Electric Motor Taxonomy — Family Index
Exhaustive catalog of rotary and linear electric motor topologies, their governing electromagnetics, standard frame/efficiency designations, and selection criteria. Scope is the machine itself; drive electronics are catalogued separately in [[Engineering/Tier3/motor-drive-electronics]].
1. At a glance
Electric motors fall into three top-level branches by commutation mechanism and field source:
Rotary AC — armature current alternates from the mains or an inverter; field is set by stator excitation, by rotor magnets, or by reluctance saliency.
- Induction: squirrel-cage (SCIM), wound-rotor (slip-ring)
- Synchronous: wound-field (WFSM), permanent-magnet (PMSM), reluctance (SynRM, RSM), hysteresis
- Universal (series-wound, runs on AC or DC)
Rotary DC — armature current is rectified mechanically (brushes + commutator) or electronically (inverter + rotor-position feedback).
- Brushed PM rotor (small-frame DC servos, automotive accessories)
- Brushed wound-field: series, shunt, compound
- Brushless: BLDC with trapezoidal back-EMF and 6-step commutation; PMSM with sinusoidal back-EMF and field-oriented control (FOC)
Special / non-rotary or pulse-driven
- Steppers: variable-reluctance (VR), permanent-magnet (PM), hybrid (HPM)
- Switched reluctance (SRM)
- Linear: iron-core LSM, ironless LSM, tubular linear, linear induction (LIM), voice-coil (VCM)
- Piezoelectric and ultrasonic
- Pancake/frameless torque, slotless, coreless, axial-flux, hub motor
The same physical machine often appears under different commercial names. A “BLDC” and a “PMSM” share identical iron and copper — only the back-EMF waveform and the choice of drive algorithm separate them.
2. Standards landscape
NEMA frame designations (USA, ANSI/NEMA MG 1) encode shaft height. For frames 42–449 the rule is:
frame number / 4 ≈ shaft height (D-dim, inches) × 16
Example: NEMA 184T has D = 4.5 in (184 ⁄ 4 ⁄ 16 ⁄ no, simpler: D × 16 = first two digits → D = 4.5 in for 18-frame).
Common industrial frames:
| Frame | Shaft height D | Typical rating (4-pole, 60 Hz) |
|---|---|---|
| 56C | 3.5 in / 88.9 mm | 0.5–1.5 hp (0.37–1.1 kW), face-mount |
| 143T | 3.5 in | 1.5 hp (1.1 kW) |
| 145T | 3.5 in | 2 hp (1.5 kW) |
| 182T | 4.5 in | 3 hp (2.2 kW) |
| 184T | 4.5 in | 5 hp (3.7 kW) |
| 213T | 5.25 in | 7.5 hp (5.5 kW) |
| 215T | 5.25 in | 10 hp (7.5 kW) |
| 254T | 6.25 in | 15 hp (11 kW) |
| 256T | 6.25 in | 20 hp (15 kW) |
| 284T | 7 in | 25 hp (18.5 kW) |
| 286T | 7 in | 30 hp (22 kW) |
| 324T | 8 in | 40 hp (30 kW) |
| 326T | 8 in | 50 hp (37 kW) |
| 364T | 9 in | 60 hp (45 kW) |
| 365T | 9 in | 75 hp (55 kW) |
| 404T | 10 in | 100 hp (75 kW) |
| 405T | 10 in | 125 hp (90 kW) |
| 444T | 11 in | 150 hp (110 kW) |
| 445T | 11 in | 200 hp (150 kW) |
Suffixes: T = current T-frame (post-1964 standard), C = C-face mount, D = D-flange, JM/JP = pump (close-coupled), U = pre-1964 U-frame (still seen on legacy gear), HP/HPH = high-thrust vertical (well/pump service).
IEC 60072 frame uses shaft-height in mm directly. The preferred series: 56, 63, 71, 80, 90, 100, 112, 132, 160, 180, 200, 225, 250, 280, 315, 355, 400, 450, 500, 560, 630, 710, 800. A IEC 132M motor has H = 132 mm and M (medium) shaft-end. Letters S/M/L = short / medium / long stator length within a frame.
IE efficiency classes (IEC 60034-30-1):
| Class | Name | NEMA equivalent | Typical 4-pole 11 kW η |
|---|---|---|---|
| IE1 | Standard | NEMA Standard | 88.7 % |
| IE2 | High | NEMA Energy Efficient | 90.5 % |
| IE3 | Premium | NEMA Premium | 91.4 % |
| IE4 | Super-Premium | (no NEMA equivalent) | 92.6 % |
| IE5 | Ultra-Premium | (no NEMA equivalent) | 93.6 % |
IE3 minimum is mandatory for most 0.75–375 kW industrial motors in EU (since 2017 / 2021) and the USA (DOE 10 CFR 431, IE3 minimum effective 2010 for general purpose). IE4 / IE5 typically require synchronous-reluctance, PM, or large-frame copper-rotor induction. ABB SynRM and WEG W22 Magnet are common IE5 commercial offerings.
Other standards: NEMA MG 1 (general), IEC 60034 series (rotating machines), IEEE 112 (test methods), CSA C390 (Canada), GB/T 18613 (China, mirrors IEC).
3. AC induction (asynchronous)
The workhorse of industrial motors. Stator excites a rotating field at synchronous speed; rotor lags by slip, induced currents create a rotor field, torque develops. Robust, brushless, cheap.
Synchronous speed:
n_s = 120 · f / P (rpm, with f in Hz and P = pole count)
At 60 Hz: 2-pole = 3600 rpm sync, 4-pole = 1800, 6-pole = 1200, 8-pole = 900. At 50 Hz: 2-pole = 3000, 4-pole = 1500, 6-pole = 1000.
Slip:
s = (n_s − n) / n_s
Typical full-load slip 1–5 %. So a 4-pole 60 Hz “1800 rpm nameplate” motor actually spins ~1750–1770 rpm.
3.1 Squirrel-cage induction (SCIM)
Solid cast-aluminum or copper bars short-circuited by end rings. Most-produced motor in history. NEMA MG 1 defines four torque/slip designs:
| Design | Locked-rotor torque | Breakdown torque | Slip | Typical app |
|---|---|---|---|---|
| A | High | Very high | Low (≤5 %) | Injection molders, high-inertia fans (older) |
| B | Normal (≥150 %) | High (200–250 %) | Low (≤5 %) | General-purpose pumps, fans, machine tools — the default |
| C | High (200–250 %) | Normal (190–225 %) | Low (≤5 %) | Conveyors, crushers, compressors with high starting load |
| D | Very high (275 %) | (equals locked-rotor) | High (5–13 %) | Punch presses, oil-well pumps, hoists — high-inertia high-slip |
3.2 Wound-rotor / slip-ring induction
Rotor has 3-phase winding brought out through slip rings; external resistance varies starting torque and lets you trade torque for slip. Largely displaced by VFD-fed SCIM, but still used in crane hoists, ball mills, wind turbines (doubly-fed induction generator, DFIG — the slip-ring being driven by a partial-rating converter; GE 1.5 MW class).
3.3 Single-phase induction variants
Single-phase mains has no rotating field. A second (auxiliary) winding offset by 90° electrical and a phase-shifted current creates the rotating field at start.
| Variant | Start mechanism | Start torque | Run | Use |
|---|---|---|---|---|
| Split-phase | High-resistance aux winding; centrifugal switch disconnects | ~150 % | Main only | Small fans, light machinery |
| Capacitor-start (CSIR) | Series cap on aux; centrifugal switch | 250–400 % | Main only | Compressors, pumps |
| Capacitor-start / capacitor-run (CSCR) | Two caps, run cap stays in | 250–400 % | Both windings | Industrial single-phase up to 5 hp |
| Permanent-split-capacitor (PSC) | Run cap stays in, no aux switch | Low (~50 %) | Both windings | HVAC blowers, ceiling fans — quiet |
| Shaded-pole | Copper shading-ring on a salient pole offsets field phase | Very low (~50 %) | — | Small clocks, fan motors, fractional hp |
4. AC synchronous
Rotor locks to the stator field — no slip at steady state. Field on the rotor comes from a DC winding, permanent magnets, or reluctance saliency.
4.1 Wound-field synchronous (WFSM)
DC-excited rotor (via brushes + slip rings, or brushless exciter). Used historically for:
- Paper-mill section drives (constant ratio gang of motors)
- Ship propulsion (Azipod with WFSM ~20 MW)
- Large pumps and compressors > 1 MW where over-excitation provides plant power-factor correction
- EV traction (Renault Zoe ZE40, Audi e-tron rear, BMW iX5 — avoids rare-earth PM cost)
Field current is the second control degree-of-freedom: it sets the back-EMF magnitude and hence the power factor. A WFSM operated leading-PF acts as a synchronous condenser.
4.2 Permanent-magnet synchronous (PMSM)
Rotor has NdFeB, SmCo, or ferrite magnets. The mechanical machine is identical to a BLDC — the line between PMSM and BLDC is purely the back-EMF waveform and drive scheme:
- Sinusoidal back-EMF + sinusoidal-current FOC drive = PMSM
- Trapezoidal back-EMF + 6-step block commutation = BLDC
Subtypes by magnet placement:
- Surface PM (SPM): magnets glued to rotor OD. Cheap, low saliency (L_d ≈ L_q). Easy to model. Used in industrial servos (Yaskawa Sigma-7, Mitsubishi MR-J5).
- Interior PM (IPM): magnets buried in rotor laminations. High saliency (L_d ≠ L_q) gives reluctance torque on top of magnet torque. Field-weakens well above base speed. Tesla Model 3/Y rear drive, Prius MG2, most modern EV traction.
- Inset PM: hybrid, magnets in surface slots — between SPM and IPM.
4.3 Reluctance machines
No rotor windings, no magnets — torque comes from the rotor preferring to align with the stator field along its low-reluctance axis.
- Synchronous reluctance (SynRM): stator is a standard distributed AC winding; rotor is a stack of laminations with deep flux barriers cut transverse to a chosen axis, creating L_d/L_q ratio of 4–10. Driven sinusoidally by a VFD. ABB IE5 SynRM (with optional PM-assist), KSB SuPremE pump motors. No rotor losses → very efficient, cool, simple to manufacture, rare-earth-free.
- Reluctance synchronous motor (RSM): older salient-pole synchronous machine with no field winding — small servo applications. Largely displaced by SynRM and PMSM.
4.4 Hysteresis motor
Rotor is a hardened ferromagnetic ring (cobalt steel) with high coercivity. Magnetic hysteresis lag between rotor and stator field produces torque. Self-starts smoothly and runs at exact synchronous speed. Tiny, low-torque, but vibration-free → instrument clocks, gyroscopes, tape-recorder capstans, lab stirrers.
4.5 Universal motor
Brushed series-wound machine designed to run on AC or DC. The series field reverses with the armature current each half-cycle, so torque is unidirectional. Operates 5 000 – 30 000 rpm — much faster than line-frequency-locked AC motors of the same frame. Drawbacks: brush wear, RFI, noise.
Applications: hand power tools, vacuum cleaners, kitchen mixers, dental drills, older washing-machine spin motors. Increasingly displaced by BLDC (“brushless tools”) for life and efficiency.
5. Brushless DC (BLDC) and PMSM detail
Same machine, different control. The trapezoidal-vs-sinusoidal distinction comes from winding distribution: concentrated-tooth windings make trapezoidal back-EMF, distributed windings make sinusoidal.
Structural variants:
- Inrunner: rotor inside stator (classic layout). Low rotor inertia → fast dynamic response. Servo motors, robot joints.
- Outrunner: stator inside, rotor (with magnets) outside. High pole count, high torque density at moderate speed, high inertia. Drone propellers, hub motors, gimbals.
- Slotless: stator iron has no teeth — windings sit in airgap on a smooth back-iron. Zero cogging, very low torque ripple. Maxon EC-i, Moog brushless DC.
- Coreless / ironless: stator winding is a self-supporting basket inside the magnet cup; no iron in the armature at all. Effectively no cogging, no iron losses, very low inductance — fast response. Faulhaber, Maxon EC, Portescap, Allied Motion. Small frames (≤100 W) for medical pumps, prosthetics, instrumentation.
- Axial-flux: see §10.
Commutation feedback options in order of cost: Hall-effect sensors (3 digital, 60° resolution — fine for BLDC 6-step), incremental encoder, absolute encoder (single- or multi-turn), resolver (aerospace/automotive, rugged), sensorless (back-EMF zero-crossing or observer for FOC — cost-free but poor at zero speed).
6. Brushed DC
Mechanical commutator (copper segments on rotor) and stationary brushes (carbon or metal-graphite) rectify the rotor current. Torque is proportional to current; speed is proportional to applied voltage minus IR drop.
6.1 PM-rotor brushed DC
Stator has permanent magnets, rotor has the wound armature. Field is fixed. Linear V-vs-speed and I-vs-torque curves make it the simplest motor to model. Range: nano-watt watch motors to ~5 kW automotive starter/blower motors. Examples: Maxon RE/DCX, Faulhaber 2342, Portescap 22N28, Pittman, MicroMo, Johnson Electric.
6.2 Wound-field brushed DC
Field winding on stator; topology determines characteristic curve.
| Type | Field connection | Speed-torque | Use |
|---|---|---|---|
| Series | Field in series with armature | High starting torque, runs away at no-load | Starter motors, traction (early electric trams, Lionel toy trains), winches, cranes |
| Shunt | Field in parallel with armature | Nearly constant speed vs load | Constant-speed industrial drives (pre-VFD era), machine tools |
| Compound (cumulative) | Both series + shunt fields aiding | High start torque, stable run | Elevators, rolling mills (legacy) |
| Compound (differential) | Series and shunt opposing | Unstable; rarely used | (avoid) |
| Separately excited | Field on its own supply | Two-quadrant speed/torque control | Large industrial drives, regenerative cranes |
Mostly historical except in automotive (12 V starter motors are still series-wound) and traction-replacement contexts. AC induction + VFD has displaced wound-field DC for industrial speed control.
7. Steppers
Open-loop position by counting drive pulses. Each pulse advances the rotor one defined step angle.
| Type | Construction | Step angle | Notes |
|---|---|---|---|
| Variable-reluctance (VR) | Toothed soft-iron rotor, no PMs, salient stator poles | 15°, 7.5° | Earliest type, low torque, audible. Largely obsolete. |
| Permanent-magnet (PM) | Multi-pole magnetized rotor | 7.5°, 15° | “Can-stack” small steppers, 28 mm tin-can frame |
| Hybrid (HPM) | PM rotor with toothed iron pole pieces (50 teeth typical) | 1.8° (200 step/rev), 0.9° (400 step/rev) | The modern industrial stepper. NEMA 11/14/17/23/24/34/42 frames. Bipolar 2-phase wiring. |
NEMA stepper frame sizes (faceplate dimensions, not the same NEMA MG 1 system):
| Frame | Faceplate | Typical holding torque |
|---|---|---|
| NEMA 8 | 20 × 20 mm | 0.018 – 0.04 N·m |
| NEMA 11 | 28 × 28 mm | 0.04 – 0.1 N·m |
| NEMA 14 | 35 × 35 mm | 0.05 – 0.2 N·m |
| NEMA 17 | 42 × 42 mm | 0.2 – 0.65 N·m (3D printers, small CNC) |
| NEMA 23 | 57 × 57 mm | 0.7 – 3.0 N·m (CNC mills, plotters) |
| NEMA 24 | 60 × 60 mm | 1.5 – 4 N·m |
| NEMA 34 | 86 × 86 mm | 4 – 13 N·m (industrial CNC) |
| NEMA 42 | 110 × 110 mm | 18 – 50 N·m |
Microstepping subdivides each full step electronically by sine-cosine current shaping in the two phases. Drivers offer 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128, 1/256. Trinamic TMC2209 / TMC5160, DRV8825, Allegro A4988 are common drivers. Microstep position accuracy is limited by detent torque and load — fine microstep ≠ fine actual resolution.
Stepper vendors: Nanotec, Anaheim Automation, Oriental Motor (Vexta), MOONS’, LeadShine, Lin Engineering, Sanyo Denki, Applied Motion.
8. Switched reluctance (SRM / SR)
Both rotor and stator are salient (toothed). No PMs, no rotor windings. Phases are pulsed by a power-electronics controller in sequence so the rotor pole sees a torque pulling it toward alignment with the nearest excited stator pole. Stop excitation just before alignment; jump to next phase.
Characteristics:
- Rotor is the simplest of any motor — stack of laminations
- High temperature tolerance (no magnets to demagnetize, no rotor copper)
- Torque ripple and acoustic noise are notorious — modern designs mitigate with profiled current commands
- Inverter is non-standard: each phase needs its own asymmetric-bridge, two switches and two diodes per phase
- Single-phase rotation requires extra cleverness (offset pole arc, mechanical bias spring)
Modern commercial SRM: Nidec SR drive systems (formerly Switched Reluctance Drives Ltd, UK), Land Rover Defender mild-hybrid generator/starter, Dyson V-series vacuum motors (a single-phase SRM running >100 000 rpm). Some appliance-compressor designs. Aerospace generator applications.
9. Linear motors
Take a rotary stator, unroll it — the rotor becomes a “forcer” sliding along a “track”. Force, not torque.
| Type | Stator (track) | Forcer | Force | Use |
|---|---|---|---|---|
| Iron-core linear synchronous (LSM) | Permanent magnets on steel track | Iron-cored windings | High (kN range) | Machine-tool axes, gantries, press feeds |
| Ironless linear synchronous | PM track, U-channel | Coil array, no iron | Lower, very smooth | Semiconductor wafer stages, optical scanners, scribing |
| Tubular linear | Cylindrical magnet rod (slider) | Coil ring assembly around rod | Moderate | Pick-and-place, packaging |
| Linear induction (LIM) | Aluminum reaction rail (long) | Primary with 3-phase coil | Moderate, end-effects significant | Maglev (Transrapid German, JR Maglev), airport people-movers, baggage handling, theme-park launchers |
| Voice-coil (VCM) | Magnet cylinder | Bobbin with a single coil | Linear in current, short stroke | Loudspeakers, hard-disk head positioner (legacy), wafer-stage fine-positioning, photolithography focus |
| Linear stepper | Toothed platen + forcer | Phased coil pair | Position by step count | Older XY tables, plotters |
Vendors: ETEL (now ETEL Heidenhain), Akribis, Aerotech, LinMot (tubular), HIWIN linear, IntelLiDrives, Tecnotion, Yaskawa Sigma Linear, Bosch Rexroth IronCore/Ironless lines, Parker Trilogy.
10. Special / niche topologies
Pancake / frameless torque motor. Large-bore axial-short PMSM optimized for direct drive at low speed and high torque, no gearbox. Hollow shaft accommodates cabling, optics, or fluid through-flow. Robot joints, camera gimbals, telescope mounts, turntables. Vendors: Kollmorgen TBM and KBM series, Tecnotion QT series, ETEL TMB/ILM, Allied Motion Megaflux, Aerotech S-series, Moog frameless. Often paired with a strain-wave (harmonic) gear or used direct-drive.
Gimbal motor. Outrunner BLDC optimized for low cogging and smooth low-speed operation, paired with a closed-loop FOC drive for stiffness against perturbation. Drone camera gimbals (DJI Ronin), telescope mounts. Often coreless or slotless.
Axial-flux motor. Flux travels parallel to the rotation axis instead of radially. Rotor is a thin disk of magnets; stator is a disk of coils on the opposite face (or two stator disks sandwiching the rotor — YASA topology). Very high torque density, very short axial length, ideal for in-wheel and aircraft propulsion.
- YASA (acquired by Mercedes-Benz 2021): YASA 750R/P400 axial-flux, used in Ferrari SF90, McLaren Speedtail hybrids
- Magnax: yokeless and segmented armature (YASA) variants
- Equipmake (UK): high-density EV traction
- Phi-Power: small-aircraft propulsion
Hub motor. Outrunner PMSM/BLDC built into a wheel hub. Eliminates driveshaft. E-bikes, scooters, in-wheel EVs (Protean Electric, Elaphe, Lordstown Endurance prototype). Penalty: unsprung mass.
Printed-circuit (pancake) motor. Brushed or brushless, stator winding etched on a PCB. The Kollmorgen ServoDisc and PMI Motion U9M/U12M are classic examples — very thin axial length, low rotor inertia.
Piezoelectric motors. Piezo stacks driven at resonance produce ultrasonic ovular oscillation in a contact element; friction pushes a rotor or slider. No magnetism — can run in MRI bores, vacuum, cryogenic.
- PI (Physik Instrumente) N-310 NEXACT, M-661 NEXLINE
- Cedrat APA / SA series amplified actuators
- New Scale Technologies Squiggle
Ultrasonic motors. Travelling-wave standing-wave variants. Shinsei USR-series (60 mm OD ring, used in older camera autofocus). Canon EF-USM and Nikon AF-S lenses use ring-USM elements. Newer SDM (Standing-wave Drive Motor) and SIDM (Smooth Impact Drive Mechanism, Konica Minolta) for compact lenses and microscope stages.
Electrostatic / MEMS motors. Force from electric field between charged plates. Practical only at MEMS scale. Hard-disk-drive head-positioner predecessors used VCM not electrostatic — but MEMS electrostatic motors exist in optical scanners and chip-scale gyroscopes.
11. Comparison table
Approximate values; ranges depend strongly on frame and rating.
| # | Type | Power range | η typ | Torque density Nm/kg | Speed range | Cost | Control complexity | Typical use |
|---|---|---|---|---|---|---|---|---|
| 1 | SCIM (3-ph IE1) | 0.1 kW – 50 MW | 75–93 % | 0.5–2 | 750–3600 rpm | Low | Low (DOL) – High (VFD) | Pumps, fans, conveyors |
| 2 | SCIM (3-ph IE3) | 0.75 kW – 1 MW | 88–96 % | 0.6–2.2 | 750–3600 rpm | Low–Med | Low–High | Industrial default |
| 3 | SCIM (3-ph IE5 copper-rotor) | 1 kW – 200 kW | 91–97 % | 0.7–2.5 | 750–3600 rpm | Med | High (VFD) | EU compliance, premium efficiency |
| 4 | Wound-rotor IM | 5 kW – 30 MW | 85–95 % | 0.4–1.5 | 0–3600 rpm | High | Med | Crane hoists, DFIG wind |
| 5 | Single-phase PSC | 50 W – 2 kW | 50–70 % | 0.2–0.6 | 1700/1100 rpm | Very low | None | HVAC blowers |
| 6 | Single-phase cap-start | 0.2–4 kW | 60–75 % | 0.3–0.8 | 1750/1100 rpm | Low | None | Compressors, well pumps |
| 7 | WFSM (large) | 1 MW – 100 MW | 95–98 % | 1.0–2.5 | 100–3600 rpm | High | High | Ship propulsion, paper mills |
| 8 | PMSM (SPM industrial) | 100 W – 100 kW | 90–96 % | 1.5–4 | 0–6000 rpm | Med–High | High (FOC) | Servo positioning |
| 9 | PMSM (IPM EV traction) | 50–400 kW | 93–97 % | 4–10 | 0–18 000 rpm | High | High | EV main drive |
| 10 | SynRM (IE5) | 1 kW – 350 kW | 92–97 % | 1.0–2.5 | 0–3600 rpm | Med | High | Pumps, fans, premium efficiency |
| 11 | PM-assisted SynRM | 5 kW – 200 kW | 94–97 % | 2–5 | 0–6000 rpm | Med–High | High | EV traction (Toyota, BMW) |
| 12 | Hysteresis | 1 – 50 W | 30–50 % | <0.1 | Sync only | Med | None | Clocks, gyros |
| 13 | Universal | 50 W – 2 kW | 35–60 % | 0.3–1 | 5 000–30 000 rpm | Very low | None–Med | Power tools, vacuums |
| 14 | BLDC outrunner | 5 W – 30 kW | 80–93 % | 1–6 | 1000–50 000 rpm | Low–Med | Med | Drones, hub motors, gimbals |
| 15 | BLDC inrunner | 5 W – 50 kW | 80–93 % | 0.8–4 | 0–60 000 rpm | Med | Med–High | RC, automotive accessories, servos |
| 16 | Slotless BLDC | 5 – 500 W | 75–90 % | 0.4–1.5 | 0–60 000 rpm | High | Med | Medical, optical |
| 17 | Coreless / ironless brushless | 1 – 200 W | 80–92 % | 0.3–1.0 | 0–80 000 rpm | High | Med | Precision instruments |
| 18 | Brushed PM DC | 1 mW – 5 kW | 70–88 % | 0.3–1.5 | 0–10 000 rpm | Very low | Very low | Automotive accessories |
| 19 | Brushed wound-field DC | 100 W – 5 MW | 80–93 % | 0.4–1.5 | 0–4000 rpm | High | Med | Legacy traction, industry |
| 20 | Hybrid stepper (NEMA 17) | 0.1 N·m – 0.65 N·m | 30–60 % | 0.4–1 | 0–1500 rpm | Low | Low | 3D printers, plotters |
| 21 | SRM | 0.1 kW – 250 kW | 85–94 % | 1–3 | 0–100 000 rpm | Low (motor) Med (drive) | High | Dyson vacuums, appliance, aero |
| 22 | Axial-flux PMSM | 5 kW – 500 kW | 93–97 % | 5–15 | 0–10 000 rpm | High | High | Hybrid supercar, aero, in-wheel |
| 23 | Linear PMSM (iron-core) | 50 N – 30 kN | 70–90 % | (N/m specific) | 0–10 m/s | High | High | Machine tools, gantries |
| 24 | Linear PMSM (ironless) | 5 N – 1 kN | 70–85 % | (N/m specific) | 0–5 m/s | High | High | Semi wafer stages |
| 25 | Voice-coil | 0.1 N – 200 N | (V·A linear) | (short stroke) | μm to cm | Med | Low | HDD heads, lithography focus |
12. Cooling and enclosure classes
IEC IC codes (60034-6) describe cooling method.
- IC01 / IC11 — open machine, self-ventilated (ODP, open-drip-proof)
- IC411 — totally-enclosed, fan-on-shaft cooling external fins (TEFC) — the industrial workhorse
- IC416 — totally-enclosed, separately-driven external fan (forced ventilation; constant cooling regardless of motor speed — important for VFD-fed motors at low speed)
- IC418 — totally-enclosed, air-over (the motor is in the airstream, e.g., direct fan mount)
- IC81W — water-jacket / liquid cooling
- IC511 — air-water heat exchanger (TEAAC, large motors)
NEMA enclosure types (overlapping concept, focused on environment):
- ODP (open drip-proof) — vented frame, drip-shielded
- TEFC (totally-enclosed fan-cooled) — sealed frame, external fan
- TENV (totally-enclosed non-ventilated) — small frames, conduction cooling
- TEAO (totally-enclosed air-over) — relies on a separately-driven airstream
- WPI / WPII — weather-protected I / II (large open motors, baffles against ingress)
- XP / EXP — explosion-proof (NEC Class I Div 1 / 2 or ATEX zones)
- TEXP — totally-enclosed explosion-proof
- Submersible — IP68 well-pump motors
IP rating (IEC 60529) — first digit solids (0–6), second digit water (0–8 plus 9K). Industrial TEFC is typically IP54 or IP55; submersible IP68.
EV traction inverters drive motors that are commonly oil-cooled (rotor or stator jacket), with end-winding spray cooling for high power density. Dyno test cells use water-jacket cooled units rated for thermal cycling.
13. Insulation classes
Maximum sustained winding temperature under rated load + ambient.
| Class | Max winding T | Allowable rise (40 °C amb) | Typical insulation |
|---|---|---|---|
| A | 105 °C | 60 K | Cotton, paper impregnated with oil |
| E | 120 °C | 75 K | Polyester enamel |
| B | 130 °C | 80 K | Mica, glass-fiber, polyester |
| F | 155 °C | 105 K | Glass-mica, modified polyester — industrial default |
| H | 180 °C | 125 K | Silicone, polyimide |
| N | 200 °C | 145 K | Polyimide, hi-temp epoxies |
| R | 220 °C | 160 K | Polyimide, mica-glass-ceramic |
| S | 240 °C | — | Specialty high-T |
| C | > 240 °C | — | Ceramic, mica without organic binder |
Ambient is normally 40 °C unless nameplated otherwise. Halving the temperature margin above the class limit roughly doubles winding life (Arrhenius: every 10 K reduction ≈ 2× insulation life). NEMA Class F motors operated within Class B rise (i.e., 25 K margin) are a common reliability standard.
14. Service factor and duty cycles
Service factor (SF) — NEMA continuous overload capability without exceeding Class B rise. 1.0 (no overload), 1.15 (15 % overload, the industrial default for general-purpose motors), 1.25, 1.4 (fractional HP). VFD-fed motors are typically declared SF 1.0 because thermal headroom is consumed by switching losses.
IEC duty types (60034-1):
| Duty | Description | Use case |
|---|---|---|
| S1 | Continuous running | Pumps, fans, conveyors at steady load |
| S2 | Short-time, defined duration then cool to ambient | Lock-gate winches, sluice gates |
| S3 | Intermittent periodic, no starting effect on heating | Periodic on/off loads |
| S4 | Intermittent periodic with starting | Cranes, hoists |
| S5 | Intermittent periodic with starting and braking | Reversing duty |
| S6 | Continuous with intermittent load | Compressors with cycling |
| S7 | Continuous with starting and braking | Frequent reversing |
| S8 | Continuous with related load/speed changes | Pole-changing motors |
| S9 | Non-periodic load/speed | Random duty |
| S10 | Discrete constant loads with discrete times | Sampled duty |
Match the duty type to the load profile to avoid undersizing (thermal) or oversizing (capital).
15. Selection heuristics
| Application | Recommended topology | Rationale |
|---|---|---|
| Pump / fan / conveyor (constant or variable torque) | IE3/IE4 SCIM + VFD | Lowest capex, broad supply, VFD gives speed control and soft-start |
| Premium-efficiency pump or fan | SynRM IE5 + VFD | Rare-earth-free, lower losses than SCIM at part-load |
| Industrial servo positioning | PMSM SPM + 24-bit absolute encoder + FOC drive | Best torque-to-inertia, mature ecosystem |
| Collaborative robot joint | Frameless torque motor (Kollmorgen TBM/KBM) + harmonic gear + dual encoder | Direct-drive feel, hollow shaft for cabling, high stiffness |
| Auto traction (passenger BEV) | IPM PMSM (Tesla 3/Y, most VW, Hyundai E-GMP) or WFSM (Renault Zoe, Audi e-tron rear, BMW iX5) | IPM = highest peak efficiency; WFSM = no rare earths, controllable field |
| Auto traction (hybrid super-car) | Axial-flux PMSM (YASA in Ferrari SF90, Mercedes-AMG) | Best torque density, fits in transaxle space |
| Aerospace fuel pump / actuator | BLDC with redundant 3-phase windings (independent DO-160 channels) | Brushless reliability + fault-tolerant winding partition |
| Drone propulsion | Outrunner BLDC + ESC | High torque at low speed, propeller direct-drive |
| Camera gimbal | Slotless or coreless outrunner BLDC + FOC | Low cogging is mandatory for video stability |
| Precision linear stage (semi wafer, scribing) | Ironless linear PMSM | Zero cogging, sub-micron positioning |
| High-force machine tool axis | Iron-core linear PMSM | kN-class force without ballscrew backlash |
| Domestic washing machine drum | BLDC direct-drive (LG, Samsung) or PSC SCIM + belt (legacy) | DD eliminates belt; lower noise, longer life |
| Domestic refrigerator compressor | Single-phase SCIM (PSC) or BLDC (inverter compressor — Embraco VNEK / VEGT) | Inverter BLDC is now the efficiency standard |
| Hand power tool | Universal motor (cheap) or BLDC (premium) | BLDC for cordless, brushless tool lines |
| Stepper (3D printer XY/Z, small CNC) | NEMA 17/23 hybrid stepper + microstepping driver | Open-loop simplicity, sufficient for low-force apps |
| Hard-disk head positioner | Voice-coil motor | Linear, low-mass, microsecond response |
| Clock / instrument timing | Hysteresis or synchronous PM | Exact line-frequency lock |
| Vacuum cleaner (high-end, e.g., Dyson) | Single-phase SRM at 100 000+ rpm | High specific power, no brushes |
16. Vendor landscape
Large industrial AC (SCIM, SynRM, WFSM) — Siemens (Simotics), ABB (M3BP, IE5 SynRM), WEG (W22 / W22 Magnet), Toshiba EQP Global, Nidec (acquired Leroy-Somer 2019, Embraco 2018, Mitsubishi Heavy Industries motors 2021), TECO-Westinghouse, Marathon (Regal Rexnord), Baldor-Reliance (ABB-owned), Hyundai Electric, Brook Crompton (Wolong), Hitachi Industrial, GE Power Conversion.
Industrial servo (PMSM SPM/IPM, FOC drives) — Yaskawa (Sigma-7, Sigma-X), Mitsubishi (MR-J5 / Melservo), Fanuc (αi/βi servo), Bosch Rexroth (IndraDyn / IndraDrive), Kollmorgen (AKM, AKM2G, AKD drive), Siemens (Simotics S, Sinamics S210), Allen-Bradley (Kinetix / MP-series), Beckhoff (AM8000 / AX5000), ABB (BSM / MicroFlex), Parker (NX / PSD), Estun, Inovance.
Precision DC / brushless small-frame — Maxon (RE, DCX, EC, EC-i, EC-flat), Faulhaber (DC-micromotor, BX4, BHx), Portescap (Athlonix, Mini-Motor), Allied Motion (Megaflux, Megafit, NEMA frameless), Moog (BN, BLDC), MicroMo (US Faulhaber arm), Pittman (AMETEK), Johnson Electric.
Steppers — Nanotec, Anaheim Automation, Oriental Motor (Vexta), MOONS’, LeadShine, Lin Engineering, Sanyo Denki SanMotion, Applied Motion, Trinamic (drives), Allegro (drivers), Texas Instruments (drivers).
Axial-flux — YASA (Mercedes-Benz), Magnax (Belgium), Equipmake (UK), Phi-Power (Switzerland), DHX (USA, aerospace), Whylot (France).
Linear — ETEL (Heidenhain), Akribis Systems (Singapore), Aerotech (USA), LinMot (NTI AG, tubular), HIWIN (linear motor stages), Tecnotion (frameless + linear), IntelLiDrives, Bosch Rexroth (IronCore/Ironless), Parker Trilogy, Yaskawa Sigma Linear.
Switched reluctance — Nidec SR (formerly SRD Ltd, UK), Dyson (in-house for V-series). Some industrial-compressor OEMs.
Piezo / ultrasonic — PI (Physik Instrumente), Cedrat Technologies (France), Shinsei Corporation (Japan, USR), Canon (in-lens USM/STM, not sold separately), New Scale Technologies (SquiggleMotor), Xeryon (Belgium).
17. Cross-references
[[Engineering/electric-motors]]— Tier 2 conceptual overview (this index is the exhaustive Tier 3 catalog)[[Engineering/Tier3/motor-drive-electronics]]— VFD / inverter / FOC topologies; the companion to this note[[Engineering/Tier3/bearings-taxonomy]]— rotor bearings: deep-groove, angular-contact, sleeve, magnetic, ceramic hybrid[[Engineering/Tier3/gears-taxonomy]]— speed reduction for direct-drive vs geared servo design[[Engineering/Tier3/couplings-taxonomy]]— shaft couplings for motor-to-load[[Robotics/motors-electric]]— robotics-specific motor selection (cobots, AGVs, drones)[[Engineering/power-electronics]]— IGBT / SiC / GaN inverter switches that drive these motors[[Engineering/classical-control]]— FOC, vector control, sensorless observers
18. Citations
- Hughes, A. & Drury, B., Electric Motors and Drives: Fundamentals, Types and Applications, 5th ed., Newnes / Elsevier, 2019. ISBN 978-0-08-102615-1.
- Krishnan, R., Switched Reluctance Motor Drives: Modeling, Simulation, Analysis, Design, and Applications, CRC Press, 2001. ISBN 978-0-8493-0838-1.
- Hanselman, D. C., Brushless Permanent Magnet Motor Design, 2nd ed., Magna Physics Publishing, 2003. ISBN 978-1-881855-15-6.
- Pyrhönen, J., Jokinen, T. & Hrabovcová, V., Design of Rotating Electrical Machines, 2nd ed., Wiley, 2014. ISBN 978-1-118-58157-5.
- Boldea, I. & Nasar, S. A., The Induction Machines Design Handbook, 2nd ed., CRC Press, 2010. ISBN 978-1-4200-6668-5.
- Miller, T. J. E., Brushless Permanent-Magnet and Reluctance Motor Drives, Oxford University Press, 1989. ISBN 978-0-19-859369-8.
- NEMA MG 1-2021 — Motors and Generators, National Electrical Manufacturers Association.
- IEC 60034 series — Rotating electrical machines (parts 1, 2-1, 5, 6, 7, 9, 11, 12, 14, 25, 30-1).
- IEC 60072 — Dimensions and output series for rotating electrical machines.
- IEC 60085 — Electrical insulation — Thermal evaluation and designation.
- IEC 60529 — Degrees of protection provided by enclosures (IP code).
- IEEE 112-2017 — Standard Test Procedure for Polyphase Induction Motors and Generators.
- DOE 10 CFR 431 Subpart B — Electric motors energy conservation standards (US).