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 spinning armature at the right moments. A BLDC puts the magnet on the rotor and the windings on the stator, then removes that mechanical commutation. Watch the stator windings switch on in turn with the rotor position. What takes over the job the commutator and brushes did?
A BLDC is a DC machine turned inside out
A DC motor used the commutator and brushes to commutate the spinning armature winding and make one-directional torque. A BLDC turns this layout inside out: the permanent magnet that makes the flux goes on the rotor, and the current-carrying winding on the stator. The rotor position is read by a sensor, and the inverter feeds current only to the stator winding that pushes the rotor forward. As the rotor turns, the energized winding is switched in turn, doing electronically what the commutator did. The torque-speed character resembles a DC motor, but with no brushes the wear and sparking are gone and the life is long.
A stepper counts position in pulses
A stepper moves the rotor one notch by a fixed step angle for each pulse. The step angle is one revolution divided by the number of steps, that is 360° over the step count. Counting pulses gives position and the pulse rate gives speed, accurately and without a sensor (open loop). So it is used where something must move a precise amount, like printers and CNC. But if the load exceeds the torque a step can provide, it misses a notch and loses position — a step-out.
A servo nails it with feedback
A servo motor measures position and speed with an encoder, feeds those signals back to a controller (closed loop), and squeezes the error between target and actual toward zero. Its body is often a BLDC or a small synchronous machine, but the heart of a servo is not the motor — it is feedback control. It is used in robot joints and machine tools that need fast response and high precision. Placing the three side by side, all are one family that governs winding excitation by rotor position; they differ only in aim: BLDC for continuous torque, stepper for discrete position, servo for closed-loop precision. The idea of commutation that began at the DC machine’s commutator, meeting circuits and sensors, carries into the modern machine that governs position.
Back to the first screen
What took the place of the commutator and brushes was the position sensor and the inverter. A BLDC turns the DC machine inside out — magnet on the rotor, winding on the stator — and erases the mechanical contact with electronic commutation, the inverter feeding the right winding by rotor position. A stepper turns the same position idea into discrete positions, one pulse per notch, and a servo feeds that position back through an encoder to nail it in a closed loop. The three branch from one stem — governing winding excitation by rotor position — and aim respectively at continuous torque, discrete position and closed-loop precision. The story of this subject that started at the DC machine’s commutator closes, meeting circuits and sensors, in the modern machine that governs position.
For both BLDC and servo, the heart is the inverter that feeds the right current to the stator windings by rotor position. How that inverter turns DC into AC of the wanted magnitude and frequency — the basics of switching and PWM — is the subject of the last unit. The V/f control of induction machines, the variable speed of synchronous machines and the electronic commutation of BLDC all run on the same inverter (MC-E2).