Concentrated vs. Distributed Windings

Concentrated vs. Distributed Windings: Why Short-Axial BLDC Motors Are Moving to Concentrated Topologies

If you specify a BLDC motor for an axial fan, pump, or direct-drive system, you know the classic design dilemma: motors that fit the allowable axial envelope often lack dynamic smoothness, while ultra-smooth distributed motors exceed the depth allowance.

Nine times out of ten, this constraint stems from stator winding topology.

In compact, short-axial machines, concentrated tooth windings are rapidly becoming the dominant industry standard. Not because concentrated windings are universally superior, but because they maximize active electromagnetic volume within constrained installation envelopes. Here is the engineering logic behind this shift.

What Actually Changes Inside the Stator

The fundamental physical distinction lies in coil span:

• Distributed winding: Each phase coil spans multiple stator teeth. While this creates a smooth, sinusoidal magnetomotive force (MMF) wave, the copper end-turns extend significantly beyond the laminated iron stack on both sides.

• Concentrated winding: Each coil winds tightly around a single tooth down one slot and back up the adjacent one. Coil overhang is dramatically minimized.

Figure 1 Stator winding topology and axial dimension comparison

The Core Dimensional Reality: Coil-end height in a concentrated winding is roughly 40% lower. This reduces overall motor axial length by ~25% at identical outer diameter while cutting copper mass by 18%.

That's the whole short-axial story in one sentence.

Table 1 — Winding geometry and electrical comparison

The Axial Length Advantage, Quantified

This geometric reduction creates compounding system benefits. Research on package-constrained traction motors demonstrates that as axial depth is restricted, concentrated machines achieve superior torque density and operational efficiency over distributed counterparts.

Because end-turns represent dead copper that produces resistance without generating torque, shrinking them cuts phase resistance to approximately 60% of a comparable distributed design. Even though concentrated coils need roughly 10% more turns to offset a lower winding factor, the dramatic reduction in conductor length delivers net copper savings.

Figure 2 Visualizing the short-axial torque advantage

Thermal Headroom Impact: Lower phase resistance directly reduces I²R resistive heating. Less heat dissipation translates into greater thermal headroom, allowing higher sustained continuous torque in enclosed applications.

Where the Trade-Offs Land

Concentrated windings involve two distinct engineering compromises:

1. Lower Winding Factor: Concentrated windings exhibit a slightly lower fundamental winding factor (typically 0.866 vs. 0.955). For an identical active iron stack, peak torque is lower. However, designers easily offset this penalty because the axial space saved from end-turns allows extending the laminated iron stack.

2. Space Harmonics and Torque Ripple: Individual tooth coils generate rich harmonic spectrums in the airgap, elevating cogging torque and torque ripple. For fans, blowers, and pumps, impeller inertia readily dampens these fluctuations. For low-inertia precision servos, unmitigated ripple poses challenges.

The engineering breakthrough is slot-pole optimization. A Fractional-Slot Concentrated Winding (FSCW) configured with the proper slot/pole ratio (such as 12-slot/10-pole) staggers tooth forces around the stator circumference. This phase offset cancels parasitic harmonics, lowering torque ripple by up to 85% to rival distributed smoothness.

Figure 3 Torque ripple suppression mechanism in Fractional-Slot Concentrated Windings (FSCW)

Table 2 — Performance trade-offs by winding type

Application Suitability in Short-Axial Designs

Specific applications benefit disproportionately from concentrated topologies:

• Direct-Drive Axial Fans: Duct geometry restricts axial depth. Concentrated windings maximize active iron stack within the duct, maximizing airflow torque while preserving clearance for bearings.

• Close-Coupled Pumps: Direct-bolted pump motors require minimal axial overhang. Shorter concentrated stacks eliminate mechanical vibration and reduce heat transfer into the fluid.

• Wheel Motors & Axial-Flux Machines: High pole-count direct-drive wheels utilize concentrated coils to maintain high torque density without increasing chassis track width.

• Where Distributed Wins: Precision servo axes requiring ripple-free low-speed control under light inertia remain better served by distributed windings.

Table 3 — Application fit by constraint type

What This Means for Motor Selection

When selecting a short-axial BLDC motor, evaluate according to your primary physical boundary:

• Priority 1 — Fixed Axial Depth & Maximum Torque: Choose Concentrated Windings. The ~40% reduction in end-turn height frees space for laminated iron, while lower phase resistance boosts continuous thermal torque capability.

• Priority 2 — Uncompromised Motion Smoothness: Choose Distributed Windings if axial envelope permits. Pure sinusoidal back-EMF provides ripple-free positioning.

• Priority 3 — Compact Envelope with Ultra-Smooth Operation: Choose Optimized Fractional-Slot Concentrated Windings (FSCW). Selected 12S/10P or 18S/16P combinations deliver short axial integration with ripple suppressed by up to 85%.

Figure 4 BLDC motor winding selection decision guide

Need engineering support selecting or customizing a fractional-slot concentrated BLDC motor for your short-axial fan or pump application? Contact Volcanomotor at info@volcanomotor.com for optimized slot/pole configurations.

FAQ

Q1: Is a concentrated winding always shorter than a distributed winding?

A1: Yes. Because coil ends do not cross adjacent slots, end-turn height drops by ~40%, reducing total motor length by roughly 25% at identical outer diameter.

Q2: Does a concentrated winding inevitably produce higher torque ripple?

A2: No. While basic concentrated designs have higher ripple, selecting an optimized fractional-slot combination (like 12-slot/10-pole) cancels dominant harmonics, reducing ripple by up to 85%.

Q3: Why is copper loss lower despite a lower winding factor?

A3: Elimination of long end-turns cuts inactive copper length significantly. Total phase resistance drops to ~60% of distributed windings, yielding lower overall I²R heating.

Q4: Can concentrated motors replace distributed motors in existing housings?

A4: In most fixed-envelope retrofits, concentrated windings are the only feasible way to increase iron stack length and torque rating without modifying the external machinery flange.