Selecting a ESC for your RC model
Always start with a power meter connected and increase the throttle while monitoring the current and ESC temperature occasionally. What appears to be a small change in prop size can cause quite a big increase in current draw that sneaks up on you.
Even though you may have a 40A rated motor, if the prop is too big, it can draw a lot more than 40A. If this continues, you will overheat the motor and ESC until one of them fails.
I had a 30A ESC fail recently after 2 years of reliable flying. On a full battery it was handling only 15-18 Amps. 3 of its 18 mosfets are fried. Stuff happens.
BEC, Linear and SBEC
The BEC provides the 5V power to operate the RC Receiver and Servos.
Typical smaller or cheaper ESCs have Linear BECs. This is a simple circuit that drops the voltage from the main battery and wastes the excess as heat.
Larger or more expensive ESCs may have switching BEC (SBEC) circuits. These are more complex but transform the voltage with much less waste and therefore much less heat generated. Be aware that SBEC circuits generate electrical noise and interference that may affect other systems on your model ie. cameras or FPV transmitters.
If your ESC is suitably rated but gets hot and there is not enough cooling air flowing over it, you may find that the BEC shuts down in flight. This causes a loss of control and probably a crash.
A typical small ESC may have a BEC rating of 3A. This does not mean you can get 3A reliably from it for any length of time; a few seconds only. Running a 4-cell battery will generate more heat in the BEC than a 3-cell battery because the BEC has to drop more voltage and therefore dissipate more power = more heat.
Always ensure good cooling of the ESC/BEC in flight.
The technical bit
ESC reliability is mostly related to current rating and heat. The mosfets switching the motor current drop some voltage which becomes heat. Even ESCs with alloy heatsinks are usually wrapped in plastic heatshrink which keeps the heat in. The more alloy, the more heat it can store or the slower the temperature rise. But if the heat is not being removed, even the biggest ESC will eventually overheat. The better ones will shut-down and protect themselves when hot. The heat transfer from the mosfets to the alloy is usually pretty poor, so if the mosfets heat up fast they can be a lot hotter than the heatsink.
The bigger the mosfets (higher Amp rating) the less heat they produce for the same current. A cheap 12A mosfet can have a 60A pulsed (short term) current rating, and a 25A ESC may have 3 of these in parallel per switch. So, 36A or 180A peak; if everything was perfect.
The biggest problem I’ve found with ESCs is the contact between heatsink and mosfets. If it’s not as good as it could be due to uneven mosfet mounting, you end up with one getting hotter. Although in theory mosfets share the load, in practice one goes poof and then you get a chain reaction failure.
Generally: The hobby power meters we use show the average current, not the switching peaks. But this is OK because the average current is closely related to power dissipation and temperature rise.
Experience has shown that even 400A worth of mosfets with 2000+ A peak rating can fail on a 50-200A load if the current sharing, temperature and a few other techie things are not quite right. And you need a change of shorts when they go off.