Motors 101 — Selection Guide

Motors convert electrical energy into motion. Choosing the right type comes down to control method, torque/speed profile, precision, and budget. Use this page as a quick lesson and a practical field guide.

AC Motors

Induction (Asynchronous): rugged workhorse; rotor torque is induced by a rotating stator field.

Synchronous (PMAC/Servo): rotor locks to stator field (often permanent magnets); used in many modern servo systems.

VFD (Induction) Servo Drive (Synchronous)

DC Motors

Brushed DC: simple voltage control; brushes/commutator handle commutation.

Brushless DC (BLDC): electronic commutation via driver; long life & high efficiency.

From Brushless: two common control styles

Stepper: open-loop step control (pulse/dir). Encoder optional.
Servo (BLDC/PMSM): closed-loop with encoder/resolver.

Stepper Drive Servo Drive

Direct-Drive Linear

Flat or tubular linear motors produce force without screws/belts. Consider them when you need high dynamics and zero backlash.

Servo Drive Scale/Encoder Feedback

Motor Decision Flowchart

Start at the left and follow the prompts.

Do you need closed-loop precision (exact position/torque at speed)?

If you must guarantee accuracy under varying loads or sync multiple axes, choose closed-loop.

Yes → Servo No → Open-Loop Options

Is the motion mostly constant-speed?

Fans, pumps, conveyors with modest accuracy.

Yes → AC Induction No, need indexing/holding → Stepper

Need simplest low-cost variable speed?

Portable tools, lab rigs, hobby fixtures.

→ Brushed DC

AC Induction (Asynchronous)

Control: Across-the-line for fixed speed; add a VFD for variable speed/vector control.

Feedback: None required; optional encoder for advanced vector control.

Cost: $ lowest lifetime cost; rugged.

Stepper (Brushless, Open-Loop)

Control: Stepper drive (pulse/dir, microstepping).

Feedback: Optional encoder for stall detect/closed-loop hybrids.

Cost: $$ economical precise indexing.

Brushed DC

Control: Direct voltage or PWM H-bridge; simplest setup.

Feedback: Optional (tach/encoder) if you need regulation.

Cost: $–$$ but brushes wear (maintenance).

Servo (AC PMSM or BLDC)

Control: Servo drive with current/velocity/position loops.

Feedback: Encoder/Resolver required.

Cost: $$$–$$$$ highest performance.

Linear Motor (Direct-Drive)

Control: Servo drive; often high-resolution linear scale.

Feedback: Required for precision.

Cost: $$$$ specialized, zero backlash.

What to Specify

Control & Drive

AC Induction: across-the-line or VFD for speed control.

Stepper: stepper drive (pulse/dir, microstep).

Servo: servo drive (fieldbus/analog/pulse).

Brushed DC: voltage or PWM H-bridge.

Feedback

Required: Servo, Linear motors.

Optional: Stepper (for stall detect/closed-loop), AC Induction (for vector control), Brushed DC (for regulation).

Application Fit

Define load/inertia, speed & accel, duty cycle, precision, environment (IP/temp), and mechanical interface (frame/shaft/gearbox).

Rule of thumb: Pick the simplest motor/drive that meets performance. If accuracy under load or high dynamics matter, move to servo. For constant-speed economy, use AC induction + VFD. For precise indexing at low/medium speed, choose stepper.

Quick Comparison

Type	Control / Drive	Feedback	Torque & Speed	Precision	Typical Use	Cost
AC Induction	Across-the-line (fixed) or VFD for variable/vector	None (optional encoder for vector)	Good continuous torque; wide speed with VFD	Moderate	Fans, pumps, conveyors, mixers	$
Stepper	Stepper drive (pulse/dir, microstep)	Optional encoder (hybrid closed-loop)	High holding torque low speed; torque drops at RPM	High repeatability (indexing)	Pick-and-place, labelers, small gantries	$$
Brushed DC	Voltage/PWM via H-bridge	Optional tach/encoder	High starting torque; moderate max speed	Low–Moderate	Actuators, tools, test rigs	$–$$
BLDC (non-servo)	BLDC driver (trapezoidal; Hall or sensorless)	Halls optional; encoder optional	Efficient; wide speed range	Moderate	AGVs/AMRs, pumps, fans, compact modules	$$
Servo (PMSM/BLDC)	Servo drive (current/velocity/position)	Encoder/Resolver required	High torque density; excellent dynamics	Very high	Robotics, CNC, high-speed packaging	$$$–$$$$
Linear Motor	Servo drive; linear scale	Required	High continuous thrust; zero backlash	Micron-level	Semiconductor, inspection, metrology	$$$$

Optional: Typical Voltage Ranges

Show common supply voltages (for teaching)

AC Induction: 120–480 VAC (1- or 3-phase), global variants exist.

Servo (AC PMSM): 200/230 VAC or 400/480 VAC 3-phase drives (low-voltage servos 24–60 VDC exist).

BLDC/Stepper/Brushed DC: typically 12–60 VDC (industrial up to ~90–170 VDC with appropriate drives).

Always match motor insulation, drive rating, and local codes.

Teaching Notes

Inertia Matching

For fast moves, keep load inertia / motor inertia within manufacturer guidelines (often <10:1 for servos). Gearboxes help.

Thermal vs Peak

Size to continuous (thermal) torque with margin; ensure peaks (accel/settle) stay within drive/motor limits.

Environment

IP rating, temperature, washdown coatings, and bearing type matter as much as torque/speed.

Have a spec you want checked? Start a Build Motor request and we’ll validate drive compatibility, feedback, and sizing.