Motor Type Comparison
| Stepper | Brushed DC | BLDC | Servo | |
|---|---|---|---|---|
| Use case | Open-loop positioning, 3D printers, CNC, low-speed indexing | Simple variable-speed drives, low-cost actuation | Fans, drones, pumps, continuous variable-speed duty | Precision closed-loop motion: robotics, CNC axes |
| Commutation | Electronic, open-loop | Mechanical (brushes + commutator) | Electronic, Hall sensors or sensorless BEMF | Same as underlying motor (usually BLDC) + closed-loop control |
| Efficiency | ~30–65% | ~75–85% | ~85–90%+ | = underlying motor efficiency |
| Control complexity | Simple open-loop (step/direction) | Simplest (PWM voltage) | Moderate (commutation, no PID needed for basic speed) | Highest (feedback loop tuning, gains) |
| Torque-speed curve | High torque at low speed, falls sharply with speed | Roughly linear decrease with speed | Flat across rated range | Flat across rated range + closed-loop hold at zero speed |
A "servo" is not a distinct motor type — it's a control architecture: a BLDC or brushed DC motor combined with a position/velocity feedback sensor and a closed-loop controller.
See the Motor Torque/Speed/Power and Motor Efficiency calculators to solve these relationships for a specific motor.
Stepper Motor Reference
| Step Angle | Steps/Rev | Motor Type |
|---|---|---|
| 1.8° | 200 | Hybrid stepper (most common) |
| 0.9° | 400 | Hybrid stepper (fine resolution) |
| 3.6° | 100 | Hybrid, less common |
| 7.5° | 48 | Permanent-magnet (PM) can-stack |
| 15° | 24 | Permanent-magnet (PM) can-stack |
| NEMA Size | Frame Width (mm) | Typical Holding Torque (N·m) |
|---|---|---|
| NEMA 8 | 20 | ~0.02–0.04 |
| NEMA 11 | 28 | ~0.06–0.17 |
| NEMA 14 | 35 | ~0.11–0.4 |
| NEMA 17 | 42 | ~0.13–0.65 |
| NEMA 23 | 56.4–57 | ~0.5–3 |
| NEMA 24 | 60 | ~1.5–4.2 |
| NEMA 34 | 85–86 | ~2–12 |
| NEMA 42 | 110 | ~12–30 |
NEMA numbers specify frame/faceplate size only, not torque, current, or winding — torque varies substantially by body (stack) length and model within a given frame size. Microstepping (1/8, 1/16, 1/32) subdivides each full step into finer commanded increments, mainly improving motion smoothness and reducing resonance/audible noise — it does not proportionally improve real positioning accuracy, since the rotor follows a sinusoidal torque-vs-displacement curve (holding torque can drop to ~70% of full-step value at the worst-case microstep).
Servo/BLDC Sizing
| Quantity | Formula | Symbols |
|---|---|---|
| Required motor torque | Treq = Tload + Taccel | Tload = torque to drive the load |
| Acceleration torque | Taccel = Jtotal · α | α = angular acceleration |
| Total inertia at the motor shaft | Jtotal = Jmotor + Jreflected | Jmotor = rotor inertia |
| Load inertia referred to the motor shaft | Jreflected = Jload / N2 | N = gear ratio |
| System Type | Max Recommended Inertia Ratio (Jload / Jmotor) |
|---|---|
| Open-loop stepper (small frame) | 5:1 |
| Open-loop stepper (large frame) | 10:1 |
| Closed-loop stepper | 30:1 |
| Servo, auto-tuning | 50:1 |
| Servo, manual tuning | 100:1 |
See the Rotational Inertia/Torque calculator to compute Taccel from a shape’s inertia and angular acceleration.
Motor Constants
| Quantity | Formula | Symbols |
|---|---|---|
| Torque from current | τ = Kt · I | Kt = torque constant (N·m/A), I = current |
| Speed from voltage | ω = Kv · V | Kv = speed constant (RPM/V), ω in RPM |
| Torque constant from speed constant | Kt = 9.5493 / Kv | 9.5493 = 60/2π; in SI units Kt = back-EMF constant Ke |
See the Back-EMF calculator to relate these constants to a motor’s back-EMF voltage at speed.
Linear Actuator Types
| Type | Typical Stroke | Typical Force | Typical Use |
|---|---|---|---|
| Leadscrew/ballscrew | Tens of mm up to ~2–6 m | ~400 N to 25,000+ N | Precision linear positioning, CNC axes, heavy load lifting/pushing |
| Voice coil | Sub-mm up to ~125 mm | Low force, constant across stroke (single- to double-digit N up to ~100s of N) | High-bandwidth precision positioning, optical/lens focus |
| Solenoid | ~1–50 mm (practical <25mm) | ~0.03 N up to ~600 N, decays sharply with stroke | Fast on/off actuation: valves, latches, relays |
See the Solenoid Force calculator to estimate pull force from coil turns, current, and air gap.