DC Motors
A brushed DC motor is one of the most useful actuators for learning mechatronics. It converts current into torque and rotation into back EMF. That simple relationship explains starting current, speed control, braking, and driver selection.
Learning Objectives
By the end of this lesson, you should be able to:
- explain torque constant and back EMF;
- read a basic DC motor torque-speed curve;
- estimate stall current and power risk;
- control speed with PWM through a driver;
- choose protection and feedback checks.
Motor Model
A simplified DC motor has winding resistance R, inductance L, back EMF E, and torque proportional to current.
$$
V = IR + L\frac{dI}{dt} + E
$$
$$
E = K_e\omega
$$
$$
\tau = K_t I
$$
At startup, speed is zero, so back EMF is zero. Current can be very high and is limited mainly by winding resistance and the driver.
Worked Example
A 12 V motor has winding resistance 3 ohm.
$$
I_\text{stall}=\frac{V}{R}=\frac{12}{3}=4\ A
$$
The driver and supply must tolerate this during startup or current-limit it. A 1 A driver is not safe just because the motor normally runs at 300 mA.
Practical Checks
- Measure winding resistance with the motor disconnected.
- Estimate stall current.
- Use a current-limited supply for first tests.
- Verify PWM frequency does not cause audible or thermal problems.
- Check driver temperature at load.
- Add encoder or tachometer feedback if speed accuracy matters.
Common Mistakes
- Sizing the driver from no-load current.
- Ignoring stall current and startup surge.
- Driving the motor directly from a GPIO.
- Forgetting inductive current paths in the driver.
- Assuming PWM duty equals exact speed under all loads.
Summary
DC motor torque is controlled by current, while speed produces back EMF. PWM controls average voltage, but load, friction, supply limits, and back EMF determine actual speed.
Further Reading
- Texas Instruments brushed DC motor driver fundamentals.
- Pololu DC motor and gearmotor selection guides.
- Maxon DC motor formulas and torque-speed curves.