Methods of Sensorless 120° Commutation Driving Startup 1: Startup on Detection of Induced Voltage from Synchronous Operation

In the previous article, basic 120° commutation driving was used to explain sensorless driving of a 3-phase full-wave brushless motor. Here, startup methods specific to sensorless driving of 3-phase full-wave brushless motors are explained. As basic startup methods, two methods will be explained over the course of two articles: a method in which the induced voltage from synchronous operation is detected for startup, and a method in which the position of the stopped permanent magnet is detected to perform startup.

Problems with Startup of Sensorless 120° Commutation Driving of a 3-phase Full-Wave Brushless Motor

Methods of Sensorless 120° Commutation Driving Startup 1: Startup on Detection of Induced Voltage from Synchronous Operation

As stated previously, in sensorless driving, the position of the permanent magnet when stopped cannot be determined, and so it cannot be known what currents should be passed in the three coils upon startup. As one means to deal with this, synchronous operation is used to rotate the permanent magnet, the induced voltage that occurs is detected, and 120° commutation driving is then performed; this is explained using the diagram below.

This operation is continued, and the time until the next switching is shortened little by little, so that the permanent magnet rotation rate increases, and gradually the induced voltage rises to a level at which detection is possible. When a state is reached in which the induced voltage can be detected, normal sensorless 120° communication driving commences.

In other words, in this method a synthetic magnetic field is created in a rotation direction regardless of the position of the permanent magnet, and by switching at fixed times to start rotation of the permanent magnet, an induced voltage occurs and is detected, and normal control is begun.

In this method, if at the start the synthetic magnetic field switching occurs too quickly, the permanent magnet may not be able to follow, and so a method must be used in which the switching period begins slowly and is gradually quickened.

Function Block and Operation Waveforms of Induced Voltage Detection from Synchronous Operation

Below are waveform examples illustrating the series of operations described above. The ST_CLK signal and BEMF_DET signal as well as the signal resulting from their addition (see the block diagram), and the output voltage waveforms A1 to A3, are relevant to the present explanation. The fact that the period of the BEMF_DET signal becomes shorter and shorter indicates that the rotation rate is increasing.

This startup method has the following issues.

• ・A synthetic magnetic field is created regardless of the position of the permanent magnet, so that depending on the magnet state, a reverse-direction torque may act, and for some stopped positions of the magnet, time may be required for startup.
• ・Originally, the relative positions of the permanent magnet and the created synthetic magnetic field should differ by 90° in order to generate a large amount of torque. But because the synthetic magnetic field is created irrespective of the permanent magnet position, motion may start from an angle such as 70° or 60°, so that the required large startup torque is not obtained.

The other method mentioned at the beginning of this article addresses these problems; in this method, the stopped position of the permanent magnet is detected to perform startup. The next article will describe this method.