In DC machines, interpoles (also known as commutating poles) are small poles located between the main poles. Their primary function is to improve commutation, which is the process where current in the armature coils is reversed as they pass under the brushes. Poor commutation can lead to sparking at the brushes, reduced efficiency, and damage to the commutator and brushes.

Understanding Interpole Connection and Function

The need for interpoles arises because of armature reaction. Armature reaction is the effect of the armature's magnetic field on the main magnetic field. It distorts the main field and also induces a voltage in the coils undergoing commutation (known as reactance voltage). This reactance voltage opposes the reversal of current in the coils, hindering proper commutation.

Interpoles are designed to counteract both the distortion of the main field in the commutating zone and the reactance voltage. They create a magnetic field in the interpolar region that is opposite in direction to the armature reaction field and is proportional to the armature current.

Why Interpoles are Connected in Series with the Armature

For the interpole field to effectively neutralize the armature reaction field and the reactance voltage, its strength must vary automatically with the load on the machine. The armature reaction flux is directly proportional to the armature current. Therefore, the interpole flux must also be proportional to the armature current.

Connecting the interpole windings in series with the armature winding achieves this proportionality. When the armature current increases (due to increased load), the current flowing through the series-connected interpole winding also increases. This increases the magnetic flux produced by the interpoles, ensuring it is always proportional to the armature reaction flux that needs to be neutralized.

Let's consider the alternatives:

• Parallel connection: If connected in parallel with the armature, the current through the interpoles would depend primarily on the voltage across them and their resistance, not directly on the armature current. This would mean the interpole field strength would not vary proportionally with the load, rendering them ineffective in neutralizing armature reaction at different load levels.
• Both series and parallel or Half series and half parallel: These combinations would not provide the necessary direct proportionality between the interpole flux and the armature current across the operating range of the machine.

Therefore, the series connection is essential for the automatic and effective operation of interpoles in improving commutation under varying load conditions.

Impact of Correct Interpole Connection

Connecting interpoles correctly in series with the armature:

• Neutralizes the armature reaction in the commutating zone.
• Induces a voltage in the coil undergoing commutation that opposes and cancels the reactance voltage.
• Ensures sparkless commutation over a wide range of load variations.

In summary, the winding of interpoles is always connected in series with the armature to ensure their magnetic field is proportional to the armature current, thereby effectively counteracting armature reaction and improving commutation.

Revision Table: DC Machine Windings

Additional Information: Commutation and Armature Reaction

Commutation: The process by which the current in an armature coil is reversed as the commutator segments connected to it pass under the brushes. Ideal commutation involves smooth current reversal without sparking.

Armature Reaction: The magnetic field produced by the armature current. This field interacts with the main field, causing:

• Distortion of the main flux (cross-magnetizing effect).
• Reduction in the net flux per pole (demagnetizing effect).
• Induction of reactance voltage in coils undergoing commutation.

Interpoles specifically address the cross-magnetizing effect and the reactance voltage in the commutating zone.