Electronic speed control (ESC) is an essential component of any RC vehicle that uses an electric motor. An ESC regulates the power delivered to the motor to control the speed and direction of the vehicle. ESCs are widely used in radio-controlled cars, drones, and other similar applications. In this blog post, we will discuss in detail what PWM is, how it is used in ESCs, how an ESC works. We will also discuss a formula that you can use to set PWM frequency.
What is PWM?
Pulse-width modulation (PWM) is a technique used to control the average voltage or power delivered to a load by rapidly switching a voltage source on and off at a fixed frequency. The width of each pulse (i.e., the duration of time the voltage source is turned on) is varied to change the average voltage or power delivered to the load.
Use of PWM in electronics:
PWM is a commonly used technique in electronics to control the speed of motors, the brightness of LEDs, and the voltage supplied to other types of loads. By varying the duty cycle of the PWM signal, it is possible to control the average power delivered to the load. Note that duty cycle is the ratio of on-time vs total time. Using a variable resistor to control the voltage is less efficient than this because the resistor dissipates power as heat, resulting in wastage.
How Does an ESC Work?
Overview of an ESC:
An ESC is a component in an RC vehicle that controls the speed of an electric motor by regulating power. An ESC typically consists of a microcontroller, power electronics, and other components that facilitate communication with the motor and the input signal from a remote control device. This is the radio.
Explanation of the operation of an ESC:
The input signal processor receives the input signal from the RC radio and then sends it to the microcontroller. The microcontroller then generates a PWM signal with a fixed frequency and duty cycle and sends it to the motor driver. The motor driver amplifies the PWM signal and sends it to the motor. This is where it controls the average voltage or power delivered to the motor and hence the motor speed.
The microcontroller adjusts the duty cycle of the PWM signal based on input from the RC radio to regulate the motor speed. The microcontroller can also monitor the motor current and temperature to prevent damage to the motor and the ESC.
PWM in an ESC
PWM frequency and its impact on ESC performance:
The frequency of the PWM signal generated by the ESC can affect its performance. A higher PWM frequency can result in a smoother motor operation and better efficiency. However, it can also increase the heat generated by the ESC. What is interesting is that the ESC may increase in temperature at a higher PWM rate but the system overall will become more efficient. This is because the efficiency of the motor is increasing more than the efficiency of the ESC is decreasing. Overal there is a net increase in the amount of efficiency.
A lower PWM frequency can reduce heat generation in the ESC but may result in a less smooth motor operation and reduced system efficiency.
Explanation of how PWM works in an ESC:
The microcontroller in the ESC generates a PWM signal with a fixed frequency, typically between 6kHz and 48kHz, depending on the application. The width of each pulse in the PWM signal is varied by the microcontroller to control the average power delivered to the motor. For example, a duty cycle set to 50% will produce motor power for 50% of the period and be off for the remaining 50%. This will result in an average voltage or power delivered to the motor that is 50% of the maximum.
By varying the duty cycle of the PWM signal, the ESC can regulate the motor speed. A higher duty cycle will result in a higher average power delivered to the motor. Hence a higher motor speed. Similarly, a lower duty cycle will result in a lower average power delivered to the motor. Hence a lower motor speed.
What to Set PWM Frequency on the ESC?
Luckily for us there is a formula that makes this fairly simple. The formula is as follows:
Frequency (Hz) = Kv x Vnom x Pole Count / 20.
Round up to the next available frequency and set this in the ESC.
Try it out and see how it performs.
PWM Tips
PWM frequency cannot really be too high, it only leads to a slight increase in (switching) losses in the controller. The higher the PWM chopping frequency, the smoother the current. A smoother motor current results in lower losses, lower temperature.
Conclusion
An ESC is a fancy piece of equipment that allows us access to the performance a Brushless motor offers. PWM allows us to control speed in our RC vehicles. Now that we understand PWM further, we can use this to alter the factory set PWM rates. Doing so can increase the overall efficiency of our system at only a small fraction of cost. This cost will be additional heat in the ESC.