Special Machines

Steppers, BLDC, servo, switched-reluctance, universal and linear motors — the machines behind printers, drones, robots and appliances. Learn how each works, its control method and how to choose between them.

Beyond classic industrial motors lies a family of machines optimised for precision, speed, size or cost. Most are driven by power electronics, so the motor and its controller are designed together.

Choosing the right one — stepper vs servo, brushed vs brushless — is a routine engineering decision in robotics, automation, consumer products and aerospace.

An analogy

A stepper motor is like climbing stairs one exact step at a time; a servo motor is like a ramp with someone constantly checking your position and correcting it.

Stepper motors

A stepper moves a fixed angle for each pulse — 1.8° (200 steps/rev) for a typical hybrid stepper. Open-loop positioning is simple: count pulses. Microstepping drivers shape the phase currents to subdivide each step (e.g. 16 microsteps) for smoother motion.

Brushless DC (BLDC)

A BLDC is a permanent-magnet synchronous motor whose windings are commutated electronically using Hall sensors or back-EMF sensing, instead of brushes. It is efficient, long-lived and quiet, and powers drones, fans, pumps, hard drives and power tools.

Servo motors and closed loop

A servo system is a motor (usually PMSM) + encoder + drive running position, velocity and current control loops. It corrects errors continuously, delivers high peak torque and never 'loses steps' — the choice for CNC machines and industrial robots.

Other special machines

Each trades off cost, robustness and control complexity.

• Universal motor: series-wound, runs on AC or DC, very high speed — vacuum cleaners, blenders, drills.
• Switched reluctance: salient iron rotor, no magnets or windings on rotor — rugged, cheap, but noisy torque ripple.
• Linear motors: an 'unrolled' motor producing direct linear thrust — maglev, wafer steppers, high-speed pick-and-place.
• Shaded-pole and PSC single-phase motors: low-cost fans and small appliances.

Stepper limits: resonance and torque–speed

Stepper torque falls with speed because winding inductance limits how fast current can rise each step. Higher driver voltage (with current control) extends speed. Mid-band resonance can make a stepper stall at certain speeds, which microstepping and damping mitigate. Missed steps go undetected in open loop, so closed-loop 'hybrid servo' steppers add encoders.

Choosing a motor

Key questions: required torque and speed range, positioning accuracy, duty cycle, dynamic response, environment, cost and control complexity. Build a torque–speed envelope of the load (including acceleration torque J·α) and pick a motor whose continuous and peak curves enclose it with margin.

Formulas

• Stepper step angle: θ_s = 360° / (steps per revolution)
• Linear resolution: Δx = lead / (steps/rev × μ)
• BLDC electrical frequency: f_e = (P/2) · rpm / 60
• Kv–Kt relationship (SI): K_t ≈ 9.55 / K_v (N·m/A, K_v in rpm/V)

Worked example: Stepper-driven lead screw

Given: 1.8° stepper (200 steps/rev); 16× microstepping; Lead screw 8 mm/rev; Target speed 50 mm/s

Find: Linear resolution and required step-pulse frequency.

• Microsteps/rev = 200 × 16 = 3200
• Resolution = 8 mm / 3200 = 2.5 µm per microstep
• Speed: 50 / 8 = 6.25 rev/s → 6.25 × 3200 = 20 000 pulses/s (20 kHz)

Answer: 2.5 µm/microstep, 20 kHz step rate

Worked example: BLDC drone motor

Given: K_v = 920 rpm/V; 4S LiPo = 14.8 V nominal; 14 magnet poles

Find: No-load speed, electrical frequency and torque constant.

• No-load speed ≈ 920 × 14.8 = 13 616 rpm
• f_e = (14/2) × 13 616 / 60 = 1589 Hz
• K_t ≈ 9.55 / 920 = 0.0104 N·m/A (10.4 mN·m per amp)

Answer: ≈ 13 600 rpm, 1.59 kHz commutation, K_t ≈ 10.4 mN·m/A

Common mistakes

• Wrong: Choosing a stepper by holding torque alone. — Right: Check torque at the required speed on the pull-out curve — at speed a stepper may deliver a fraction of its holding torque.
• Wrong: Assuming microstepping multiplies accuracy by the same factor. — Right: Microstepping improves smoothness; accuracy is limited by motor tolerance (±5 % of a full step) and load.

Applications

• 3D printers & CNC routers: Steppers with microstepping drivers give low-cost, open-loop positioning for hobby and light industrial machines.
• Drones & e-bikes: BLDC outrunner and hub motors with ESCs/FOC controllers deliver high power density and efficiency.
• Industrial robots: AC servo motors with high-resolution encoders deliver micrometre-level repeatability.

Key takeaways

• Steppers: open-loop, fixed step angle, torque falls with speed.
• BLDC/PMSM: electronically commutated, efficient; Kt ≈ 9.55/Kv.
• Servos: closed-loop, high dynamics, never lose position.
• Select motors from the load's full torque–speed envelope, not holding torque alone.

Going further

• Sensorless BLDC control: At speed, the undriven phase's back-EMF zero-crossing reveals rotor position, so Hall sensors can be dropped. At standstill there is no back-EMF, so drives use open-loop ramp-up or high-frequency injection.
• Cogging and torque ripple: Magnet–slot interaction produces cogging torque; skewing slots or magnets and fractional-slot windings minimise it. Precision gimbals and servo tables need near-zero cogging.
• Halbach arrays and axial-flux motors: Axial-flux machines with Halbach magnet arrays achieve very high torque density — attractive for in-wheel EV motors and electric aircraft.