From Michael Faraday’s (uncaged) discovery mathematicians tell us, the force on a conductor (wire) in a magnetic field is proportional to the current going through the wire, and the length of wire in the field. (I’m sure some cows or sheep may be in there too).
It didn’t take long to work out if you put the same wire through the magnets twice the force doubled, and hence a coil with 10 turns through the magnets gave 10 times the force (for the same current). Force times distance is torque. (distance being the distance of the conductor from the shaft). So, good motors have big (strong) magnetic field and lots of windings to give lots of torque to turn propellers. NOT QUITE.
In most motors used in models the magnetic field is fixed (permanent magnets) and can’t be varied, so you get what you pay for, high quality permanent magnets aren’t cheap but modern ceramic magnets are pretty good.
So lots of windings for a good motor. Again NOT QUITE.
The number of times the wire is wrapped round the armature pole is limited by the space available, but if you use thinner wire you can get more turns in. ( good lots of turns =lots of torque). However Georg Simon Ohm (of ohm’s law fame) gets into the act he tells us that ( among other things) that the resistance of a conductor(wire not musician) is R= γ*l/a (γ is resistivity of the material, usually copper) l, is length and a, cross section area of conductor. Problem long lengths of thin wire have high resistance and will limit the current through the motor, also thin wire has a habit of melting (like fuse wire) if you pass too much current through it. Also thin wire is not very strong and may be thrown off the armature by centrifugl force as the motor spins (it could be glued in place, but cost matters)
Motor makers juggle to give motors that :-
will curry the current without burning out (thick enough wire)
are small enough to fit in the model (not too thick wire)
are reasonably cheap (not too complicated to make)
I think it was “slot car racers” who first found that if they unwound the armature coils on normal production motors and rewound them with less turns of thicker wire they could get the edge on their competitors (less resistance more current) and from there the interest in the number of turns was developed, then with the development of electronic speed control (remember slot cars had variable resistor controllers) the internal resistance of the motor became important. So the number of turns is a non technical indication of internal resistance. Remember the lower(smaller) the resistance of the motor the greater the potential current, so the less turns the larger the current.
FET’s used in ESC’s are prone to being “spiked” by pulses of high current that motors take when starting up ( one Ripmax ESC 120 amp continuous 440 amp peak 18 to 36 turns ) so when using low resistance (small number of turns) it is possible to spike ( kill, damage, render useless) the FET’s and the controller with starting and reversing currents while not exceeding the carefully measured normal load current
.
Instead of an ESC manufacturer saying do not use with motors of less than 0.01ohm (a purely random number on my part) they tend to say suitable for 6 to 12 turn motors.
Unfortunately not all motor manufacturers tell you how many turns are on there motors. OOPs
I hope my slightly askew explanation (or perhaps rambling) doesn’t give too many “experts” hypoplexia and if nothing else gives a few wry smiles.
Fair weather and calm water
Tom