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 source 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, power supply, etc. 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). Hargrave Technologies 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 is that the sensor is without any RPM, and starting a large load (like a traction application) is very difficult due to the lack of signals from the motor.

Hargrave 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 for Hargrave ESCs. During the motor braking, power is returned to the battery so that the overall operation time is increased. Alongside this, the 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 health of the system and applying appropriate protection mechanisms in the event of a fault. All Hargrave Technologies ESCs have the following protection mechanisms:

• Over Temperature Protection

  • If an ESC exceeds the over-temperature threshold, the output power to the motor is dropped. When the temperature drops to manageable levels, the ESC returns power. The end result is that the ESC will try to continue driving the motor in extreme conditions.

• Over Current Protection

  • Over-current protection is split into input current (bus) and output current (phase) limits. Hargrave 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 overcharging it. Hargrave ESCs 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. Hargrave Technologies units tackle this by actively tracking the state of the motor and modulating the drive accordingly to remain in sync.

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. Hargrave Technologies' ESCs are rated by their phase current output ability, and this distinction is important.

Our 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.

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, Hargrave Technologies' 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 the 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

Hargrave Technologies rates ESCs by the phase current they can output under continuous and peak operating conditions. The amount of heat generated by an ESC is mostly dependent 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.