BLDC, Stepper, Servo and Electronic Commutation

In a BLDC, what takes the place of the commutator and brushes?

A DC motor kept turning one way only because the commutator and brushes reversed the current in the rotating armature at just the right moments. The BLDC motor swaps the places, putting the magnet on the rotor and the winding on the stator, and then does away with this mechanical commutation. As the slider turns the rotor, the stator coils switch on one after another to match its position; what now does the job of the commutator and brushes is the question of this lesson.

A BLDC is a DC machine turned inside out

A DC motor reversed the current in its rotating armature winding with a commutator and brushes to produce torque in one direction. The BLDC motor turns this structure inside out: the permanent magnet that makes the flux sits on the rotor, and the winding that carries current sits on the stator. A sensor such as a Hall sensor reads the rotor position, and the inverter sends current only into the stator windings that push the rotor forward, switching windings in turn as the rotor turns. A real BLDC motor usually energizes two phases together and changes the combination every 60 electrical degrees in a 6-step scheme, but the figure simplifies it by switching on a single coil. Its torque-speed behavior resembles a DC motor's, and with no brushes there is no wear or sparking, while the winding on the outer stator also makes heat easy to remove.

A stepper counts position in pulses

A stepper motor moves its rotor one notch, a fixed step angle, for each pulse. The step angle is one full turn of 360° divided by the number of steps, so the common 1.8° motor takes 200 steps per turn. Counting pulses gives the position and the pulse rate gives the speed without any sensor (open loop), so steppers are used where something must move by a set amount precisely, as in printers and CNC machines. If the load exceeds the torque the motor can deliver, though, it skips a step and loses its position, and in open loop there is no way to catch that slip.

A servo nails it with feedback

A servo motor measures position and speed with an encoder, feeds that signal back to the controller (closed loop), and narrows the error between target and actual to 0. The motor itself is often a BLDC or permanent-magnet synchronous motor, but the heart of a servo is the feedback control more than the motor. Servos are used in robot joints and machine tools that need fast response and high precision. Put the three machines side by side and they are one family that governs winding current with rotor-position information, differing only in what they aim for: continuous torque for the BLDC, discrete position for the stepper, closed-loop precision for the servo. The idea of commutation that began with the DC machine's commutator has met electronics and sensors and led on to today's machines that govern position.

Back to the first screen

What replaced the commutator and brushes was a position sensor and an inverter. The BLDC motor turns the DC machine inside out, with the magnet on the rotor and the winding on the stator, and removes the mechanical contact through electronic commutation, in which the inverter sends current into the right winding to match the rotor position. The stepper turns the same idea of position into discrete positions, one pulse per notch, and the servo feeds that position back through an encoder to hold it in a closed loop. The three branch from a single stem, governing winding current by rotor position, and aim at continuous torque, discrete position and closed-loop precision respectively; the story of this course, which started from the DC machine's commutator, arrives at today's machines that govern position by meeting electronics and sensors.

Whether BLDC or servo, the heart is the inverter that sends the right current into the stator windings to match the rotor position. How that inverter turns DC into AC of the size and frequency you want, the basics of switching and PWM, is the subject of the last unit. The V/f control of the induction machine, the variable speed of the synchronous machine and the electronic commutation of the BLDC all run on the same inverter (MC-E2).