Have you ever wondered how the brushed DC motors powering everyday devices can transform into reliable generators? This exploration reveals the technical nuances of utilizing brushed DC motors for energy conversion.

Engineers have long recognized that both brushed and brushless DC (BLDC) motors possess inherent generator capabilities. Brushed DC motors excel in applications requiring direct current output, while BLDC motors are better suited for alternating current generation. Converting BLDC output to DC requires additional rectification circuitry, whereas brushed motors need DC-to-AC conversion electronics for alternating current production. This analysis focuses on the fundamental relationships governing brushed DC motors in generator mode—particularly the interplay between rotational speed, voltage, torque, and current.

When a motor's rotor spins within a magnetic field, electromagnetic forces induce voltage across the windings—a phenomenon called back electromotive force (Back EMF). The back EMF constant (Kᴇ), typically measured in millivolts per RPM, serves as a critical specification. The induced voltage (Uᵢ) relates proportionally to angular velocity (ω) through the equation:

Uᵢ = Kᴇ × ω

In generator operation, an external power source rotates the motor shaft, causing rotor coils to cut through sinusoidal magnetic flux. Each coil turn generates sinusoidal voltage proportional to speed and flux density. A single-turn coil produces pure sine waves with periods matching electrical cycles.

Brushed DC motors typically feature rotors with odd-numbered coil segments (e.g., 3, 5, 7) powered via carbon brushes. During generation, back EMF appears at output terminals with voltage ripple usually below 5% of total output—a consequence of the segmented coil design.

The back EMF constant dictates voltage output relative to shaft speed. When existing Kᴇ values prove insufficient, gear reducers can increase effective RPM—provided they respect maximum speed limitations. Motor selection must account for both thermal and mechanical constraints, particularly maximum continuous torque and speed ratings.

Unloaded generators produce terminal voltage (Uᵢ) directly proportional to angular velocity with zero current flow. Introducing load resistance (R _Load ) creates current (I _Load ), causing voltage drop per the equation:

U _T = Uᵢ − (I _Load × R _Rotor )

where R _Rotor represents internal winding resistance. At fixed speeds, increasing load current progressively reduces terminal voltage until back EMF equals resistive drop—theoretically reaching zero voltage at maximum current:

I _Max = Uᵢ / R _Rotor

Maximum power output occurs when terminal voltage equals half of Uᵢ and load current reaches half of I _Max :

P _Max = (Uᵢ × I _Max ) / 4

However, practical generator designs should target actual power requirements rather than theoretical maxima, often necessitating motors with higher ratings. System efficiency is calculated as:

η = P _Actual / P _Mechanical

With a 1.17 mV/RPM back EMF constant, this motor generates 5.85V at 5,000 RPM. Its 8.3Ω winding resistance permits 0.70A maximum current—exceeding the 0.55A continuous rating. Intermittent operation may tolerate this excess, but continuous use requires load resistors exceeding 3Ω.

This 0.70 mV/RPM unit produces 7.0V at 10,000 RPM. Its 14.9Ω resistance limits short-circuit current to 0.47A—safely below the continuous rating—making it suitable for direct generator applications without supplemental resistors.

Peak efficiency often occurs below maximum power output. The 16C18 demonstrates highest efficiency at moderate currents, with approximately 50% efficiency at full output. Optimal generator operation requires balancing electrical and mechanical parameters—a process where experienced application engineers provide valuable guidance for specialized uses like tachometer generators or energy harvesting systems.