[Episode 4] A Sudden Closeness?! New Things the Two Have in Common

2025.08.22

table of contents

Up until the last time, I had been explaining motor driving circuits and timing charts as part of the fundamentals of brushless motor driving. Now I’ll talk about brushless motor characteristics.

Contents of Episode 4:

Brushless Motor Characteristics

Brushless motor characteristics include the characteristics of the motor mechanism itself, circuit operation, and the motor output (revolution rate, torque). This is of course essential information when developing a motor, so please take the time to master this material.

The actual waveforms of motor driving signals exhibit behavior that does not appear in timing charts, which are theory-based design documents. Among this behavior are the points below that I will explain, since they may not be easy to understand based only on what I have discussed so far.

■Actual Signal Waveforms in Brushless Motor Driving

Actual Signal Waveforms in Brushless Motor Driving

Oscilloscopes are used to observe the electrical behavior of motors. An oscilloscope can monitor the voltage and current values of a circuit. When an oscilloscope is used to monitor the signals on the circuit board shown below, the oscilloscope screen displays the following waveforms.

Oscilloscope

Actual Signal Waveforms in Brushless Motor Driving

These are ordinary signal waveforms used in motor driving; from the top, they are the U phase upper-side gate signal (UH) and lower-side gate signal (UL), the U phase winding terminal voltage, the U phase winding current, and the power supply current. These waveforms cannot be explained solely using knowledge of timing charts, which I explained earlier. When you actually run a motor, you will have the following doubts and questions.

(The numbers below correspond to the numbers appearing in the oscilloscope screen above.)

  • ① Why is the U phase current waveform shaped this way?
  • ② I understand the UH and UL signals and waveforms, but when I zoom in, the UH signal has a pulsed shape (repeated on/off switching). Why?
  • ③ Where the UH waveform is pulsed, the U phase voltage (the voltage across the U phase winding terminals) has a pulsed waveform, like the UH waveform, and where the UL signal is turned on, I understand why it is at GND level. But what is the diagonal waveform of the U phase voltage where UH and UL are both turned off?
  • ④ What does the power supply current represent? This current also has a pulsed waveform.

These points are important, both when driving a motor and in order to understand the characteristics of a motor.

① The U phase current

The “U phase current” is a current in windings. In order to understand the waveform of this current, you have to understand winding currents and induced voltages, and so I’ll explain these things, dividing them into the following four topics.

Key points of this article

・Brushless motor characteristics, which must be understood in order to drive a brushless motor, will be explained.

・The characteristics to be considered are the characteristics of the motor mechanism itself (excluding the circuitry), the various operations of circuits, and the motor output (revolution rate, torque).

・The driving waveforms observed in actual brushless motors are different from the signal waveforms one would expect from an explanation of timing charts.

・These waveforms and their behavior are important for an understanding of motor characteristics.

Next page: Brushless Motor Winding Currents and the Principle of Induced Voltage Occurrence

    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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