startup methods for sensorless motor control

To run a BLDC motor, the rotor position is needed so that the stator field can be set orthogonal to the rotor field. Most sensorless motor controllers rely on the back-emf generated in the stator winding to obtain the rotor position. Back-emf is generated only when the magnetized rotors are moving. So when the rotor is at rest, there is no back-emf generated, and the rotor position cannot be inferred. Thus, motor controllers implement a startup method for sensorless control.

Open loop startup

In open-loop startup, the controller generates a rotating magnetic field by forcing current in the stator winding. The generated magnetic field is not orthogonal to the rotor field but still generates some torque to rotate the rotor. After the rotor reaches sufficient speed to generate back-emf, the controller can measure the rotor position and start field oriented control.


Open loop startup method. A rotating magnetic field is generated by the controller and the rotor rotates with the field. After the rotor reaches sufficient speed to generate back-emf, the controller starts FOC

The controller generates a rotating magnetic field without knowing the rotor position. So the generated magnetic field is not orthogonal to the rotor magnetic field. The torque generated during this process is thus inefficient.

The generated magnetic field needs to be large enough to overcome the inertia and load of the rotor.

The common method to implement this is V/F startup. In V/F startup, a 3-phase sine wave is fed to the motor. The voltage and frequency can be linearly varied. The voltage magnitude and frequency depend on the motor, motor load, inertia, etc.

Rolling start

Rolling start method waits for the rotor to be rotated by an external torque. After the external torque brings the rotor to sufficient speed, the controller detects the rotor position and delivers torque.


Rolling start method. An external torque rotates the rotor and the controller delivers torque after the rotor reaches sufficient speed

Rolling start works best in torque control mode, where the controller regulates the I_q current.

Rolling starts for ebikes

A common application of rolling start is in sensorless control of direct-drive HUB motors for ebikes. The ebike is gently moved forward and the controller delivers torque after detecting the wheel rotation.

Compared to the open-loop startup method, this method reduces the starting current and stress on the battery.

Saliency tracking

In some motors, the inductance of a coil varies as a function of rotor position. This variation is due to the rotor geometry. Saliency tracking exploits this variation in inductance to estimate the rotor position.

This method of rotor position detection does not depend on the back-emf. So the rotor position can be detected even at zero speed, and the controller can generate torque at standstill.