Failure
Failure may not be an option to most Mayhemmers, who love their motors like helianthus loves the sun, but failure of Brushless systems are possible and if you know what causes it you will be better equipped to avoid doing that thing!
BL motors - No brushgear, almost universally ballraced, rare earth magnets high electrical efficiency – what can go wrong?
Well, abuse can lead to overheating, and this generally takes a characteristic pattern.
What is abuse, and how do I avoid it?
Like all electromagnetic devices the worst form of abuse is running them slowly and overloaded! In Boat terms this means overload due to too big a prop (or other load), or possibly high friction or misalignment in the propshaft, or a bent motor shaft which allows the rotor to catch or rub on the stator. None of these forms of abuse are very likely in boats – since gravity is kind to them, and loss of control seldom involves tent-peg landings and re-kitting; to which flight motors are (occasionally) subject.
When it comes to overloading unfortunately the efficiency of the BL motor and ESC combination can hide the distress of the motor.
This is actually far MORE likely among Boat people and especially Mayhemmers (because a lot of advice is passed out to rate ESCs for the STALL CURRENT of the motor. This means that the ESC often has a current capability WELL beyond what the motor will survive, and the thermal protection of the ESC is useless.
This is probably the subject for another thread, but it can contribute to distress of the motor. I’m happy to say that our shining experts don’t do this oversizing.
The pattern of failure of a BL motor is:
• Overloaded motor slows and gets hotter
• Boat owner compensates by adding throttle
• Motor gets hotter still, and the coils gradually transfer heat into the rotor
• Rotor heats up and the magnets lose a little magnetism
• Boat owner compensates by adding throttle
• Efficiency drops off
• and the same throttle opening leads to MUCH more heat (and less power)
• Eventually
• The magnets lose a LOT of magnetism
• Motor efficiency and speed fall
• One or more stator coils burn out
Failure is always blamed on the burnt out coil but it is generally due to the thermal demagnetisation that led up to it.
Inrunner, outrunner, do they fail the same?
Every word above applies equally to both styles of motor, but there are differences that are worth mentioning.
Inrunners
Remember that inrunners have the rotor completely inside the stator.
T
he rotor is always VERY small diameter and has the magnets locked into the surface of the rotor. The windings of the stator are outside this and can be cooled by passing air, or water but in a boat its likely they sit in stagnant, warm air.
Also: Inrunners can and indeed must run very fast to make any power at all (cos the torque of this little rotor is small) and speeds of 30,000 rpm are not unusual (under load!).
SO they overheat very fast if they are abused!
Inrunners must spin fast
They must sound like a dememted dentists drill to be happy and efficient.
Feigao make BL inrunners from 12mm diameter up to “600 replacement” size – the 12mm size comes in KV up to 7500 rpm/volt, so 10V is 75 THOUSAND RPM unloaded. Rotor is probably about 6mm diameter. If you want to make a plastic magic 1/72nd hydrofoil this is a fair place to start
Because they have a smooth and stationary stator they are easy to mount (see next BLB spiel).
For the same reason they can be water-cooled just as easily as a brushed motor – wrap some aluminium or copper pipe round and round them and run cooling water through them
Even if you have a 380 replacement inrunner don’t assume the case diameter is the same as a 380 brushed motor – the manufacturer is only saying that the shaft diameter and fixing holes in the front face are the same as a 380!
There are some inrunners made with built in water cooling – Martin has pictured some in earlier discussions. Might be worth considering but please see the next statement as well:
But – and this is an andrew-opinion - Inrunners would be better uncooled and spinning like a screaming dervish, than heavily cooled and slogging!
AndyN has a high-speed Feigao inrunner for his tiny BL speedster. I think this is a 30 mm diameter motor – often referred to as 380 or 400 replacement. Andy will be running this with a tiny racing prop and I’m perfectly certain it will spin at astronomical speeds and shift the boat nicely. Was it 6s or 7S lipos, Andy?
Outrunners
Remember on these the stator is on the inside, and the magnets are fixed to the rotor which whirls through the air - so the bits which need to be cooled are moving and close to the place they can get the cooling from air.
Air cooling is much easier – many of the heavy-duty outrunners are made with integral fans to keep some air moving over the outside.
Remember boat-people that if you want air cooling you must provide cooling air! Think IC engines and allow air into the boat – open a window at the front and allow the hot air out of the back – fit a computer fan if you wish!
Water-cooling, conversely is more difficult. If you have read Hannu’s posts you will see he has contrived water-cooled engine mounts for his big BL motors. This is good it cools not only the drive –end bearing but also the most massive bit of metal in the motor, and the path through which all the heat has to pass to get to the magnets! Hyvva, Hannu.
andrew
Next chapter - mounting BLs in boats
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Topic: Brushless Basics (Read 140038 times)