# Aircraft Electrical Motors and Control Principles

> Principles of DC motors, torque, back EMF, and failure modes (seizure, runaway).

- **Subject:** 021 — Airframe, Systems, Electrics, Powerplant
- **Section:** 021-09 — Electrics
- **Topic code:** 021-09-05
- **Last reviewed:** 2026-03-24
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/airframe-systems-electrics-powerplant/electrics/electrical-motors/

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## 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.*

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.