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I think I understand the operating principles of a brushless motor and a stepper motor, but I'm a little confused about the difference. Is a brushless DC motor a very basic stepper motor? With proper controls, could a brushless DC motor be operated as a stepper motor? If not, how do they differ?

For an electronics newbie, can someone highlight the similarities and differences between stepper motors and DC brushless motors?

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  • \$\begingroup\$ Not an answer to the question, but this link indicates Hurst AC Synchronous motors are "identical in construction to the HURST® stepping motors". \$\endgroup\$ – Tut Aug 19 '13 at 18:32
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The two are largely the same, fundamentally. However, they differer in intended application. A stepper motor is intended to be operated in, well, steps. A BLDC motor is intended to be operated to provide smooth motion.

Since stepper motors are used for motion control, repeatability of the steps is desirable. That is, if you start at one step, then to another, then back to the first, it should ideally return to exactly where it was previously. Various things can mess this up; slop in the bearings, friction, etc. BLDC motors are optimized for smooth torque between steps, not repeatability.

Stepper motors are designed to maximize holding torque, the stepper's ability to hold the mechanical load at one of the steps. This is accomplished by keeping the winding current high even though the rotor is aligned with the stator. This wastes a lot of energy, because it generates no torque unless the load tries to turn out of position, but it does avoid the need for any feedback mechanism.

On the other hand, BLDCs are typically operated with the rotor lagging the stator so that applied current always generates maximum torque, which is what a brushed motor would do. If less torque is desired, then the current is decreased. This is more efficient, but one must sense the position of the load to know how much torque to apply. Consequently, stepper motors are usually bigger to accommodate the additional heat of operating the motor at maximum current all the time.

Also, for most applications, people expect a stepper to be capable of small steps for precise motion control. This means a large number of magnetic poles. A stepper motor typically has hundreds of steps per revolution. A BLDC will usually have many less. For example, recently I was playing with a BLDC from a hard drive, and it has four "steps" per revolution.

Stepper motors are usually designed for maximum holding torque first, and speed second. This usually means windings of very many turns, which creates a stronger magnetic field, and thus more torque, per unit of current. However, this comes at the expense of increased back-EMF, thus reducing the speed per unit voltage.

Also, stepper motors are usually driven by two phases 90 degrees apart, while BLDCs typically have three phases, 120 degrees part (though there are exceptions in both cases):

stepper motor
stepper windings

BLDC
BLDC windings

Despite these differences, a stepper can be operated like a BLDC, or a BLDC like a stepper. However, given the conflicting design intentions, the result is likely to be less than optimal.

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  • \$\begingroup\$ Most BLDC controllers I've seen haven't been designed for the same sorts of positional accuracy as steppers, but is there any reason why BLDC motors shouldn't offer control to whatever precision the rotational sensor offers? If I want to rotate something by exactly 12.25 turns as quickly as possible, I would think a BLDC motor should be able to do the job better than a stepper, since a stepper needs to be driven with a pessimistic acceleration profile but a BLDC motor would have no such restriction. \$\endgroup\$ – supercat Aug 19 '14 at 17:07
  • \$\begingroup\$ @supercat No particular reason. But with the BLDC and a rotational sensor, what you have is a servo, and you need some kind of feedback loop to control it. The same can be accomplished with a brushed motor. Steppers usually don't have rotational sensors and they are driven without any kind of feedback loop (except limit switches to find the end of the range of motion), so they depend on not missing any steps for positional accuracy. That simplifies the driver, but also puts limits on speed and torque. \$\endgroup\$ – Phil Frost Feb 11 '16 at 14:08
  • \$\begingroup\$ One would achieve smoothest motion with a smart servo controller, but if one wants to move a BLDC motor by 12.25 turns and one doesn't particularly care about smoothness, would there be any particular problem with simply running the motor forward until it reaches a spot just before the destination, then setting it for the destination phase, and running it backward if it overshoots too far? Stepper motors are horribly inefficient, but it would seem that adding a rotary encoder and a brake could improve efficiency enormously. \$\endgroup\$ – supercat Feb 11 '16 at 16:19
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    \$\begingroup\$ @supercat I think that still counts as a servo. There's no problem with it in particular, but if the servo controller is "simple", then it probably won't be as accurate or repeatable, or have as much holding torque as a stepper of equal cost, and even a "simple" servo controller is more complex than a stepper which requires no controller at all. I suppose for a battery-powered device the inefficiency of a stepper would be a big problem, but for anything plugged into the wall, electrical power is cheap and abundant. \$\endgroup\$ – Phil Frost Feb 11 '16 at 16:42
  • \$\begingroup\$ If holding torque is required, it will be necessary to use either a stepper or a brake. Efficiency isn't only important for reasons of power usage, though; a more efficient motor can be smaller and lighter than one which needs to be able to dissipate more power as heat than it will ever be called upon to produce mechanically. I would think a combination of a BLDC (or intermittent-duty stepper) plus rotary encoder plus brake could in many cases be smaller, lighter, and cheaper than a stepper motor with the same top speed and usable torque. \$\endgroup\$ – supercat Feb 11 '16 at 17:02
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A stepper motor is a form of brushless DC motor, but with a specific physical arrangement of coils and stator so as to achieve a fixed number of stops or detents subdividing the full circle of rotation.

The number of poles of a stepper motor determine the step size or number of subdivisions, or "full steps", if you like.

However, with some fancy footwork in the energizing of the stepper motor coils, modern stepper motors with suitable controllers can often provide rotation in partial steps, known as micro-stepping.

TL;DR: Stepper motors are (typically) a subset of the Brushless Motor family.


Switched Reluctance motors are another form of stepper motor, somewhat different from the standard BLDC stepper.

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protected by Community Dec 9 '15 at 6:02

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