Electric motors are built from seven named subassemblies: rotor, stator, air gap, armature, commutator, shaft, and bearings [S3]. The engineering bill of materials (EBOM) that describes a motor is typically a spreadsheet pulled from CAD data, with one row per part plus a quantity property, a unique part number, and metadata that lets purchasing, assembly, and quality teams all read the same document [S4].
Standardized electric motors power everything from watch movements to marine propulsion systems; the largest units exceed 100 MW output, while small fractional-horsepower designs drive disk drives and household appliances [S3]. The BOM must therefore scale with the motor class, with a 100 MW marine unit listing hundreds of lamination sheets, copper bars, and insulation parts, and a small brushless DC unit listing only a handful of subassemblies. The shaft is one component that appears in both ends of that range, since torque is always transmitted through a rotating shaft regardless of motor size.
Stator: Frame, Core, and Winding
Statores are the stationary components that, when energized with alternating current, produce the magnetic field which interacts with the rotor to create rotation [S2]. The three structural parts of a stator are the frame (outer yoke), the core, and the winding. The core is built up from thin metal sheets precisely cut and stacked into a lamination stack; the sheet thickness and alloy are chosen to keep eddy-current losses down and permeability high [S2]. Stator material is generally iron, steel, or, for low-power designs, a printed circuit board (PCB) that can be made smaller, lighter, and less noisy than a laminated iron core [S2].
The winding is the electrical heart of the stator: copper conductors are laid through slots on the inside of the stator core in a complex pattern, and the insulation system around those conductors is what limits stator service life. Broken winding insulation is the most common cause of stator faults, so the EBOM typically lists the conductor, slot liner, top-stick, and varnish as separate line items with their own part numbers and material callouts [S2]. A wound stator is functionally related to other electromagnetic assemblies used in industrial control, including the electric actuator and the electric ball valve, which use the same lamination-and-winding building blocks at smaller scale.
Rotor: Core, Shaft, and Winding or Magnets
The rotor is the moving part of the motor that delivers mechanical power through the shaft; rotor conductors carry currents on which the stator's magnetic field exerts a Lorentz force [S3]. The three structural parts of a rotor are the rotor core, the shaft, and either a wound winding or a permanent-magnet array [S2]. Permanent-magnet rotors are common in internal permanent magnet (IPM) traction motors, which hold high power density and stay efficient over a wide operating range; induction-motor rotors use a squirrel-cage or wound conductor instead, traded off against lower power density and lower peak efficiency [S1].
The rotor's magnetic-property tolerances are looser than the stator's because the rotor sees less flux; however, the rotor's mechanical properties (balance grade, allowable runout, and lamination stacking force) are tighter because the part spins [S2]. In the BOM this shows up as separate rows for the lamination stack, the magnet set or cage bars, the end rings (for induction), the shaft forging, and any retention sleeve or banding wrap, each with its own part number and quantity [S4]. A related reference that covers the upstream material question for these parts is the Electric Motor Raw Material Sourcing: Steel, Copper, Aluminum, and Magnet Spec Map guide.
Air Gap, Armature, and Commutator
The air gap is the narrow radial space between the rotor and the stator, and it is the magnetic circuit element that sets the machine's torque-per-ampere ratio; the gap is not a purchased part but is defined on the BOM as a designed-in clearance with a tolerance callout [S3]. The armature is the power-carrying winding, which is attached either to the rotor (in most motor topologies) or to the stator (in some configurations), with the field magnets on the opposite member; one of the two is reversed compared to a generator, but the magnetic circuit is the same [S3].
The commutator is a mechanical rectifier used in brushed DC motors: a segmented copper cylinder rotating with the shaft, paired with stationary carbon brushes that reverse current direction in the armature at the right angular position. Brushed topologies are still widely used because they are simple and cheap, although brushless designs are displacing them in traction and servo applications where maintenance access is poor [S3]. A typical brushed-motor BOM therefore carries a commutator row with material (copper or silver alloy), segment count, and a matching brush set with grade, spring force, and lead-wire termination specified separately [S4].
Shaft, Bearings, and Mechanical Interfaces
The shaft is the rotating mechanical output member of the motor, supported by bearings and connected to the driven load through a coupling, gear, or pulley [S8]. A shaft-key or keyway is one of the standard ways to transmit torque from the shaft to the mounted component, and the shaft key is therefore a line item on most motor-output BOMs. Bearing selection is driven by radial and axial load, speed (dn value), lubrication method, and expected service life; the EBOM typically lists the drive-end bearing and the non-drive-end bearing as separate lines because they are often different sizes or types [S4].
Other mechanical-interface items that appear on the BOM but are often overlooked are the housing, end shields or bearing caps, fasteners, terminal box or connector, cooling fan or water-jacket fittings, and the nameplate. Each gets a unique part number and a quantity, with units of "Each" for parts, kilograms for raw casting or steel bar stock, or length units for copper magnet wire [S4]. A clean EBOM is the foundation for accurate purchasing and correct final assembly; missing rows on the BOM are a frequent cause of stalled motor builds and field returns [S4].
EBOM vs. MBOM: What Each Row Carries
The engineering bill of materials (EBOM) describes engineering intent: every part, subassembly, and raw material in the design, with quantities, part numbers, descriptions, thumbnails, and reference designators where applicable [S4]. The manufacturing bill of materials (MBOM) is a derivative of the EBOM that adds process data: where a part is made or bought, its cost, the operations needed to assemble it, and any packaging or kitting instructions [S4]. A standard part-number convention is the simplest way to keep the two BOMs in sync, with each part carrying a distinct identifier, usually a hyphenated number such as 12-3456, assigned once and never reused while the part is active [S4].
For an electric motor, the EBOM typically carries rows for the stator subassembly, rotor subassembly, frame, end shields, bearings, shaft, fan, terminal box, nameplate, fasteners, and any accessories (encoder, brake, cable gland). The MBOM then breaks the stator subassembly into the lamination stack, winding, insulation, impregnation varnish, and termination, with each operation routed to a specific work center [S4]. A common engineering error is to treat the stator and rotor as single line items and only explode them at the MBOM stage, which leaves purchasing blind to long-lead items such as grain-oriented electrical steel or rare-earth magnet blanks.
Selection Criteria and Comparison of Motor Types
Three motor topologies dominate modern industrial and traction specifications, and each maps to a different BOM shape. Internal permanent magnet (IPM) motors offer the highest power density and the highest efficiency over a wide operating range, but they rely on rare-earth magnets and a fabricated rotor stack, which raises unit cost and exposes the supply chain to rare-earth price swings [S1]. Induction motors use a more mature, lower-cost supply chain, with high starting torque and high reliability, but their power density and peak efficiency lag IPM designs; this gap is unlikely to close given that the technology is mature [S1].
Switched reluctance motors offer a lower-cost, rugged, easily manufactured alternative that tolerates high temperatures and speeds, but the trade-off is more acoustic noise and vibration, lower efficiency than IPM or induction, and a need for extra rotor-position sensors and a more complex controller [S1]. On a four-axis selection table, IPM leads on power density and efficiency, induction leads on cost and supply-chain maturity, switched reluctance leads on thermal and speed headroom, and induction leads on control simplicity, with the right choice set by the application's weight, cost, noise, and rare-earth exposure targets. The construction machinery and equipment market is one segment where the IPM-versus-induction trade-off is actively re-evaluated because of duty cycle, dust, and thermal load.
Standards, Sourcing, and Limits of the BOM
No single standard writes the entire BOM for an electric motor; instead, several standards govern the parts individually. Lamination steel grades, magnet grades (N42SH, N48SH, and the like), insulation class (A, B, F, or H), and IP enclosure rating each carry their own spec sheet that the EBOM references. Efficiency classes for industrial AC motors are typically cited against IEC 60034-30-1, which defines IE1 through IE4 efficiency bands; the motor's frame size and mounting follow IEC 60072-1 dimensional standards. Insulation systems for traction and industrial motors reference IEC 60085 for thermal class, while magnet and lamination traceability is handled under each OEM's own part-number scheme. [S2]
The practical limit of the EBOM is that it describes geometry and material, not behavior. Two motors with identical BOMs can perform very differently because of slot/pole combination, winding distribution factor, and skew, none of which appear as a single line on the BOM. A good BOM therefore also references the controlling drawing revision, the simulation report, and the test plan, so that the engineering intent captured on the spreadsheet is traceable to the performance that the design was meant to deliver [S4].
Trackable next signals: watch for updates to the Beyond Rare Earth Magnets (BREM) program at Ames Laboratory, which is investigating lower-cost permanent magnets and magnetic materials as a path around rare-earth supply risk [S1]; and watch for revised IE-class efficiency thresholds in the next IEC 60034-30 cycle, which historically tighten the IE4/IE5 bands and force BOM-level changes in lamination grade and copper fill. A useful cross-check on the raw-material side of the BOM is the Electric Motor Raw Material Sourcing: Steel, Copper, Aluminum, and Magnet Spec Map reference.