A dc simple motor (technically known as a brushed DC or BDC motor) remains the undisputed workhorse for low-cost, high-torque actuation in DIY and prototyping environments. If you need to spin a conveyor, drive a winch, or move a robot chassis without implementing complex field-oriented control (FOC) logic, a brushed DC motor is your baseline. The direct answer for selection is this: size the motor by its stall torque and continuous current rating, not by its no-load RPM. No-load specs only tell you how fast the motor spins when doing zero work.
Motor Type Comparison: Where the DC Simple Motor Fits
Before committing to a brushed design, you must understand where it sits on the torque-speed curve compared to modern alternatives. The table below maps the four most common hobbyist and light-industrial motor types against their control requirements and ideal load profiles.
| Motor Type | Torque Curve Profile | Control Complexity | Typical Cost (Unit) | Best Load Profile |
|---|---|---|---|---|
| Brushed DC (Simple) | Linear drop: Max torque at 0 RPM (stall), drops to 0 at no-load speed. | Low: Simple H-bridge or single MOSFET. PWM for speed. | $3 - $15 | Winches, basic actuators, toys, high-starting-torque friction loads. |
| BLDC (Outrunner/Inrunner) | Flat constant torque to base speed, then constant power drop-off. | High: 3-phase ESC, requires Hall sensors or sensorless back-EMF zero-crossing. | $15 - $60 | Drones, e-bikes, high-speed spindles, continuous high-efficiency drives. |
| Stepper (Bipolar) | High holding torque at 0 RPM, drops off sharply and non-linearly with speed. | Medium: Step/direction pulse driver (e.g., TMC2209) with microstepping. | $8 - $25 | 3D printers, CNC routers, precise open-loop positioning. |
| Coreless DC | Similar linear drop to BDC, but extremely low rotor inertia allows instant acceleration. | Low: Same as BDC, but requires ultra-fast current limiting to prevent demagnetization. | $20 - $80 | Medical devices, RC servos, haptic feedback, high-dynamic positioning. |
Sizing Rule of Thumb and Worked Load Example
The golden rule for sizing a dc simple motor is the "50% Stall, 80% Continuous" framework. Your expected operating load should never exceed 50% of the motor's rated stall torque, and your continuous current draw should stay below 80% of the motor's thermal limit. This provides the necessary overhead to overcome static friction (stiction) during startup without overheating the windings.
Worked Example: Sizing a Winch Motor
Let’s size a motor to lift a 5 kg mass using a winch drum with a 20 mm (0.02 m) radius, coupled through a 10:1 spur gearbox with 80% efficiency.
- Calculate Load Force: F = m × g = 5 kg × 9.81 m/s² = 49.05 N.
- Calculate Load Torque at Drum: τ = F × r = 49.05 N × 0.02 m = 0.981 N·m.
- Reflect Torque to Motor Shaft: Account for the gearbox ratio and efficiency. Motor Torque = 0.981 / (10 × 0.80) = 0.122 N·m continuous requirement.
- Apply the 50% Rule: Target a motor with a stall torque of at least 0.122 × 2 = 0.244 N·m.
- Calculate Current: If the selected motor has a torque constant (Kt) of 0.05 N·m/A, the continuous current is 0.122 / 0.05 = 2.44 A. The stall current will be roughly double (4.88 A).
Based on this math, you would select a 12V brushed gearmotor rated for at least 0.25 N·m stall torque and 3A continuous current, paired with a driver capable of handling the 5A stall spike.
Wiring and Terminal Identification
A standard dc simple motor features a simple 2-terminal interface. Terminal 1 (typically Red or marked '+') and Terminal 2 (Black or '-') connect directly to the carbon brushes. Reversing the polarity reverses the commutation sequence, instantly reversing the motor's direction.
Warning: If your motor has a third terminal, do not assume it is a ground. On many precision BDC motors, the third pin is an integrated tachometer (hall effect) output or a center-tap for a specific voltage winding. Applying full bus voltage to a 3.3V or 5V tachometer pin will instantly destroy the internal sensor.
Drive Electronics: H-Bridges, PWM, and Protection
Because a dc simple motor requires bidirectional control and speed regulation, you cannot simply use a mechanical relay. You need an H-bridge circuit. For unidirectional applications (like a simple fan or pump), a single logic-level N-channel MOSFET on the low side is sufficient.
For bidirectional control, select your driver based on your stall current calculation:
- Low Current (< 3.6A): The TI DRV8871 is a robust, integrated H-bridge. It includes internal protection against overcurrent and thermal shutdown, making it ideal for ESP32 and Arduino projects where board space is tight.
- High Current (10A - 30A): The BTS7960 module is a ubiquitous, low-cost hobbyist favorite. It uses two half-bridges to handle massive peak currents (up to 43A). However, bench experience dictates you must add adequate dead-time in your software to prevent shoot-through (both high and low MOSFETs turning on simultaneously), which will short your power supply and vaporize the driver.
The Flyback Diode Mandate
A brushed DC motor is essentially a massive, spinning inductor. When your H-bridge turns off the PWM signal, the collapsing magnetic field induces a massive reverse voltage spike ($V = L \frac{di}{dt}$). If your driver IC does not have internal clamping diodes (the DRV8871 does; raw MOSFET bridges often do not), you must install external Schottky flyback diodes across the motor terminals and to the power rails. Skipping this is the number one cause of bricked microcontrollers and melted H-bridges on the workbench.
Additionally, solder a 100nF ceramic capacitor directly across the motor's two terminals. This suppresses the high-frequency EMI generated by the carbon brushes arcing against the commutator, which can otherwise cause brownouts or reset your nearby microcontroller.
Failure Signatures: Diagnosing Hum, Overheat, and Stall
Brushed motors fail in highly predictable ways. Recognizing these signatures early will save your mechanical assembly and your drive electronics.
1. The "Hum" Without Rotation
Symptom: The motor emits a low-frequency buzz or hum, draws massive current, but the shaft does not turn.
Cause: Static friction (stiction) in your mechanical load exceeds the motor's starting torque, or the voltage is too low to overcome the cogging torque.
Fix: Do not leave it energized; it will overheat in seconds. Increase the bus voltage, reduce the mechanical bind, or implement a "kick-start" routine in your code that applies 100% PWM for 50 milliseconds before dropping to your target speed.
2. Overheat and the "Sweet" Smell
Symptom: The motor casing becomes too hot to touch, followed by the distinct smell of burning sugar or varnish.
Cause: You are running the motor above its continuous current rating. The thermal mass of the copper armature absorbs the $I^2R$ heat until the enamel insulation on the windings breaks down (typically around 150°C to 180°C for Class F/H insulation). Once the enamel melts, adjacent copper wires short together, reducing the coil's resistance, drawing even more current, and creating a thermal runaway loop.
Fix: Check your gearbox ratio. If you are gearing down heavily, the motor might be spinning fast but the reflected inertia is stalling it under load. Add active cooling or step up to a larger frame size.
3. Visible Sparking and Intermittent Stalling
Symptom: You can see blue/orange arcs through the motor's ventilation slots, and the motor stutters under load.
Cause: Carbon brush wear. The brushes are consumable friction items. As they wear down, the internal springs lose tension. The brush then "bounces" on the spinning commutator at high RPM, causing massive arcing, pitting the copper segments, and generating severe electromagnetic interference.
Fix: The motor has reached the end of its mechanical lifespan. Replace the motor. If this is a critical application, switch to a Brushless DC (BLDC) motor which eliminates the physical commutator entirely.
For a deeper dive into the physics of brushed commutation and thermal modeling, the All About Circuits DC Motor chapter provides excellent foundational theory on back-EMF and armature reaction.
