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Documentation

Electric motors* 

Summary: DC, BLDC, PMSM, hub motors, torque/speed/efficiency curves.

1) Quick overview and evolution

  • Yesterday: brushed DC motors (easy to control, brush/commutator maintenance, modest efficiency).
  • Today: BLDC/PMSM (brushless) thanks to affordable electronic controllers → higher efficiency, less maintenance, better power density.
  • Hub motor: integrates motor + reducer + sometimes the brake; compact for off-road vehicles/tools.

2) Composition of a BLDC/PMSM and the role of the controller

  • Wound stator (generates rotating field), permanent magnet rotor.
  • Sensors: Hall/encoder (position/speed) according to required precision.
  • Controller (inverter): converts 48–51.2 V DC into controlled AC phases (PWM), manages torque/speed, limits current, protects the motor, provides telemetry.
  • Necessity: impossible to properly operate a BLDC/PMSM without an appropriate controller (curves, limits, ramps).

3) Advantages / disadvantages vs thermal / hydraulic

  • BLDC vs thermal: + High efficiency, instant start, zero local emissions, reduced noise, limited maintenance. − Autonomy linked to battery, electronic management required.
  • BLDC vs hydraulic: + Often better efficiency, control precision, cleanliness. − High instant torque but sensitivity to overheating, IP/sealing to be taken care of.
  • Hydraulic advantage: high torque at very low speed without reducer, shock robustness; circuit maintenance and leakage risks to consider.
  • Complementary BLDC advantages: very fine control (speed, torque, ramps, limits), native telemetry (current, temperature, speed), easy data feedback → simplified robotisation and supervision.

4) Reducers and kinematics (low speed, high torque)

  • At low speed and high torque, use a reducer: it multiplies the torque at the wheel/output shaft and reduces speed.
  • Ratio R ≈ n_motor / n_output. Output torque ≈ Motor torque × R × η_trans.
  • Hub motor case: the reducer supports the load, multiplies and simplifies integration (beware of radial/axial loads and bearing sizing).
  • Power-off brake recommended: mechanical holding in the absence of power (spring brake), released only under tension via electromagnet → safety at stop and on slope.

5) Efficiency and losses → cooling

  • Copper losses (I²R), iron losses (hysteresis/Eddy), mechanical losses (bearings/ventilation), losses in the controller.
  • Typical overall chain efficiency: 0.65–0.9 depending on sizing, transmission quality and operating point.
  • Cooling: forced air or conduction to chassis; avoid prolonged operation beyond nominal continuous current.
  • Controller: generates heat (MOSFET, diodes, DC bus). Provide dedicated dissipation: mount on heatsink/chassis with flat surface and thermal paste, air circulation, beware of IP which reduces evacuation. Consider thermal derating and avoid mounting on insulating surfaces (foam) without thermal path.

6) Torque/speed curve (simplified BLDC case)

  • Quasi-constant torque zone (current limited): C ≈ C_nom up to a transition speed.
  • Quasi-constant power zone: C decreases as speed increases (available voltage / EMF).
  • Practical application: choose the reduction ratio to stay in an efficient zone (sufficient torque without overcurrent), especially in start-up/slope.

8) Maintenance and quick diagnosis

  • Maintenance: check connectors, tightness, cables, cleanliness; monitor abnormal noises, overheating, mechanical play.
  • Diagnosis: via controller (fault codes, temperature, current), multimeter/clamp meter; test Hall/encoder sensors if synchronization losses.

Quick check-list

  • Reduction ratio defined based on need (C, n)
  • Continuous/peak current compatible with motor + controller
  • Cooling and IP compliant with field use
  • Cables/connections tight and suitable for current


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*: The technical information presented in this article is provided for informational purposes only. It does not replace the official manuals of the manufacturers. Before any installation, handling or use, please consult the product documentation and follow the safety instructions. Torque.works cannot be held responsible for inappropriate use or incorrect interpretation of the information provided.