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How is a BLDC motor fundamentally different from a stepper motor? They both operate in the same way where you manually energize coils to force the rotor to turn.

Is there a reason you couldn't do position or speed control with BLDC motor by changing the timing on the coils in the same way you do a stepper? The control loops I have looked at for BLDC motors basically vary power with PWM while always having the rotor following in a way to maximize torque.

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  • \$\begingroup\$ One steps. One doesn't. \$\endgroup\$ Oct 28, 2021 at 17:41
  • \$\begingroup\$ The differences are typically speed and resolution. Stepper motors generally have finer resolution, but lower speed. BLDC motors generally have coarser resolution but faster speed. But you are correct, the basic principles are the same. \$\endgroup\$ Oct 28, 2021 at 17:46
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    \$\begingroup\$ The current answers don't really get at the core difference - a stepper is a brushless motor with a novel flux switching structure that provides magnetic gearing. That it why it can generate high static torque relatively efficiently. It is actually the only structure I'm aware of that can provide such 'gearing'. But this topic always causes a lot of debate, so I'll try to write up a detailed answer when I have time. \$\endgroup\$
    – Jon
    Nov 1, 2021 at 11:22

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Both use identical drivers but different algorithms and have different responses in speed, RPM and torque. A BLDC can be the best choice for small high RPM or low RPM with position servo control but are more costly with the servo feedback and wheel encoder yet have more peak torque and low idle current without a load since at idle if there is not load the current feedback drops to whatever holding current limit you choose. They respond proportionally to whatever error you choose. Acceleration step, velocity ramp or position error signal (PES).

Steppers

Stepper Motors are often designed with 200 rotor poles per revolution so they are like tiny magnetic gears to step each phase and stop at a lower RPM.

They can be microstepped to finer position angles by using a sinusoidal fractional current ratio between the two poles which are called Full stepping or microstepping when the PWM changes the ratio of current like a sine wave in N microsteps. This microstepping also reduces the holding torque and acceleration rate when turning but makes them quieter.

The Stepper needs to be sensed at some HOME position to recalibrate it's mechanical Zero position, then electrical step pulses are counted to know where it has been moved. However, you must limit current to reduce heat e.g. 1 or 2A when idle or less and ensure the acceleration rate of step pulses does not exceed the torque and mechanical inertia for the motor to move as fast as the incremental voltage pulses to apply current as the rotor turns then stops.

You can make a Stepper quieter with a rotary oil filled brass damping disk about the diameter of motor but very thin so the Eddy currents of the oil dampen the vibration and thus motion noises and resonance. But few know about this option once used by Hitachi NPL in their early 5.25" hard disk drives, but it makes them faster and quieter due to the dampened step response without ringing.

BLDC

BLDC motors run faster because they have either 2 poles for split phase or 3 poles for more torque and smooth overlapping phases currents.

You would operate them with internal or external gear or for example 8mm toothed belt drive ratios to reduce the speed like a stepper may used this 8mm toothed pulley wheel for moving, but without an index Hall sensor for 1 pulse per rev, and a wheel encoder with say 1000 pulses per rev ( optional) you do not know where it stops. But you have far more torque, albeit more complexity with the gear reduction.

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Stepper motors are reluctance machines. The rotor is made of steel, although there are also hybrid models which do have also a permanent magnet The rotor aligns to stator in a such way that reluctance (magnetic resistance) is minimal. The phase currents are always nominal controlled by chopper driver.

For the machine, this holds true : \$T=T_{max}\cdot sin\varphi\$. It means that output torque at aligned position is zero. Only at angle increase, the torque increases. So you don't have any correction in position with a stepper motor, even at nominal current. The error is +/- 1/2 full step angle, no matter what kind of stepping/microstepping you use.

The BLDC is totally different. It has a permanent magnet and a switched stator, such that stator field is always 90 degrees at right angle with respect to rotor field. \$sin\varphi\ = 1\$. The torque is controlled by the magnitude of the current. Therefore for zero torque, there is zero current - as compared with stepper where you always have nominal current.

BLDC is more dynamic, no cogging, no rattling as it doesn't have a salient reluctance paths. It needs a rotor position sensor feedback to properly switch the electronic switches, otherwise the rotor would align with the stator flux similar as stepper, but this is unwanted.

On the other hand the stepper doesn't need a feedback sensor, so it used where a low cost is main goal. It makes non sense to have a stepper motor with a feedback - you get the same price as BLDC, but lot inferior working.

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Your confusion is justified. A stepper motor is a terrible term for an electric motor. The term is the result of engineers treating the motor as a black box. For three reasons:

  1. A stepper motor may be a reluctance motor or a permanent magnet synchronous motor.
  2. If it is a permanent magnet synchronous motor (PMSM), it usually has a high number of magnet poles. Typically around 52 poles.
  3. Stepping is an open loop control method and the motor can be controlled in other ways.

In lab I have run field oriented control, including field weakening on a stepper motor's (PMSM type) and it behaves just like any other PMSM with lower pole counts. Hence you are not limited to just stepping the motor as a method of control.

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Precise motor positioning and motor control is always done with the help of an encoder no matter what kind of your motor is.

Stepper motors are used mainly as linear actuators without encoder especially in the dashboard of cars (needles and hands).

Stepper motors are used to drive capacity control valves due to the ability to keep the torque. They are now widely used in heat-transfer no-gas boilers.

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  • \$\begingroup\$ Stepper motors are most common in rotary (Floppy disk drives) then axial control like Gantry XY and 3D printers But if you need higher power, speed torque , quiet and fine position control you would always choose a BLDC motor and 1 of many feedback sensors for position sensing, like HDD's use intersector servo dibits between data sectors. These are now called Rotary Servos but originally they used Linear servo actuators which are still used today for other applications. Linear Servo motors have far greater torque abilities and never miss a step. But Steppers are cheaper and common for this. \$\endgroup\$ Oct 28, 2021 at 18:23
  • \$\begingroup\$ But a well design Stepper can exceed the toque demands and rarely miss a step, but is slower with less torque as they skip steps . My Mercedes Real hatch door uses as stepper motor and it skips steps frequently now (tick tickiety tick) when the friction is too high but stops end when the end stop switch is detected or stops if the current spikes are too frequent or reverses if it hits an object. It sounds like a plastic gear skipping yet no damage is done \$\endgroup\$ Oct 28, 2021 at 18:29
  • \$\begingroup\$ Stepper motors miss steps all the time. \$\endgroup\$ Oct 28, 2021 at 18:32
  • \$\begingroup\$ I designed at Stepper 1mx1m gantry and I limited the acceleration step rate with a constant number so it would never miss a step. Yet I could drive it up to 1m/s with a tiny 12W motor. Fast , quiet and reliable with 0.1mm resolution. But it is a common design flaw or feature when position does not matter between endpoints. \$\endgroup\$ Oct 28, 2021 at 18:37
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The first BLDC motors performed the commutation internally. They incorporated a simple four quadrant "encoder" and a few transistors to effectively replace the brushes. From the user's point of view they function exactly like brushed motors but they eliminate the unreliable and smelly brushes. FWIW, low-cost motors like this are still available. I wouldn't be surprised if incorporating Hall sensors and transistors is less expensive than carbon brushes.

Somewhere along the way, some engineers decided that they wanted to do the commutation themselves, thus the modern BLDC motor was born. Superficially, they do resemble a stepper motor with very few steps per revolution. However, they are better suited for closed-loop control.

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