Brushless Motor Winding Currents That Consider Induced Voltages

2025.08.22

Contents of Episode 4:

Based on the explanations so far, we summarize the relationship between the winding current and the induced voltage.

Brushless Motor Winding Currents That Consider Induced Voltages

Considering that an induced voltage is generated in a coil of a brushless motor, an induced voltage (Vbemf) is added to the circuit shown in “Winding Currents of Brushless Motors”. Assuming the waveforms of the applied voltage and the induced voltage shown in the figure below, the actual voltage applied to the winding coil will be the voltage in the gray area.

Winding current upon voltage application

As shown above, the actual voltage applied to the winding coil is large at first, decreases in the middle, and then increases again. Since the voltage waveform (gray area) is shaped like this, the winding current has a concave center (similar to the waveform shown in Winding Currents of Brushless Motors).

As a result, you will notice that the amount of the winding current will change as the amounts of the applied voltage and induced voltage change. While the applied voltage can be controlled by the control IC, the induced voltage varies under various conditions. We have explained that the induced voltage is the amount of change in the magnetic flux passing through the coil, and the amount (wave height value) of the induced voltage is as follows:

▶ It increases with an increase in the magnetic force of the magnet ← because the absolute amount of magnetic flux increases.
▶ It increases with an increase in the number of coil turns ← because the number of coil windings that generate electricity increases.
▶ It increases with the motor rotation speed ← because the magnetic flux changes more rapidly.

The following figure shows the change in the current waveform with a change in the amount of the induced voltage. It can be seen that the larger the induced voltage and the closer it is to the amount of the applied voltage, the more pronounced is the concavity in the center of the current (upper figure). Apart from the amount of the voltage, the current waveform also changes with the timing of voltage application (lower figure).

Induced voltage and current waveforms

This is the waveform variation for 120° conduction (sinusoidal), but the winding current varies with the amount and position of the induced voltage in other conduction patterns as well. This will affect the motor characteristics in various ways.

This concludes my explanation of question ①, Why is the U phase current waveform shaped this way? Next, I will explain question ②, Why does the UH signal have a pulsed shape?

Key points of this article

・If the amount of applied voltage and induced voltage changes, the amount of winding current will also change.

・The applied voltage can be controlled by the control IC, but the induced voltage varies under various conditions.

・The amount of induced voltage

– Increases with an increase in the magnetic force of the magnet because the absolute amount of magnetic flux increases.

– Increases with an increase in the number of coil turns because the number of coil windings that generate electricity increases.

– Increases with the motor rotation speed because the magnetic flux changes more rapidly.

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