Electronic speed control

An electronic speed control (ESC) is an electronic circuit that controls and regulates the speed of an electric motor. It may also provide reversing of the motor and dynamic braking. Miniature electronic speed controls are used in electrically powered radio controlled models. Full-size electric vehicles also have systems to control the speed of their drive motors.

An electronic speed control follows a speed reference signal (derived from a throttle lever, joystick, or other manual input) and varies the switching rate of a network of field effect transistors (FETs). By adjusting the duty cycle or switching frequency of the transistors, the speed of the motor is changed. The rapid switching of the current flowing through the motor is what causes the motor itself to emit its characteristic high-pitched whine, especially noticeable at lower speeds.

Different types of speed controls are required for brushed DC motors and brushless DC motors. A brushed motor can have its speed controlled by varying the voltage on its armature. (Industrially, motors with electromagnet field windings instead of permanent magnets can also have their speed controlled by adjusting the strength of the motor field current.) A brushless motor requires a different operating principle. The speed of the motor is varied by adjusting the timing of pulses of current delivered to the several windings of the motor.

Brushless ESC systems basically create three-phase AC power, like a variable frequency drive, to run brushless motors. Brushless motors are popular with radio controlled airplane hobbyists because of their efficiency, power, longevity and light weight in comparison to traditional brushed motors. Brushless DC motor controllers are much more complicated than brushed motor controllers.

The correct phase of the current fed to the motor varies with the motor rotation, which is to be taken into account by the ESC: Usually, back EMF from the motor windings is used to detect this rotation, but variations exist that use separate magnetic (Hall effect) sensors or optical detectors. Computer-programmable speed controls generally have user-specified options which allow setting low voltage cut-off limits, timing, acceleration, braking and direction of rotation. Reversing the motor's direction may also be accomplished by switching any two of the three leads from the ESC to the motor.

ESCs are normally rated according to maximum current, for example, 25 amperes (25 A). Generally the higher the rating, the larger and heavier the ESC tends to be, which is a factor when calculating mass and balance in airplanes. Many modern ESCs support nickel metal hydride, lithium ion polymer and lithium iron phosphate batteries with a range of input and cut-off voltages. The type of battery and number of cells connected is an important consideration when choosing a battery eliminator circuit (BEC), whether built into the controller or as a stand-alone unit. A higher number of cells connected will result in a reduced power rating and therefore a lower number of servos supported by an integrated BEC, if it uses a linear voltage regulator. A well designed BEC using a switching regulator should not have a similar limitation.

Most modern ESCs contain a microcontroller interpreting the input signal and appropriately controlling the motor using a built-in program, or firmware. In some cases it is possible to change the factory built-in firmware for an alternate, publicly available, open source firmware. This is done generally to adapt the ESC to a particular application. Some ESCs are factory built with the capability of user upgradable firmware. Others require soldering to connect a programmer. ESC are usually sold as black boxes with proprietary firmware. As of 2014, a Swedish engineer named Benjamin Vedder started an open source ESC project later called VESC. The VESC project has since attracted attention for its advanced customization options and relatively reasonable build price compared to other high end ESCs.

Large, high-current ESCs are used in electric cars, such as the Nissan Leaf, Tesla Roadster (2008), Model S, Model X, Model 3, and the Chevrolet Bolt. The energy draw is usually measured in kilowatts (the Nissan Leaf, for instance, uses a 160 kW motor that produces up to 340 Nm torque ). Most mass-produced electric cars feature ESCs that capture energy when the car coasts or brakes, using the motor as a generator and slowing the car down. The captured energy is used to charge the batteries and thus extend the driving range of the car (this is known as regenerative braking). In some vehicles, such as those produced by Tesla, this can be used to slow down so effectively that the car's conventional brakes are only needed at very low speeds (the motor braking effect diminishes as the speed is reduced). In others, such as the Nissan Leaf, there is only a slight "drag" effect when coasting, and the ESC modulates the energy capture in tandem with the conventional brakes to bring the car to a stop.