, the optimum commutation point shifts, resulting in brush arcing.
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, with armature windings (A), field windings (F), and compensation windings (C) using dot and cross convention. For each wire in the armature next to the field pole face plate, there is a wire in the face plate carrying current in the opposite direction. Figure E shows compensation wires in the field pole face plate carrying current opposed to the armature wire adjacent to the gap, restoring the flux in the gap to the condition with no armature flux.
Reader's Guide
The compensation winding is significant because it directly addresses the problem of brush arcing and erosion in DC motors under demanding operating conditions. The article describes that when armature flux and field flux are about equal, the center of flux in the gap shifts, moving the optimal commutation point. This shift is especially problematic in applications with weak fields, variable heavy loads, or reversing operation, such as steel-mill motors. By carrying armature current in the opposite direction to adjacent armature windings, the compensation winding restores the flux position at the pole face plate to what it would be with zero armature current. This restoration minimizes arcing and erosion, improving motor reliability and brush life. The article notes that the main drawback is the expense, implying that the benefit of reduced maintenance and improved performance must be weighed against the higher initial cost. The winding's legacy is as a specialized solution for heavy-duty DC motor applications where field distortion would otherwise cause significant operational issues.
More in Electric Motors 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced