Have read somewhere, that brushless DC motors have constant torque.

How can this be true?

Regard motor on the picture:

enter image description here

If constant current flows as red arrows indicate, the magnetic field appears as blue arrows indicate. The permanent magnet (not shown) tends to put it's N and S poles appropriate.

All this time, the current remains constant.

Then how the torque can be constant, as the magnet approaches it's final position? Isn't torque decreasing as this happening?

Wouldn't it be more constant torque, if I power the winding coils with three phase sinusoidal current, having phase ahead by some value relative to rotor?

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    \$\begingroup\$ ower the winding coils with three phase sinusoidal current, having phase ahead by some value relative to rotor - this is exactly how you normally drive BLDC motors. \$\endgroup\$ – pjc50 Sep 29 '15 at 12:19
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    \$\begingroup\$ This TI explanation does a pretty thorough job in clarifying things. \$\endgroup\$ – Roger Rowland Sep 29 '15 at 12:36
  • \$\begingroup\$ @pjc50 you don't excite a BLDC machine with sinusoidal currents. \$\endgroup\$ – JonRB Sep 29 '15 at 12:46
  • \$\begingroup\$ @Dims "constant torque" probably came with some conditions. A high inertial load would result in a constant shaft torque. BLDC machine do not provide a constant torque, they pulse due to commutation (compounded is quasi-squarewave control is used, better is FOC with a trap profile is used) \$\endgroup\$ – JonRB Sep 29 '15 at 12:48
  • \$\begingroup\$ BLDC has a torque ripple, but if you look the mean value is constant. It means more a property of the BLDC that the torque vs. speed doesn't change, or you have the ability to control the output torque with current, of course it is not perfectly flat at least not as flat as PMSM. \$\endgroup\$ – Marko Buršič Sep 29 '15 at 13:13

Perhaps you have misunderstood the working principle of BLDC and PMSM. The working principle is similar to DC motor. We hava an armature current and excitation current (or permanent magnet), it is important that you undarstand that both magnetic fields are right angle, as the torque between two fileds are T=T_max*sin(phi).
IN BLDC is the same, the magnet never aproaches to final angle, because the stator is switched, so that there is always almost right angle - the same is for DC, there is a brush commutator.

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    \$\begingroup\$ In average, you mean? But it jumps as switching occurs, right? \$\endgroup\$ – Dims Sep 29 '15 at 15:17

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