An Electronic Speed Controller (ESC) is a device used to electronically modulate the speed of an electric motor.
Let's give a brief historical introduction to mechanical speed controllers and how they worked, before looking at how a ESC works.
Before using ESCs
Before the era of ESCs, speed controllers were made with electric resistors placed in the battery-motor circuit; the resistor drew the current and dropped down the voltage at the motor wires.
Some simple systems, such as the TAMIYA mechanical speed controller, were servo-controlled to 7 different positions to achieve 3 forward and 3 reverse speeds, as well as neutral:
The highest resistance #1 was matched with the lowest speed, and the lowest resistance #2 was matched with the intermediate speed.
They were basic systems, but they were not particularly appreciated for two reasons: the first was the non-proportional-stepped control (the 3 speeds); the second reason was the high power consumption even at the lowest speeds, because also resistors consumed the battery capacity (that was very poor in that time batteries).
On/off operation
Direct connection between battery and motor is no doubt the most efficient method of transferring power without waste; a simple on/off switch to be operated to give or ungive power.
Toy cars, after all, have only one speed and are driven pulsing the accelerator. However, this is possible because the speed of RC toys is very low; driving the fastest models this way would require the pilot to have unnatural sensitivity and responsiveness.
Thankfully, this skills are not required but outsourced to the ESC. Let's see how.
Pulse Width Modulation – PWM
Imagine driving a toy car and accelerating for a tenth of a second: the motor will have a pulse and the car will move forward a few meters, then will stop after a while.
Imagine repeating the same action one second after the first impulse: the car is probably still moving and we will see it accelerate again, then will slow down once more.
By repeating the same action several times, we will see the car slightly increase its speed with each pulse of the accelerator.
This experiment manually reproduces so-called pulse width modulation (PWM), which is drawn with a square waveform. If the parameters of the example are used, the pulsed wave modulation has the following features:
period (T): 1 s
frequency (f): 1 Hz
duty cycle: 10%
The period T is the time between the beginning of one pulse and the beginning of the next.
Frequency (measured in Hz) is the reciprocal of the period and indicates how many periods exist in one second.
Duty Cycle is the duration of the pulse, expressed as a percentage of the period. The pulse in the example lasts 0.1 seconds, the period 1s: Duty Cycle is 0.1 s/ 1 s = 0.1 = 10%.
Waveform for duty cycle 10%
Imagine repeating the experiment by increasing the pulse duration at 0.5 s. The new parameters are:
period (T): 1 s
frequency (f): 1 Hz
duty cycle: 50%
Waveform for duty cycle 50%
Once the concept is learned, it is obvious that increasing the duty cycle by 5 times makes the car reaching a much higher speed.
We have just explained the working principle of the ESC. The voltage applied to the motor does not change, what is changing is the time of application of that voltage over time T. Acting on the throttle, the pilot just changes the duty cycle.
How a ESC works - The throttle control
The PWM frequency of an electronic speed controller is a fixed value, tipically between 1 and 32 kHz, it depends on the ESC microprocessor.
In the following image we can see what happens when the throttle is at neutral position, 10% of travel, midle travel and fully pushed, respectively.
neutral throttle matches 0% duty cycle, that is, there is no pulse and the motor is stopped;
at a 10% of throttle excursion, the duty cycle matches the same percentage;
at half throttle travel, the duty cycle is 50%;
at full throttle, the duty cycle is 100% and the motor receives full battery power.
Conclusions
You have probably noticed that this article focuses on the operation of an ESC for brushless motors. We have simplified the discussion, but the operation of an ESC for brushless motors is very similar.
The PWM signal is always applied to the same pair of wires in the case of brush motors, while brushless motors have 3 wires. The signal is still applied to 2 wires at a time, but the excited wire pair changes continuously as the rotor position changes.