05/10/2026
In DC motors, interpoles (also called commutating poles or compoles) are narrow auxiliary poles placed on the stator frame midway between the main field poles. They are connected in series with the armature winding so that their magnetic strength automatically varies with motor load.
1. Core Functions & Purposes
🌍 Neutralizes Reactance Voltage (Improves Commutation)
👍 During commutation, as a coil short-circuits under the carbon brush, the rapid reversal of current creates a self-induced voltage known as reactance voltage (e_r = L \frac{di}{dt}).
🙏 Without intervention, this induced voltage causes heavy electric arcing and sparking between the commutator segments and brushes, leading to rapid brush wear and commutator pitting.
🎉 Interpoles induce a counter-EMF (dynamic EMF) in the short-circuited coil that is equal and opposite to the reactance voltage, achieving sparkless commutation..
2. Cancels Armature Reaction in the Neutral Zone
🌍 Under heavy loads, the magnetic field produced by the armature current distorts the main magnetic field (armature reaction), shifting the Magnetic Neutral Axis (MNA).
👍 Interpoles generate a localized magnetic flux in the interpolar space that directly opposes and cancels out the cross-magnetizing armature flux, keeping the commutation zone magnetically neutral regardless of load changes.
3. Eliminates the Need to Shift Brushes
🌍 In older or non-interpole DC motors, brush position had to be shifted manually as the load varied to keep the brushes on the MNA.
🙏 Interpoles eliminate this requirement entirely, allowing fixed brush positions across all operating load conditions and during motor reversal.
4. Key Design & Operating Characteristics
🌍 Series Connection: The interpole winding carries full armature current (I_a). If load increases, the opposing interpole flux increases proportionally, providing automatic dynamic compensation.
👍 Polarity Rule for DC Motors: An interpole must have the same polarity as the main pole behind it in the direction of motor rotation (or the same polarity as the main pole ahead of it in a generator).
🎉 Air Gap: Interpoles typically feature a wider non-magnetic air gap than main poles to prevent magnetic saturation at high armature currents, ensuring a linear relation between load current and compensating flux.
