Brushless DC electric motor
Brushless DC electric motor (BLDC motors, BL motors) also known as electronically commutated motors (ECMs, EC motors) are synchronous motors powered by DC electricity via an inverter/switching power supply which produces an AC/bi-directional electric current to drive each phase of the motor via a closed loop controller. The controller times commutation (hence rpm) and creates current waveforms (hence torque). In this context alternating current does not imply but does include a sinusoidal waveform, with minimal restriction on waveform; it must be periodic, and its frequency will determine motor rpm, and the waveform does effect how smooth the generated torque is as well as the motors efficiency at transforming electrical to mechanical energy. In a well designed PMSM the air gap magnetic flux is spatial sinusoidal and the phase commutation currents are sinusoidal, ninety degrees out of phase.
The motor structural elements of a brushless motor system is typically permanent magnet synchronous motor, but can also be a switched reluctance motor, or induction motor.
Brushless motors may be implemented as stepper motors as well; however, the term "stepper motor" tends to be used for motors with a radically different design and controlled with an open loop (hence the controller cannot detect when the stepper does not stop due to too high shaft load; there is not shaft position sensor). They are frequently stopped with the rotor in a defined angular position while still producing torque. A well design power supply/controller/PMSM can also be held at zero rpm and finite torque. Two key performance parameters of brushless DC motors are the motor constants Kt ( torque constant) and Ke ( BEMF constant also known as speed constant Kv = 1/Ke ).
In SI units Kt and Ke are the same constant:
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