BLDC Motor Drives

Brushless DC (BLDC) motors are electronically commutated motors that combine the high efficiency and low maintenance of AC motors with the excellent speed-torque characteristics of DC motors. Unlike conventional DC motors, they have no mechanical brushes and the commutation is done by power electronic switches controlled by rotor position sensors. BLDC motors are widely used in electric vehicles, drones, home appliances, and industrial automation, making them a key topic in power electronics drives.

Core Concept of BLDC Motor Operation

A BLDC motor has a permanent magnet rotor and a wound stator with three-phase windings. The rotor magnets produce a fixed magnetic field. To generate continuous rotation, the stator windings must be energized in a sequence that always creates a stator field ahead of the rotor field, producing a net torque that pulls the rotor forward. In a conventional DC motor, this sequence is maintained mechanically by brushes and a commutator. In a BLDC motor, it is done electronically.

The position of the rotor is detected using three Hall effect sensors mounted 120 degrees apart inside the motor. As the permanent magnet rotor rotates, the Hall sensors produce a 3-bit binary code (H1, H2, H3) that changes six times per electrical revolution. Each 3-bit code uniquely identifies the rotor position in 60-degree sectors. The controller uses this code to switch on the appropriate pair of transistors in the three-phase inverter, energizing two stator phases at a time. This is called 6-step commutation.

The back-EMF waveform of a BLDC motor is trapezoidal, unlike the sinusoidal back-EMF of a PMSM (Permanent Magnet Synchronous Motor). The flat top of the trapezoidal back-EMF coincides with the 60-degree conduction window of each phase, allowing maximum power transfer when the phase current and back-EMF are both at their peak flat portions. This is the physical reason why 6-step commutation is the natural and efficient drive method for BLDC motors.

Mathematical Expression

The torque produced by a BLDC motor can be expressed in terms of the back-EMF constant and the phase current. For a BLDC motor running with two phases conducting at any time, the instantaneous electromagnetic power is the product of two phase back-EMFs and the common current. Dividing by mechanical speed gives the torque. The torque constant Kt relates torque directly to current in SI units.

The key motor equations are: Back-EMF E = Ke * omega, where Ke is the back-EMF constant in V/(rad/s) and omega is the mechanical angular speed. The electromagnetic torque T = Kt * I, where Kt equals Ke in SI units. The electrical equation for a conducting phase is: Vdc = 2*E + 2*I*R + 2*L*(dI/dt), accounting for two phases in series. At steady state, Vdc = 2*Ke*omega + 2*I*R, which gives the speed-torque relationship.

Given:
Vdc = 48 V, Ke = 0.05 V/(rad/s), R per phase = 0.3 ohm
Required: Find no-load speed and stall torque

Why this formula applies:
At no-load, current is negligible. At stall, speed = 0.

Formula:
Vdc = 2 * Ke * omega_nl + 2 * I * R

No-load speed (I = 0):
omega_nl = Vdc / (2 * Ke)
omega_nl = 48 / (2 * 0.05) = 480 rad/s
Speed in RPM = 480 * 60 / (2*pi) = 4584 RPM

Stall torque (omega = 0):
I_stall = Vdc / (2 * R) = 48 / (2 * 0.3) = 80 A
T_stall = Kt * I_stall = 0.05 * 80 = 4.0 Nm

Final Answer:
No-load speed = 480 rad/s (4584 RPM), Stall torque = 4.0 Nm

Exam Tip: BLDC motor back-EMF is trapezoidal and PMSM back-EMF is sinusoidal. BLDC uses 6-step block commutation with Hall sensors; PMSM uses FOC with sinusoidal currents. In GATE, the distinction between BLDC and PMSM drive methods is a common question. Kt = Ke in SI units but not in mixed units.

Mechanism Explained

• At any instant, only two of the six inverter switches are ON, energizing two motor phases. The third phase floats and its back-EMF can be sensed for sensorless commutation.
• The 3-bit Hall sensor code changes every 60 electrical degrees, triggering a new commutation step. Six steps complete one electrical cycle.
• Speed control is achieved by varying the DC bus voltage or by adjusting the duty cycle of PWM applied to the lower switches while maintaining the commutation sequence.
• Sensorless BLDC drives detect the zero crossing of the floating phase back-EMF to determine commutation timing, eliminating Hall sensors at the cost of complexity.
• The trapezoidal back-EMF and block current waveform produce torque ripple at six times the electrical frequency, which is a known limitation of BLDC drives compared to FOC-controlled PMSM drives.

Quick Revision

• BLDC motor: permanent magnet rotor, wound stator, trapezoidal back-EMF, electronically commutated using 6-step block commutation.
• Hall sensors (3 sensors, 120 degrees apart) produce a 6-state binary code per electrical revolution to trigger commutation.
• 6-step commutation: two phases conduct at a time, six 60-degree steps per electrical cycle, using 6 power switches.
• Key formulas: E = Ke*omega, T = Kt*I, and at steady state: Vdc = 2*Ke*omega + 2*I*R.
• Kt = Ke in SI units (Nm/A = V-s/rad). This equality is a common GATE formula check.
• GATE trap: BLDC is NOT the same as PMSM. BLDC has trapezoidal back-EMF and uses block commutation; PMSM has sinusoidal back-EMF and uses sinusoidal current control or FOC.
• Torque ripple occurs at 6 times electrical frequency in BLDC due to step commutation, unlike the smooth torque of sinusoidal drives.