Creating a Timing Chart for a Brushless Motor

2025.07.22

In my previous talk, I explained the operation of a brushless motor using a circuit based on a control IC. I hope that my explanation has helped you to grasp the overall workings of the motor circuit. This time, as the next step, I will talk about signal timing. Let’s suppose that you want to develop a motor driver that will efficiently and quietly drive a motor. In order to do so, you must create a timing chart that will achieve this kind of performance. In order to create a timing chart that will run a motor quietly and efficiently, an understanding of signal timing is important.

Contents of Episode 3:

Creating a Timing Chart for a Brushless Motor

Knowing the position at which the magnetic field will be generated

The first thing you will need to know is the position, relative to the rotor (a permanent magnet), at which the magnetic field of the electromagnet will be generated. For example, if you want to run your motor counterclockwise, the S pole of the electromagnet will be formed on the counterclockwise side of the N pole of the permanent magnet. However, this alone is not good enough; you also need to determine the torque (mechanical energy) needed from the motor using as little electric power (electrical energy) as possible. This is also affected by the characteristics (such as the magnetic force strength) of the motor itself, but also depends on the position of the electromagnet relative to the permanent magnet (the angle between them). Hence it is not enough to simply choose the counterclockwise side; you need to figure out at exactly what position the electromagnet should be created.

Magnetic field relative angle and torque

The permanent magnet rotor and the torque generated by the electromagnet stator are related as shown below. The torque generated by the motor depends on the relative angle θ between two magnetic fields, those of the magnet and of the windings (electromagnet), and is calculated using sin θ (assuming that the magnitudes of the magnetic field created by the windings and the magnetic field of the permanent magnet are constant).

Relative angle between two magnetic fields and resulting torque

Theoretically, the torque is greatest when the relative angle between the magnetic fields is 90°. Hence we would want to keep the direction of the windings magnetic field close to 90° relative to the rotor magnetic field direction.

Timing of generation of the windings magnetic field

Based on all of this, let’s consider the timing with which the windings magnetic field should be generated. To begin with, the polarity signals of the three Hall elements can be used to classify the electrical angle into a 60° interval (see The Number of Poles and Number of Slots in a Motor, Mechanical Angle and Electrical Angle of a Motor). Also, a windings magnetic field can be created in six different directions by combining the voltages applied to the winding terminals (see “Timing Chart for a Brushless Motor Driving Circuit (2): Electromagnet Magnetic Fields Created by Voltage Patterns” in Timing Charts of a Brushless Motor Driving Circuit). From this, you see that we can select a winding magnetic field in any one of six directions according to six rotor positions (position ranges).

Well then, what kind of relative angle should the winding magnetic field to be created have at each of the rotor positions? In order to obtain a large torque, we should set the relative angle within the range 60° to 120°. And when the rotor leaves this range, we should make the winding magnetic field change to the next direction.

Winding magnetic field changing in 60° increments

Rotor position and signal timing

Drawing on these ideas, let’s look at the timing chart below, starting at the bottom and moving upwards. That is, we’ll use the rotor position and movement to confirm the states of each of the signals.

Timing chart

Now I’ll explain the gray arrows 1 to 5 on the right edge of the timing chart.

  1. 【1】 First, the polarities of the U, V, and W windings are determined according to the rotor position. In the diagrams, the rotor positions at which the torque is greatest are shown. The same magnetic field is generated in the interval within 30° ahead of and behind the rotor position.
  2. 【2】 Next, the direction of the current that is passed to create the winding magnetic field is determined. The current direction corresponds to the direction of the voltage applied to the windings. For example, if a current is passed from U to V in order to set the N pole at the U coil and the S pole at the V coil, then a positive voltage is applied to the terminal of the U phase windings, and the V phase terminal is set to negative (grounded).
  3. 【3】 In order to apply the voltage in this way, the U phase high side transistor and the V phase low side transistor must be turned on. Hence the UH and VL signals are set to High, and all other signals are set to Low. The signal logic from UH to WL is determined similarly for the other rotor positions.
  4. 【4】 In order for the six signals, from UH to WL, to be switched as shown in the diagram, the Hall signals must change at the positions shown. I’ll also note that the positions at which there is switching between High and Low for each of the signals are important; strictly speaking, the U, V, and W Hall waveforms need not be exactly as shown in the diagram. (If six rotor positions are known, then other logic combinations may be used instead.)
  5. 【5】 In order for the Hall signals to switch at these rotor positions, the Hall elements should be installed at the positions shown in the above diagrams. Here, what is important are the angles with respect to the windings. The position in the radial direction must be studied separately.

In this way, we can create a timing chart for a brushless motor and also determine the positioning of the corresponding Hall elements. This timing chart, which results in operation similar to that of a brushed motor, is called brushless motor driving control or 120-degree conduction. Because this conduction pattern is comparatively simple, it is widely used, but various other conduction patterns have also been proposed. We could say that, in contrast with a construction that uses a commutator for mechanical switching, the conduction pattern can be adjusted by using a control IC. I will explain other conduction patterns another time.

Well then, in the course of these three talks I have explained the construction of brushless motor driving circuits and the timing charts that they use. I hope you’ve got an overall picture of things by now. Next time, I will explain actual motor characteristics in a bit more detail, in connection with the motor driving operation using this 120-degree conduction.

Key points of this article

・In motor driver design and development, in order to achieve the performance required to run a motor quietly and efficiently, creating a timing chart that represents the motor operation is vital.

・In order to run a brushless motor as intended, the installed positions of position detectors (here, Hall elements) must be based on the timing chart.

    Teacher Sugiken’s Motor Library

    Teacher Sugiken’s Motor Driver Dojo

    An Introduction to Motors

    Brushless Mortor Driver

    Motor Q&A

    THIS WEBSITE USES COOKIES.
    By continuing to browse this website without changing your web-browser cookie settings, you are agreeing to our use of cookies.
    Privacy Policy
    OK