Aircraft Electrical Motors and Control Principles

021-09-05Pablo Asensio Martínez2026-03-242 min

Basic Principles

  • Function: Convert electrical energy into mechanical energy (rotation).
  • Back EMF: A voltage generated by the spinning rotor that opposes the supplied voltage.
    • Acts as a "resistance" to current flow.
    • Proportional to speed (RPM).
    • High Speed $\rightarrow$ High Back EMF $\rightarrow$ Low Current.
    • Low Speed/Stopped $\rightarrow$ Low/No Back EMF $\rightarrow$ High Current.

Torque and Power Relationships

  • Power ($P$):
    • Electrical: $P = \text{Voltage} (V) \times \text{Current} (I)$
    • Mechanical: $P = \text{Torque} (T) \times \text{RPM}$
  • Torque ($T$): The turning force.
    • $T \propto \text{Current} \times \text{Magnetic Field Strength}$
    • From power equations: $T = (V \times I) / \text{RPM}$
    • High Current $\rightarrow$ High Torque.

Operational Changes (DC Motor)

  • Increasing Mechanical Load:
    1. Motor slows down (RPM decreases).
    2. Back EMF decreases (less opposition).
    3. Current Increases.
    4. Torque Increases (to match the load).
    5. Power output increases.

Failure Modes

Rotor Seizure (Stalled Motor)

  • Condition: The rotor is physically prevented from turning (RPM = 0).
  • Back EMF: Zero (no motion).
  • Current: Increases massively (maximum possible current flows).
  • Torque: Increases massively (maximum starting torque).
  • Risk: Overheating, melting wires/fuses, fire.

Rotor Runaway (No Load)

  • Condition: The mechanical load is suddenly removed (e.g., shaft break).
  • Response: Motor speeds up rapidly.
  • Back EMF: Increases significantly.
  • Current: Decreases (due to high Back EMF).
  • Torque: Decreases.
  • Risk: Overspeed/mechanical destruction (if series-wound).

Starter-Generators

  • Definition: A single unit acting as a starter motor (to start the engine) and then as a generator (once the engine is running).
  • Power Sources for Driving the Generator (When acting as a generator):
    • Running Engine
    • APU
    • RAT (Ram Air Turbine)
    • Hydraulic Motor
  • Power Sources for Driving the Starter (When acting as a motor):
    • Internal Battery
    • Ground Power Unit (GPU)
    • Note: RAT cannot power the starter.