[Episode 5] Passion! Which Thoughts Did Ichinose Sense from Ninomiya?

2025.08.22

The first behavior that is not described in the timing chart, “Winding currents and induced voltages,” shown in “Actual Signal Waveforms in Brushless Motor Driving,” took a long time to explain, but did you understand it? Next, I will explain the second one, “Pulsed Applied Voltages”.

Actual Signal Waveforms in Brushless Motor Driving

① Why is the U phase current waveform shaped this way?
▶Winding Currents and Induced Voltages
② I understand the UH and UL signals and waveforms, but when I zoom in, the UH signal has a pulsed shape.
▶Applied voltage pulses
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?
▶Waveforms of Winding Terminal Off Intervals
▶Freewheel Diodes
④ What does the power supply current represent? This current also has a pulsed waveform.
▶Power Supply Currents

Brushless Motors: Applied Voltage Pulses

When observing the electrical signals of a motor with an oscilloscope, the command signals that control power transistor on/off operations may be pulsed signals (the gate signal UH in the figure below). This is also an important point for controlling the motor.

Electrical signals in a circuit

Before discussing pulsed command signals, let’s consider the functions required of motors. Many systems perform their functions by varying the speed or torque of a motor. The fans in electric fans and air conditioners adjust airflow by changing their speed. Motors in trains and electric vehicles also change their speed and torque. In this way, the speed and torque of motors are controlled by adjusting the amount of the voltage applied. For example, for a motor that turns a fan, increasing the applied voltage will increase the speed and torque. For this reason, motor control requires the ability to adjust the applied voltage.

There are two main methods for adjusting the applied voltage. One method is called PAM (Pulse Amplitude Modulation) control, in which the power supply voltage to is changed to change the applied voltage (amplitude/height).

The other method is called PWM (Pulse Width Modulation) control, in which the applied voltage is pulsed to change the average value. In PWM control, the average applied voltage is generally controlled by creating High and Low periods within a reference cycle (T in the figure below) and adjusting the ratio of the High and Low periods. This period is called the PWM period or, when converted to a frequency, the PWM frequency.

Voltage adjustment

Although PAM control requires a circuit to adjust the power supply voltage, PWM control is mainly used these days to adjust the applied voltage, because PWM control only requires a function to turn power transistors on and off. PWM control allows partial adjustment of the amount of voltage, rather than uniform application of voltages as in the case of 120° conduction.

This makes it possible to control a wider range of voltages, including sine wave conduction.

The voltage adjustment ratio in PWM control is expressed in terms of duty. For example, if the power supply voltage is 100 V and the applied voltage is to be adjusted to 30 V, this is called “setting Duty to 30%.”

The pulsed waveform of UH shown in the first figure indicates that this control IC (a motor driver or controller) uses PWM-controlled 120° conduction. Since duty is mainly input from the outside (e.g., a system controller) as a control command, the motor driver has an input terminal to receive such a command.

Next time, I will discuss (3) “Waveforms of winding terminal off intervals”.

Key points of this article

・The fact that the command signals that control power transistor on/off operations are pulsed signals indicates that the control IC performs PWM control.

・Devices that use motors require speed and torque to be changed, which is adjusted by the amount of voltage applied to the motor.

・There are two methods for adjusting the applied voltage:

・PAM (Pulse Amplitude Modulation) control to change the applied voltage amplitude/height.

・PWM (Pulse Width Modulation) control in which the applied voltage is pulsed to change the average value, which is mainly used these days.

Next page: Brushless Motors: Waveforms of Winding Terminal Off Intervas,
Freewheel Diodes, Power Supply Currents

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