The switching frequency of a controller: a trade-off between heat, noise, and precision

A controller chops the battery voltage into pulses, thousands of times per second, to generate the motor currents. The number of cycles per second is called the switching frequency, or PWM frequency. This setting subtly affects the controller's heat, the motor's heat, the noise level, and the quality of the control. There is no universally "good" value; only a compromise to be found for each controller-motor combination.

What the splitting process does

Each phase of the motor is alternately connected to the positive and negative terminals of the battery. The proportion of time spent on each side determines the average voltage. The windings, thanks to their inductance, smooth out these pulses: the current rises slightly during one pulse and then decreases during the next. This creates a small, sawtooth-shaped ripple around the desired value. See the inside of a motor controller and vector modulation.

Increasing the frequency: advantages and drawbacks

What you gain

• Reduced current ripple: The current has less time to deviate between pulses. This reduces motor losses caused by this ripple.
• Reduced audible noise: Beyond approximately 20,000 cycles per second, which is the limit of human hearing, the high-pitched sound of the switching process is no longer audible.
• More precise control: The controller measures and corrects in each cycle; more cycles mean a current control loop that can be faster.

What we're paying for

• Reduced switching losses: Each switching event causes a transistor to dissipate a small amount of energy. Doubling the frequency doubles these losses. At high currents, this can necessitate reducing the allowable current.
• Increased processing load: The microcontroller must execute all commands in every cycle.
• Reduced electromagnetic interference, which must be controlled through circuit design.

The role of motor inductance

The current ripple is directly dependent on the motor's inductance: the lower the inductance, the faster the current changes with each pulse. Therefore, very powerful and compact motors with low inductance require a higher frequency to maintain a reasonable level of ripple. Conversely, a hub motor for an electric scooter, with higher inductance, can operate with a lower frequency.

That's why you shouldn't connect just any motor to any controller without checking: a motor with very low inductance connected to a controller with a low frequency can experience significant current spikes.

The connection to the motor speed

To accurately reproduce a sinusoidal waveform, a sufficient number of switching cycles per electrical revolution is required. However, the electrical frequency increases with speed and the number of poles (see our guide on Kv, Kt, and poles). At very high speeds on a motor with many poles, a switching frequency that is too low will degrade the control.

Finding the right balance

On electric vehicle controllers, the frequency typically ranges from a few thousand to tens of thousands of cycles per second. The selection process involves considering:

• The temperature of the transistors in the high-current controller
• The motor temperature at a stabilized speed;
• The noise, especially at low speeds;
• The quality of control at maximum speed.