True DC motor torque control relies on managing armature current (for brushed motors) or phase current (for brushless motors), not just adjusting voltage. While a basic PWM voltage chopper can regulate speed, it cannot guarantee consistent force against a varying mechanical load. To achieve precise torque control, you must pair a motor with a current-regulated driver and size the system based on continuous thermal limits rather than peak stall ratings.
Sizing Rule of Thumb and Worked Load Example
The most common mistake in motor selection is sizing based on peak stall torque. Stall torque is only available for a fraction of a second before the windings overheat. For continuous operation, use this rule of thumb: Size the motor’s continuous rated torque at 1.5x to 2.0x the calculated continuous load torque. This margin accounts for startup inertia, ambient temperature derating, and unexpected friction spikes.
Assume a conveyor requires 2.5 Nm of continuous torque to move the payload, and belt friction adds 0.5 Nm. Your total continuous load torque is 3.0 Nm.
Target Motor Torque: 3.0 Nm × 1.5 = 4.5 Nm continuous rating.
Current Calculation: If you select a 24V BLDC motor with a torque constant ($K_t$) of 0.15 Nm/A, the continuous current required is 4.5 Nm / 0.15 Nm/A = 30A.
Driver Sizing: Your driver must handle 30A continuous RMS current, and ideally 60A peak current for acceleration phases. If your ambient temperature exceeds 40°C, apply a 10% thermal derating factor, pushing your required continuous driver capacity to 33A.
Motor Type Comparison: Matching the Load Profile
Selecting the right motor architecture dictates your control complexity and cost. Stepper motors and closed-loop servos are fundamentally different architectures and are not interchangeable; steppers rely on open-loop magnetic detents (unless fitted with an external encoder), while servos use continuous closed-loop feedback. Here is how the primary DC motor types compare for torque-critical applications in 2026.
| Motor Type | Torque Curve | Control Needs | Typical Cost (50W class) | Best Load Profile |
|---|---|---|---|---|
| Brushed DC (BDC) | Linear drop from stall to no-load. | H-bridge with inline current shunt sensing. | $15 - $35 | High starting torque, low-speed winches, simple actuators. |
| Brushless DC (BLDC) | Flat continuous region, sharp drop near base speed. | 3-phase inverter with FOC (Field Oriented Control). | $60 - $120 | Continuous duty, high-efficiency conveyors, robotics joints. |
| Stepper | High holding torque, rapid drop-off at speed. | Chopper driver (open-loop) or closed-loop stepper driver. | $25 - $50 | Low-speed precision positioning, 3D printer axes. |
| AC/DC Servo | Constant torque up to rated speed, constant power above. | Dedicated servo drive with high-res absolute encoder. | $250 - $600+ | High-dynamic CNC axes, industrial pick-and-place. |
Wiring, Terminals, and Driver Demands
Implementing brushless DC motor control requires precise terminal identification. Miswiring phase or sensor lines will result in immediate driver faults or erratic torque output.
Brushed DC Terminal Identification
- A1 / A2 (Armature): The main power terminals connected to the rotor windings. Reversing these reverses direction.
- F1 / F2 (Field): Present only on wound-field motors (not permanent magnet). Used to control the magnetic flux. For torque control, the field current is usually kept constant while armature current is varied.
BLDC Terminal and Sensor Identification
- U, V, W (Phases): The three main power phases. The driver must commutate these in a specific sequence based on rotor position.
- Hall Sensors (H1, H2, H3, VCC, GND): Provide coarse rotor position. Gotcha: Hall sensors are typically powered by 5V. Feeding them 12V or 24V will instantly destroy the internal ICs. Always verify the motor datasheet before applying VCC.
For true torque control on a BLDC motor, basic trapezoidal commutation is insufficient due to torque ripple. You must use a driver capable of Field Oriented Control (FOC), such as an ODrive or a board running SimpleFOC. FOC uses Clarke and Park transforms to decouple the flux-producing current ($I_d$) from the torque-producing current ($I_q$), allowing you to command exact torque values regardless of rotor angle.
Diagnosing Failure Signatures: Hum, Overheat, and Stall
When a torque-controlled system fails, the electrical and mechanical symptoms provide a direct map to the root cause. Use your multimeter and thermal camera to diagnose these three primary failure modes.
Cause: Commutation failure or mechanical binding. In BLDC motors, this is almost always caused by miswired Hall sensors or an incorrect pole-pair configuration in the driver firmware. The driver is energizing the wrong phase for the current rotor position.
Fix: Disconnect power. Measure the resistance between U-V, V-W, and U-W. They should be identical (typically < 1 ohm). If electrical continuity is good, swap two Hall sensor signal wires and re-run the driver's auto-calibration routine.
Cause: Continuous RMS current exceeds the motor's thermal dissipation limit. This often happens when a load requires high holding torque at zero speed. Without back-EMF to limit current, all electrical energy converts to $I^2R$ heat in the windings.
Fix: Check your driver's continuous current limit settings. If the application requires high holding torque, you must either add forced air cooling, switch to a larger motor frame, or implement a mechanical brake to hold the load at rest.
Cause: The load torque has exceeded the motor's breakdown torque. When the rotor stops, back-EMF drops to zero. The current spikes to $V_{supply} / R_{winding}$, which is often 10x the continuous rating, triggering the driver's hardware overcurrent protection.
Fix: Review your mechanical gear reduction. Adding a 10:1 planetary gearbox multiplies your output torque by 10 (minus efficiency losses) while reducing the reflected inertia, preventing the motor from stalling under peak loads.
FAQ: DC Motor Torque Control
How do you maintain constant torque at low RPM with a DC motor?
At low RPM, a DC motor generates very little back-EMF, meaning the driver must actively limit the voltage to prevent current (and therefore torque) from spiking. To maintain constant torque, use a closed-loop current controller (like an FOC algorithm) that measures phase current via shunt resistors thousands of times per second and adjusts the PWM duty cycle to maintain the exact target $I_q$ current, regardless of how slow the rotor is turning.
Why does my BLDC motor lose torque control when using basic PWM voltage?
Basic PWM only controls the average voltage applied to the motor. Because motor torque is strictly proportional to current ($T = K_t imes I$), and current is dictated by the difference between supply voltage and back-EMF divided by winding resistance, a voltage-only command cannot account for varying mechanical loads. If the load increases, the motor slows down, back-EMF drops, and current spikes unpredictably. You must use a current-regulated driver for true torque control.
Can I use a stepper motor driver for closed-loop DC torque control?
No. Stepper drivers (like the TMC2209 or A4988) are designed to sequence current through two distinct phases (A and B) to create discrete magnetic detents. They lack the 3-phase inverter topology required to drive a BLDC motor's U, V, and W terminals, and they do not compute the sinusoidal commutation angles required for smooth DC torque production. Always match the driver topology to the motor phase count.
