ESC

Electronic Speed Controllers (ESCs)

The electronic speed controllers (ESCs) drive the motors based on input of the controller. There are different types of ESCs for different types of motors. Brushless motors (also called outrunners or inrunners) are used for multirotors and require an ESC that works with brushless motors.

How a brushless ESC works is quite complicated. Basically they control three phases to generate magnetic fields in coils to attract the permanent magnets fixed on a runner. The speed in which those magnetic fields are changed determines the speed of the motor.

For this to work an ESC has to have it's own brain - a small CPU with an internal program. This programming can be quite different depending on the kind of model it's supposed to be used for. For example the ESC for a car needs to be able to turn the motor backwards and our multirotors require extremely fast speed changes to keep the copter stable.

Because of this a lot of ESCs need to be re-programmed (flashed) with an alternative firmware to work on a multirotor. The most common one is called "SimonK" written by Simon Kirby. There are different versions for different ESCs manufacturers. Some manufacturers or merchants even offer pre-flashed ESCs. If you don't want to fiddle with this yourself, it might be a good idea to get those.

The SimonK firmware increases the rate at which the ESC sends speed information to the motor and removes some features that are bad for multirotors. For example most ESCs have an averaging function to filter quick throttle moves to reduce spikes in current. But also make sure you use a SimonK flashed ESC only in multirotors. If you have a heavy rotor like on a helicopter it will probably break the gears because of the sudden speed changes. (also see PPM filtering)

The most important detail on an ESC is its current rating in ampere (e.g. 25 A). ESCs that can handle higher currents usually are larger and heavier. Because of this you want to get an ESC that can handle the current your motors generate plus a little headroom for spikes generated by fast speed changes, but not too big to keep the weight down - after all you need one ESC for each motor. If your ESC is too small for the current it will let you know - with little sparks and smoke.

Another thing to think about is the battery eliminator circuit (BEC). If you don't want to have an extra battery or external BEC to power your main controller, you should get ESCs that have a BEC on board. ESC’s called OPTO’s don’t have a BEC built in as sometimes it is not wanted or could harm your flight controller. It is best to find out which you need and that depends on your setup.

Non multirotor ESCs have a low-voltage-cutoff which senses a voltage drop if the battery charge is getting low. This is important as draining a Lipo battery too far can destroy the battery for good. There are two kinds of low-voltage-cutoffs: A "hard cutoff" where the ESC cuts the power to the motor completely so the steering servos can use the remaining power to maintain the control surfaces and allow for an emergency landing. This is a really bad idea on a multirotor for obvious reasons. A “soft cutoff” slowly reduces the power over a 8-15 second span to allow the aircraft to settle to the ground. Although better that is still not good for a multirotor. A true multirotor ESC has no cutoff at all and the pilot relies on a timer or a low voltage buzzer to tell them to land.