Timing Charts for Brushless Motor Driving Circuits

2025.07.22

In order to drive a brushless motor, it is important to understand the meaning of certain waveform diagrams that are called timing charts. A timing chart is a graphic image that explains specifications related to motor driving control. This time, I will explain what each of the waveforms in a timing chart represents.

Contents of Episode 3:

Timing Charts of a Brushless Motor Driving Circuit

When a motor is running, the voltages across the windings and the currents flowing within them are changing. A diagram showing changes in electric signals when a motor is being driven is called a timing chart. Timing charts are also used when showing the operation of a logic circuit, but please keep in mind that the timing charts I’m talking about are used for motor driving.

A timing chart shows the major signals of each of the circuits, with the rotor position (electrical angle) along the horizontal axis. When the signals in a timing chart are digital, they are represented by two values, High and Low. In timing charts that I will talk about later, the actual voltage values are different—they may be Hall signals, UH signals, or something else. Originally these signals are supposed to represent the heights of waveforms, but what is intended for representation is High and Low signals, and so these are often shown without much elaboration.

As an example, here I’ll show you the timing chart for a brushless motor driving circuit that executes nearly the same operation as a previously mentioned brushed motor. When driving the motor in this way, it is convenient to position the Hall elements at the positions shown below (I’ll explain why later). Let me explain things using the diagram below.

Timing Chart for a Brushless Motor Driving Circuit (1): Voltages Across Winding Terminals

Timing Chart for a Brushless Motor Driving Circuit (1): Voltages Across Winding Terminals

First, let’s set the rotor position in the above diagram as the left end of the timing chart, angle 0 degrees (note the angular positions at the bottom of the chart). When the motor rotates counterclockwise from this position, the respective signals change as shown in the timing chart. Below I explain the different signals.

■Inputs to the control IC: Hall signals U, V, W

  • ・The Hall elements output High or Low signals according to the magnetic pole they face. Within the IC, the rotor position is divided into six segments according to logic processing of the High and Low signals of these three Hall elements (these are the six numbers above the chart).
    Examples: {U, V, W} = {Low, High, Low} is segment 1; succeeding segments are segment 2 {Low, High, High} to segment 6 {High, High, Low}.

■Outputs from the control IC: UH to WL

  • ・Based on the above segment information, the control IC generates signals UH to WL instructing the transistors to turn on or off.
  • ・Although not shown, the signals UH to WL are passed to the level shifter, and in the level shifter are converted into signals that the transistors can receive (that are able to drive the transistors).

■Transistors: U, V, W winding voltages

  • ・The UH to WL signals act as commands to turn the power transistors on and off (High level turns a transistor on, Low turns it off). Through this operation, the voltages across the windings are determined.
  • ・When both the high side and the low side transistors are off, the voltage across the windings is in the open state (later I will talk about the waveform that one would expect to see).

When I mentioned earlier that this motor performs “nearly the same operation as a brushed motor”, I meant that these winding voltage patterns are the same.

Timing Chart for a Brushless Motor Driving Circuit (2): Electromagnet Magnetic Fields Created by Voltage Patterns

The diagram above showed only the voltages across the winding terminals. Next, I’ll explain the positions at which an electromagnet is formed by applying those voltages, and how the rotor rotates as a result.

The first diagram below shows the relationship of the current direction to the N and S poles. Once you understand this, you can proceed to my explanation of the images following that one, showing the timing chart and the rotor positions in each of segments 1 to 6.

Timing Chart for a Brushless Motor Driving Circuit (2): Electromagnet Magnetic Fields Created by Voltage Patterns

Timing Chart for a Brushless Motor Driving Circuit (2): Electromagnet Magnetic Fields Created by Voltage Patterns

First, in segment 1 in the image, the power supply voltage is applied to the U phase windings, and the V phase windings are connected to ground. Hence current flows from the U phase to the V phase.

Further, if the U phase windings are the N pole, then the V phase windings are the S pole (the windings are connected in this way). When the rotor position is in segment 1, if these are the winding polarities, then the rotor will rotate in the counterclockwise direction.

When the rotor rotates and the polarity of the Hall signal W switches, the motor transitions to segment 2. Here, current flows from the U phase to the W phase, and the winding magnetic field switches as shown. Here also, the rotor moves in the counterclockwise direction.

In this way, the rotor continues to rotate, repeating the sequence “magnetic field is created” → “rotor rotates” → “Hall signal switches” → “next magnetic field is created” → “rotor rotates” → “Hall signal switches”. This is the operation indicated by the timing chart.

Up to this point, perhaps some of you were thinking that my explanation has things backwards. Sure, I have been explaining the circuit operation, but from the standpoint of running the motor, perhaps the sequence should be “wanting to create a magnetic field at a position to which the rotor will turn” → “control transistors turn-on/turn-off accordingly” → “position the Hall elements where they need to be”. In my next talk, I’ll explain this approach—that is, how to create a timing chart.

Key points of this article

・A timing chart is a graph explaining specifications used to control motor driving; understanding the meaning of the waveform diagrams in a timing chart is important.

・A timing chart indicates the pattern in which voltages are applied to run a motor. The electromagnet magnetic field created by these voltages was described.

Next page: Creating a Timing Chart for a Brushless Motor

    Teacher Sugiken’s Motor Library

    Teacher Sugiken’s Motor Driver Dojo

    An Introduction to Motors

    Brushless Mortor Driver

    Motor Q&A

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