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misan
If I understand correctly, torque is proportional to coil current...
For any particular permanent magnet motor, yes. More generally, for permanent magnet motors, it is proportional to the ampere-turns of the winding, i.e. the product of current and the number of turns. This applies to both steppers and BLDCs.
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misan
, so theoretically you could get the desired torque at any speed as far as you can supply the desired current to the windings. Unfortunately, there are two reasons that is not possible:
1) the cycle time a coils is energized is reduced as rotational speed increases, this makes it more difficult to achieve the desired coil current, specially when you have a voltage limited power supply.
2) the rotation of the motor induces a an opposite voltage on the coils (counter electromotive force) that reduce the maximum current that go through the coils. That effect is more important the faster the rotation speed is, thus limiting the maximum speed the motor can reach.
Both of those can be countered by using constant current drive and increasing the drive voltage as necessary with increasing speed. But there is a third factor:
3) there is a maximum current (the rated current of the motor) that you can put through the windings without the motor overheating, because of the resistance of the windings. To allow more current, you have to make the motor bigger so that you can use thicker wire having lower resistance.
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misan
So usually you are left with real motors, that achieve the maximum torque at low speeds, but with the proper control can keep a good torque value for higher speeds.
Real motors can maintain the same torque as you increase the speed, provided you increase the voltage to counter the back emf of the windings so as to maintain the current. Of course there is a limit to this, because eventually other effects become important such as bearing heating, the voltage being more than the insulation can stand, and increased losses in the electronics.
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misan
... If you add turns to the coils you increase the torque at the expense of reducing the maxim speed.
No. If you add turns, you have to use thinner wire to fit the extra turns in, which increases resistance; hence you have to reduce the current - which to a first approximation (i.e. assuming that the same percentage of winding area is filled with copper in either case) exactly cancels out the effect of the extra turns on the torque. What you have done is to turn a high current low voltage motor into a lower current higher voltage motor.