A brushless DC motor relies on an electronic controller (driver) to perform “electronic commutation”—the process of energizing stator coils in sequence to create a rotating magnetic field that pulls the rotor. To do this, the controller must know the current position of the rotor (which is a permanent magnet).
“Sensored” and “Sensorless” systems are the two fundamentally different technical solutions to answer this critical question: “How do we know the rotor’s position?”
1. Sensored Brushless Motors
A sensored system uses physical position sensors mounted inside the motor to get a direct, accurate reading of the rotor’s position.
How it Works: Hall Effect Sensors
The most common method is to embed three Hall-effect sensors within the stator. As the rotor’s magnets (North or South poles) pass over these sensors, they output a digital HIGH or LOW signal.
By reading the combined pattern of these three sensors, the driver knows exactly which electrical sector the rotor is in at all times—even when the motor is at a complete standstill. This allows the driver to energize the correct coils at the perfect time.
Core Advantages
- Excellent Startup Performance: This is the greatest strength of a sensored system. By knowing the rotor position at 0 RPM, the motor can start smoothly and powerfully, generating 100% of its available torque instantly.
- Precise Low-Speed Control: Even at extremely low speeds, the motor maintains smooth, precise control without “cogging” or “stuttering.”
- High Starting Torque: Ideal for applications that must overcome significant static friction or start under a heavy load.
Limitations
- Cost and Complexity: The motor requires extra sensors and more wires (typically 3 motor phases + 5 sensor wires = 8 wires).
- Reliability: The sensors are additional electronic components. They represent a potential point of failure in harsh environments with high temperatures, vibration, or dust.
2. Sensorless Brushless Motors
As the name implies, a sensorless system has no position sensors in the motor body. It uses a more clever (but also more challenging) physical principle to “calculate” the rotor’s position.
How it Works: Back-EMF
As the motor’s rotor (a permanent magnet) spins, it “induces” a voltage in the stator coils that are not currently energized. This voltage is called Back-Electromotive Force.
The voltage of this Back-EMF is directly proportional to the motor’s speed. More importantly, the driver can monitor this Back-EMF signal (specifically, when it crosses zero) to indirectly calculate the rotor’s position and speed.
Core Advantages
- Low Cost and Simplicity: The motor has fewer wires (only the 3 phase wires) and a simpler construction.
- High Reliability (Robustness): With no delicate sensors to fail, sensorless motors are extremely robust and ideal for harsh industrial environments with heat, oil, dust, or high vibration.
- High-Speed Performance: At high speeds, the Back-EMF signal is strong and clear, making sensorless control very efficient and stable.
Limitations
- The Startup Problem: This is the greatest weakness of sensorless systems.
- At standstill (0 RPM), there is no rotation, and therefore zero Back-EMF.
- The driver is “blind” at startup; it doesn’t know where the rotor is.
- To start, the driver must “guess” a position and “force” current in an “open-loop” sequence. This causes the motor to “jerk” or “vibrate” for a moment before it can “lock on” to the rotor and begin generating Back-EMF, at which point it switches to closed-loop operation.
- No Low-Speed Performance: At very low speeds, the Back-EMF signal is extremely weak and easily lost in electrical noise. This prevents the driver from accurately determining the position. Therefore, sensorless systems have a minimum operating speed and cannot provide smooth low-speed control or hold a position at zero speed.
3. Core Comparison: Sensored vs. Sensorless
| Feature | Sensored Brushless Motor | Sensorless Brushless Motor |
| Position Detection | Direct (via Hall Sensors) | Indirect (Calculated from Back-EMF) |
| Startup Performance | Smooth, high-torque | Jerk or “jolt” on startup |
| Zero-Speed Torque | Yes (Can hold 100% torque) | No (Cannot hold torque at 0 RPM) |
| Low-Speed Control | Excellent (Smooth, precise) | Poor (Has a minimum speed limit) |
| Reliability | Good (Sensors can fail) | Excellent (Robust construction) |
| Cost | Higher (Sensors + extra wiring) | Lower (Simpler construction) |
| Wiring | Complex (Often 8 wires) | Simple (Only 3 phase wires) |
Conclusion: How to Choose for Your Application
The choice between sensored and sensorless depends entirely on your application’s requirements.
1. Scenarios That Require “Sensored” Motors:
- Robotics & Servo Applications: Require precise position control, smooth low-speed motion, and zero-speed holding (e.g., robotic arms, camera gimbals).
- Heavy-Load Starts: Need high starting torque (e.g., electric vehicles, e-bikes, loaded AGVs).
- Variable Loads: When the load at startup is unpredictable (e.g., conveyor belts).
2. Scenarios That Can Use “Sensorless” Motors:
- Continuous High-Speed Operation: Applications that, once started, run at a consistent high speed (e.g., fans, blowers, pumps).
- Harsh Environments: The application involves dust, oil, water, or high temperatures/vibration.
- Cost-Sensitive: Applications where cost is a primary driver and low-speed performance is not needed (e.g., RC models, drones).