Motor Control Design Guide
Brushed, BLDC, Stepper, Servo, H-Bridge & PID Control
Motor Control Design Guide
The complete engineering reference for Brushed DC, BLDC, Stepper, and Servo motor control. Covers H-Bridge topologies, PWM strategies, PID tuning, BEMF sensing, and torque-speed analysis — with worked examples you can apply immediately.
/calculators/. Bookmark this page.Quick Answer: How Does Motor Control Work?
Motor control regulates speed, torque, position, and direction by manipulating voltage, current, and commutation timing:
- Brushed DC — simplest. DC voltage; speed ≈ proportional to voltage. PWM for variable speed. Used in power tools, toys, automotive.
- BLDC — electronic commutation via 6-step or FOC. Needs rotor feedback (Hall or sensorless BEMF). Used in drones, e-bikes, HVAC.
- Stepper — discrete angular steps, open-loop positioning. Used in 3D printers, CNC, medical pumps.
- Servo — closed-loop with encoder + PID. Used in robotics, industrial automation.
Universal building blocks: power stage (H-Bridge or 3-phase inverter), modulation (PWM, SVPWM), feedback (current sense, encoder, BEMF), control loop (PID or cascaded PI).
Core Formulas, Worked Example & Parameter Table
Fundamental Motor Equations
E = Ke × ω … (2) Back-EMF [V]
V = I × R + E … (3) Terminal voltage
ω = (V − I × R) / Ke … (4) Speed under load; no-load: ω₀ ≈ V / Ke
Pmech = T × ω … (5) Mechanical power [W]
η = Pmech / (V × I) … (6) Efficiency
Note: In SI, Kt ≈ Ke when Ke in V·s/rad.
PWM Speed Control (Brushed DC)
ω ≈ (D × Vbus − I × R) / Ke … (8) Speed under load
Pdiss = I² × Rds(on) × 2 … (9) H-Bridge conduction loss
BLDC Commutation Timing (6-Step)
Tcomm = 60 / (RPM × 6) … (11) Time between commutations [s]
Stepper Step Angle
RPM = (θstep × fpulse × 60) / 360 … (13) Speed from pulse rate
Common: 1.8° (200 steps/rev).
PID Control Law (Discrete)
Worked Example: 24V BLDC Ceiling Fan
Specs: Ke=0.012 V/rpm, Rphase=0.8 Ω, 8 poles, target 1200 RPM, load 0.15 N·m.
Step 1 — Required current:
I = T / Kt = 0.15 / 0.1146 ≈ 1.31 A
Step 2 — Bus voltage (trapezoidal):
E = Ke × ω = 0.1146 × 125.66 ≈ 14.4 V (L-N BEMF peak)
Vbus ≥ 2×(E + I×R) = 2×(14.4 + 1.31×0.8) = 30.9 V → 36 V bus recommended
For 24 V bus, use FOC+SVPWM (~1.15× margin).
Step 3 — PWM frequency:
Rule: fPWM ≥ 20×felec → ≥ 1600 Hz → choose 20 kHz (above audible).
Step 4 — Loss (Rds(on)=15 mΩ @ 100°C):
Psw ≈ 0.5×36×1.31×50e-9×20000 ≈ 0.024 W/FET → negligible
Key Parameter Reference
| Parameter | Symbol | Typical Range | Motor Type | Notes |
|---|---|---|---|---|
| Torque constant | Kt | 0.01–2 N·m/A | All | Larger for high-torque low-speed |
| Voltage constant | Ke | 0.001–0.2 V/rpm | BLDC/Brushed | Ke≈Kt in SI |
| Phase resistance | R | 0.1–50 Ω | All | Low R = efficient but high stall current |
| PWM frequency | fPWM | 8–50 kHz | Brushed/BLDC | >16 kHz avoids audible whine |
| Step angle | θstep | 0.9°–7.5° | Stepper | 1.8° (200 steps/rev) most common |
| PID Kp | — | 0.01–10 | Servo/BLDC | Start low, increase till oscillation, halve |
| PID Ki | — | 0.1–100 s⁻¹ | Servo/BLDC | Eliminates steady-state error |
| PID Kd | — | 0.0001–0.1 s | Servo/BLDC | Damps overshoot; use LPF |
| Current loop BW | — | 500–5000 Hz | BLDC(FOC) | ~10× slower than PWM |
| Speed loop BW | — | 50–500 Hz | BLDC(FOC) | 5–10× slower than current loop |
Common Mistakes in Motor Control Design
1. Forgetting Flyback / Recirculation Diodes
2. PWM Frequency Too Low
Many beginners use 500 Hz “because the example code did.” At audio frequencies, windings act as loudspeakers — irritating whine. Always use fPWM ≥ 16 kHz (brushed) / ≥ 20 kHz (BLDC). Bonus: higher PWM reduces current and torque ripple.
3. Ignoring Dead Time in Half-Bridges
In any half-bridge (two FETs between Vbus and GND), both FETs must never conduct simultaneously — this is shoot-through, a dead short that destroys the FETs. Insert 100–500 ns dead time. Most modern gate drivers (TI DRV, Infineon EiceDRIVER) provide programmable dead time.
4. Sensorless BLDC Startup Without Alignment
BEMF sensing only works when the rotor is spinning. At standstill there is zero BEMF — applying commutation blindly causes vibration or reverse rotation. Fix: alignment stage — energize two phases with limited current to lock rotor, then ramp open-loop before switching to closed-loop.
5. Integral Windup Without Anti-Windup
When the motor saturates (hits current/speed limit), the integrator keeps accumulating error while output is clamped. When the setpoint becomes reachable, the wound-up integrator causes massive overshoot. Always implement clamping anti-windup: freeze or back-calculate the integrator during saturation.
6. Overlooking Fast Over-Current Protection
Stall current = V/R can be 5–20× rated current. Without cycle-by-cycle protection, the power stage fails in microseconds. Use low-side shunt + op-amp + comparator, or dedicated current-sense amps (INA, ACS). Set hardware trip at 1.5–2× rated peak current.
7. Bad DC Bus Capacitor Placement
The DC bus capacitor supplies high-frequency current pulses that wiring cannot deliver. Place a low-ESR electrolytic (100–1000 µF) + ceramic (0.1–1 µF) as close as possible to MOSFET drains. Minimize the Vbus→HS FET→LS FET→GND loop area to cut parasitic inductance and EMI.
Frequently Asked Questions
Related Calculators
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