Electrical Engineering: Armature Reaction

Armature Reaction

• The effect of the magnetic field produced by armature current on the distribution of the flux under main poles is known as armature reaction.
• This effect leads to reduced generated voltage and sparking at the brushes.

Magnetic Neutral Axis (MNA)

• The axis along which no EMF is produced in the armature conductors as they move parallel to the flux lines.
• It is also known as the axis of commutation.

Armature Reaction Effects

• Demagnetizing: The armature MMF weakens the main pole flux.
• Cross Magnetizing: The armature MMF distorts the main flux, concentrating it at the trailing pole tips and weakening it at the leading pole tips.

Commutation

• The process of reversing the current in a short-circuited armature coil as it crosses the magnetic neutral axis.
• The brief period during which the coil is short-circuited is known as the commutation period.

Factors Affecting Commutation

• Self-induced EMF (Reactance Voltage): This voltage is generated in the armature coil due to its inductance and opposes the current change during commutation, hindering ideal commutation.

Ideal Commutation

• When the current reversal completes within the commutation period, resulting in a smooth, spark-free transfer of current.

Sparking

• Occurs when the current reversal is not completed within the commutation period.
• A difference in current between the coil undergoing commutation and its neighboring coils creates a spark as current jumps through the air between the commutator segment and the brush.

Commutation in Electrical Machines

• Short circuit in DC Motors: Even small voltages can produce large currents through the coil due to low resistance during short circuit.
• Self-induction: Even when coils undergo short-circuit in the magnetic neutral plane, they still experience self-induction, resulting in sparking at the brushes.
• Improving commutation: Techniques to enhance performance, reduce sparking, and increase lifespan of commutator and brushes.

Methods to Improve Commutation

• Brush Shifting:
  • Adjusts brush position slightly from the neutral axis.
  • Alters current reversal timing in coils.
  • Advantages: Simple, cost-effective.
  • Disadvantages: Limited effectiveness, not suitable for severe sparking.
• Interpoles (Compoles):
  • Auxiliary poles between main poles.
  • Wound with heavy wire, connected in series with the armature.
  • Advantages: Neutralizes armature reaction, significantly improves commutation.
  • Disadvantages: Adds complexity and cost.
• Compensating Windings:
  • Placed in slots of main pole faces, connected in series with the armature.
  • Generate opposing magnetic field to armature reaction.
  • Advantages: Effective in large machines with significant armature reaction.
  • Disadvantages: Increased complexity and cost.
• High-Resistance Brushes:
  • Reduce current density at commutator segments, enabling smoother current reversal.
  • Advantages: Reduced sparking and commutator wear.
  • Disadvantages: Higher losses due to increased resistance.
• Brush Grade Selection:
  • Choose appropriate brush material (carbon, graphite, metal-graphite composites) based on application.
  • Advantages: Optimal brush and commutator performance, lifespan.
  • Disadvantages: Requires careful selection based on operating conditions.
• Magnetic Neutral Axis (MNA) Adjustment:
  • Adjusting the MNA (where magnetic field is zero) improves commutation.
  • Achieved through field winding design and pole shoe shaping.
  • Advantages: Reduced sparking, improved commutation.
  • Disadvantages: Requires precise design and adjustments.
• Split Commutator:
  • Divides commutator segments into smaller parts.
  • Enables more gradual commutation, reducing sparking.
  • Advantages: Reduced sparking, improved commutation.
  • Disadvantages: Added complexity, manufacturing cost.

Armature Reaction

• The magnetic field generated by the armature current affects the distribution of the main field in a DC generator.
• This effect is called armature reaction and it can be categorized into two main effects:
  • Demagnetisation: The armature field weakens the main field, resulting in a reduced generated voltage.
  • Cross-magnetisation: The armature field distorts the main field, leading to sparking at the brushes.

Commutation

• Commutation is the process of reversing the current flow in a short-circuited armature coil as it passes the magnetic neutral axis (M.N.A.) in a DC machine.
• The M.N.A. is the axis where no EMF is induced in the armature conductors because they move parallel to the magnetic field lines.
• The commutator helps achieve this reversal by switching the current path at the appropriate time.
• The period during which the coil is short-circuited is known as the commutation period.
• Ideal commutation occurs when the current reversal is completed within the commutation period, resulting in smooth current flow.
• However, due to factors such as self-induced EMF in the coil, current reversal may not always be perfect and the result will be sparking at the brushes.
• This sparking can lead to damage to the commutator and brushes.

Factors Affecting Commutation

• Self-induced EMF: The coil undergoing commutation has self-inductance, which generates a reactance voltage that opposes changes in current flow. This makes it harder for the current to reverse quickly.
• Brush position: The position of the brushes relative to the M.N.A. plays a crucial role in commutation. The brushes should be placed along the M.N.A. to ensure smooth current reversal.
• Brush material: The type of material used for the brushes, such as carbon or graphite, affects the contact resistance and commutation performance.
• Armature winding design: Different armature winding types, such as lap and wave winding, have varying commutation characteristics.
• Armature reaction: The magnetic field distortion caused by armature reaction can also influence commutation.

Significance of Good Commutation

• Reduced sparking: Smooth commutation minimizes sparking, reducing wear and tear on the commutator and brushes.
• Increased efficiency: Improved commutation leads to lower power losses and higher efficiency in the DC machine.
• Longer lifespan: Reduced sparking extends the lifespan of the commutator and brushes, reducing maintenance requirements.
• Better performance: Optimized commutation improves the overall performance of the DC machine, including its voltage and torque characteristics.

Commutation in Electrical Machines

• Commutation is the process of reversing the current direction in the armature coils of a DC motor as the commutator segments change contact with the brushes.
• Short circuit occurs when the coil under the brush is directly connected to the supply, leading to a high current and sparking.
• Spark at the brushes is due to the self-induced electromotive force (EMF) that occurs during current reversal.
• Improving commutation reduces sparking, increases the lifespan of the commutator and brushes, and enhances the performance of electrical machines.

Methods to Improve Commutation

• Brush Shifting: Move brushes slightly from the neutral axis to alter the timing of current reversal, simple and cost-effective but only for minor adjustments.
• Interpoles (Compoles): Small auxiliary poles between the main poles, connected in series with the armature, providing a magnetic field to neutralize armature reaction, significantly improving commutation but adding complexity and cost.
• Compensating Windings: In the slots of the main pole faces, connected in series with the armature, opposing the armature reaction and improving commutation, effective in large machines but increasing complexity and cost.
• High-Resistance Brushes: Reduce current density at the commutator segments, leading to smoother current reversal, reduces sparking and commutator wear but has slightly higher losses.
• Brush Grade Selection: Choose the right brush material (carbon, graphite, metal-graphite composites) based on the application, optimizing performance and lifespan but requiring careful selection.
• Magnetic Neutral Axis (MNA) Adjustment: Adjusting the MNA using field winding design and pole shoe shaping reduces sparking and improves commutation, but requires precise design and adjustments.
• Split Commutator: Divides the commutator segment into smaller parts, allowing for a more gradual commutation process, reducing sparking but adding manufacturing complexity.