Understanding the confusing terminology of brushless DC (BLDC) motors

This is a condensed discussion of the terminology of DC electric motors, using object design’s class terminology.

The technology of motors has a long history.   Terminology used over that history is not always consistent.  Even at a moment in time, different sources may use differing terminology.

No warranty: this may be incorrect, and it is incomplete.

Brushed versus brushless DC motors

BrushedCommutated and BrushlessCommutated are subclasses of the class Commutated.

Brushed motors: mechanical commutation.

Brushless motors: electronic commutation.

Brushless motors are more reliable (have longer life) because electronics last longer than mechanical, generally speaking.  For example, the brushes may be copper fingers rubbing against copper rings.  Rubbing wears the fingers away.  Also, electrical current sparking across the gap between the fingers and the commutation ring eats them away.

Stepping motors versus brushless DC motors

SteppingBrushless and ContinuouslyDrivenBrushless are subclasses of BrushlessCommutated.

The distinction is one of purpose and control.  Otherwise, instances of the two classes may  have similar mechanical design.

Stepping:

  • purpose is precise angle of rotation
  • control is a finite sequence of steps

Brushless DC:

  • purpose is continuous rotation
  • control is a continuing sequence of steps

Magnetic poles

A single magnet has two poles, North and South.  But an arrangement of many magnets is also said to have many poles.  For example, a ring of four magnets, when viewed either from the inside or outside of the ring, can be said to have four poles.

The Poles class gives a Geometry.  Poles is a mixin behaviour of a CompoundMagnet (which includes a single magnet.)

Magnetic field concentration

Some materials, such as iron, concentrate the field of a magnet.  A particular configuration of such material may be called an armature or a yoke.   The concentrations it produces may also be called poles, although it might be better to call them lobes.

A coil (electromagnetic) may produce many poles (lobes) because of an armature.

Poles is also a mixin behaviour of a Compound of a CompoundMagnet and an Armature.

Axially wound versus radially wound

DC motors have coils, part of electromagnets.

The coils have a certain geometrical relation to the axis of rotation of the motor:

  • Axially wound: coil wound around the axis of the motor.  Line of axis of coil is coincident with line axis of rotation of the motor.
  • Radially wound: Line of axis of coil is orthogonal to line axis of rotation of the motor.

An axial coil may produce a radial pole.  It is non-intuitive that an axially wound motor would work, except that an armature concentrates the magnetic field of the coil into poles (lobes) that are NOT axial to the motor, but orthogonal (radial) to it.

In other words, the Poles behaviour of a compound of a magnet and armature need NOT delegate: the combination of magnet and armature may produce a different geometry than either of the parts.

Unipolar versus Bipolar

These terms describe a sequence of energizing coils.  They both refer to a motor having two coils (and any number of lobes, due to an armature!).  These terms describe the control or driving of a motor, not the mechanical design, nor the configuration of magnetic poles.

  • unipolar: only one coil energized at a time
  • bipolar: two coils may be energized at the same time

A bipolar driven motor is denser (produces more torque per mass) because the coils are energized more often.  In other words, at any given time, a unipolar driven motor has about half its weight (one coil out of two) that is dead weight because it is not energized.  A bipolar driven motor is generally 1.4 times as powerful (torque) as a unipolar motor of the same weight.

Windings versus coils versus bobbins

(These definitions may not be correct to all readers.)

A coil is one or many, concentric,  circular turns of wire.  Coils may be wound on a bobbin, which holds it in place.  Coils may also be wound around an armature.

Two coils in series makes one winding.  For example, the coils may be on opposite sides of the motor, but part of the same winding.

Two coils in series but with a third wire (a tap in the middle of  the long wire) from the junction between them, makes two windings.

A common example

For example, a common type of brushless PC fan motor.

It has one bobbin, axial.  Two wires are wrapped axially around the bobbin.  At first glance, it looks like one coil, but it has four wire ends.  Thus it is two coils and two windings.

The motor is driven unipolar.  In a unipolar driving circuit, two wire ends, one from each winding, are at the same electrical potential, tied together.  Thus you could say what is on the bobbin is a tapped wire.

The electronic commutation control circuitry is on a circuit board at one end of the motor.  It drives the motor continuously whenever power is supplied.  (Different circuitry could just as well drive it as a stepping motor, although it might not be precise as another design.)

The motor has a Y-shaped yoke or armature at each end.  The Y-shaped yokes are not aligned with each other, not at the same angular position.  These shape the magnetic fields of the windings into many lobes.  (Other motors have toothed cups to shape lobes.)

The motor has permanent magnets in a ring outside the windings.  The material of the magnets is one topological donut shape (flattened) of refrigerator magnet composition.  The donut has many magnets established in it, that is, the donut has many poles.

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