Sensorless BLDC Motor Speed Controller using Back-EMF

BLDC motors dominate drones, EVs, pumps and appliances because they combine DC-like control with AC-motor robustness — but they cannot run on plain DC: the stator windings must be energized in a rotating sequence synchronized to the rotor position. The usual answer is Hall-effect sensors inside the motor; the sensorless answer is to exploit the motor itself as the sensor. As the rotor's magnets sweep past the un-energized winding, they induce a back-EMF whose zero-crossing marks the exact commutation instant. Sensorless control reads this signal, filters it, and commutates from it — cheaper, more robust, and a genuinely instructive control problem because the scheme must start the motor blind (no back-EMF at standstill) and hand over to closed-loop sensing once spinning. This project implements that full cycle on an Arduino driving a 3-phase bridge.

1. The 3-phase MOSFET bridge energizes two of the three motor windings at a time in the six-step sequence, leaving the third floating.
2. The back-EMF induced in the floating winding is divided down and compared against a virtual neutral point built from the three phases.
3. Each zero-crossing of this signal marks 30 electrical degrees before the ideal commutation instant; a timer schedules the next step from it.
4. At standstill there is no back-EMF, so the firmware first runs a timed open-loop ramp, increasing commutation frequency until the motor spins fast enough for reliable zero-cross detection.
5. Control then hands over to closed-loop sensorless commutation, with the potentiometer setting the PWM duty cycle (speed).
6. Commutation frequency is converted to RPM and displayed on the LCD with the duty cycle and direction.
7. Overcurrent or loss of commutation feedback triggers shutdown with a fault indication until reset.

Tech stack:

• Arduino Uno/Nano (ATmega328P)
• 3-phase MOSFET bridge with gate drivers
• Back-EMF sensing network (dividers + virtual neutral)
• Comparator/ADC zero-cross detection in firmware
• Current sensing for overcurrent protection
• 16x2 character LCD
• Arduino IDE (C/C++ firmware)
• Sensorless BLDC test motor with coupled demo load

• [Back-EMF zero-crossing commutation] The floating phase's back-EMF is compared against a virtual neutral; each zero-crossing schedules the next commutation — true sensorless operation with no Hall sensors.
• [Open-loop start-up ramp] From standstill the firmware runs a timed open-loop commutation ramp that spins the motor up until back-EMF is strong enough for closed-loop handover, demonstrating the classic sensorless start problem.
• [PWM speed control] A potentiometer sets the duty cycle applied to the bridge, giving smooth speed control across the motor's range.
• [Direction control] A switch reverses the commutation sequence for bidirectional running.
• [Overcurrent limiting] Phase current sensing folds back or trips the drive on overcurrent, protecting the MOSFETs during stalls.
• [RPM display on LCD] Speed is computed from the commutation frequency and shown on a 16x2 LCD alongside duty cycle and running state.
• [Stall detection] If commutation feedback is lost (locked rotor), the drive shuts down instead of cooking the windings.

• Working sensorless BLDC controller prototype (Arduino, 3-phase bridge, sensing network, LCD, test motor)
• Complete firmware source code (start-up ramp, zero-cross commutation, PWM, protections, LCD)
• Circuit and wiring documentation with bridge and sensing design notes
• Component list with voltage/current ratings
• Tuning procedure (buyer-run: set ramp profile, verify handover, calibrate RPM display)
• Project report PDF (BLDC theory, sensorless principles, methodology, test procedure)
• PPT presentation for final review
• Viva Q&A preparation document (back-EMF, six-step commutation, virtual neutral, sensorless start problem)
• Setup and demonstration guide

• This is an academic demonstration drive at modest power; it is not an EV or industrial servo drive.
• Start-up reliability, speed range and handover behavior are design behaviors tuned by the buyer's own procedure — the drive ships with default constants, not measured performance.
• Sensorless control has weak low-speed torque by principle; precise low-speed positioning needs sensored control, noted as a documented limitation.
• The exact RPM and current figures depend on the motor and supply used; the report gives the measurement method rather than pre-claimed numbers.
• Sustained stall or overload needs the thermal and current notes followed; the MOSFETs need heatsinking.