Main Content

AC7 - Sensorless Brushless DC Motor Drive During Speed Regulation

This example shows the AC7 Sensorless Brushless DC Motor Drive during speed regulation.

Souleman Njoya M., Louis-A. Dessaint (Ecole de technologie superieure, Montreal))


This circuit uses a modified version of the AC7 block of Specialized Power Systems library. It models a sensorless brushless DC motor drive with a braking chopper for a 3HP motor. The AC7 which requires speed and hall sensors is made sensorless. The motor speed and position are estimated from terminal voltages and currents using a back-emf observer [1]. The commutations signals (equivalent to hall effect signals) are generated from the rotor position every 60 electrical degrees.

The permanent magnet synchronous motor (with trapezoidal back-EMF) is fed by a PWM voltage source inverter, which is built using a Universal Bridge Block. The speed control loop uses a PI regulator to produce the torque reference for the current control block. The current control block computes the three reference motor line currents, in phase with the back electromotive forces, corresponding to the torque reference and then feeds the motor with these currents using a three- phase current regulator.

Motor current, speed (reference, real and estimated), and torque signals are available at the output of the block.


Start the simulation. You can observe the motor stator current, the rotor speed, the electromagnetic torque and the DC bus voltage on the scope. The speed set point and the torque set point are also shown.

At time t = 0 s, the speed set point is 300 rpm. Observe that the speed follows precisely the acceleration ramp.

At t = 0.5 s, the full load torque is applied to the motor. You can observe a small disturbance in the motor speed, which stabilizes very quickly.

At t = 1 s, the speed set point is changed to 0 rpm. The speed decreases down to 0 rpm following precisely the deceleration ramp.


1) The power system has been discretised with a 2 us time step. The speed controller uses a 140 us sample and the current controller uses a 20 us sample time in order to simulate a microcontroller control device.


1. TS Kim, BG Park, DM Lee, JS Ryu, DS Hyun, "A New Approach to Sensorless Control Method for Brushless DC Motors", International Journal of Control, Automation, and Systems, vol. 6, no. 4, pp. 477-487, August 2008.