What is an ESC?
You may have heard them called motor controllers, or even plain old inverters. An Electronic Speed Controller (ESC) is a purpose-built device designed for controlling the speed of an electric motor. Using a specialised combination of hardware and firmware, ESCs drive motors to a commanded speed. They maintain motor speed under various circumstances, such as the dynamic load of a propeller.
While different ESC types exist for different motor types, the focus here is on Brushless DC (BLDC) motors. BLDC ESCs convert power from a DC supply into a dynamic voltage to drive BLDC motors. This conversion process is flexible, varying the voltage to increase or decrease the motor speed.
The input supply can take the form of a battery or power supply. Using a centralised control system, inputs from a user or an autopilot system are mapped into throttle setpoints for each connected ESC.
Features of an ESC
An ESC can track the motor's real-time position and speed using one of two methods. The first is using external sensors attached to the motor (sensored systems). The second is by taking voltage measurements from the motor itself (sensorless systems). APD ESCs are typically sensorless, which improves reliability by removing the sensor as a potential failure point. Other advantages include higher maximum RPMs, and compatibility with most off-the-shelf powertrains. A downside to using the motor as the sensor is without any RPM, starting a large load (like a traction application), is very difficult due to lack of signals from the motor.
APD ESC's functionality extends motor control, allowing for quick reversals of the motor, regenerative braking, and telemetry during operation. Quick reversals are utilised in systems where both forward and reverse operations are needed in quick succession, such as robotics. Regenerative braking is another advanced feature standard to APD ESCs. During braking of the motor, power is returned to the battery such that overall operation time is increased. Alongside this, efficiency of the ESC is improved while reducing thermal demands on the ESC itself. Operational telemetry is a must-have feature for modern flight control systems, providing a full picture of the powertrain.
During operation, the ESC is responsible for tracking the overall system health and applying appropriate protection mechanisms in the event of a fault. All APD ESCs have the following protection mechanisms:
Over-Temperature Protection
If an APD ESC exceeds the over-temperature threshold, the output power to the motor is dropped. When the temperature drops to manageable levels, the ESC will return power. The result is that the ESC will try to remain driving the motor in extreme conditions.
Over-Current Protection
Over-current protection is split into input current (bus) and output current (phase) limits. APD ESCs will quickly change the output power to the motor if an over-current event is detected, returning back to normal operation once the event is passed.
Over-Voltage Protection
While recharging the battery with braking power, it is important to avoid over charging it. APD ESCs will actively track the input voltage and avoid returning power when the voltage rises too high.
Desynchronisation Protection
Remaining in sync with a sensorless motor is one of the challenges of an ESC. APD units tackle this by actively tracking the state of the motor and modulating the drive accordingly to remain in sync.
Bus Current vs Phase Current
A common question received about BLDC ESCs is the difference between bus current and phase current. Bus current is the current that is supplied to the ESC from a battery or power supply. Phase current is the current that the ESC outputs to the motor. APD ESCs are rated by their phase current output ability, and this distinction is important.
APD BLDC ESCs control the motor's speed by changing the voltage on the outputs to the motor (called the phase voltage). A duty cycle is output from 0-100%, which modulates voltage using a technique called pulse width modulation (PWM). This duty cycle determines the motor's speed.
$$ \text{Motor RPM} = \text{KV} \times \text{Duty Cycle %} \times \text{Input Voltage} $$
Why is this important? An ESC cannot create power, which means that the power into an ESC (from the power source) must equal the power out to the motor. There is a small amount of loss in the ESC itself. However, APD units are typically >95% efficient.
The output voltage is always less than the input voltage. The ESC adjusts the phase current to compensate for the difference (power in must equal power out). The relationship between duty cycle and phase current becomes important when designing a BLDC system. If the duty cycle is too low, phase currents can become very large very quickly (for example, a propeller that is too big for a specific motor kV).
For example, consider a system that is drawing 50 V 100A from a battery. The ESC is commanded to operate at 50% duty cycle. Therefore, the output voltage to the motor is 25 V (50% duty cycle). The ESC will now compensate by increasing the output current to 200A (power in must equal power out; Power = Voltage x Current).
Rating an ESC
APD rates ESCs by the phase current they can output under continuous and peak operating conditions. The amount of heat generated by an ESC is dependent mostly on the current output at any given time. Heat loss is equal to phase current squared times the resistance of the unit. Further details about powertrain matching can be found in this blog post. Higher voltages should be used where possible, as for a given power, the current will be less.
$$ \text{Phase Current Multiplier} = \frac{\text{KV} \times \text{Input Voltage}}{\text{RPM}} $$
$$ \text{Power Loss Multiplier} \approx \text{Phase Current Multiplier}^2 $$
