real electric motors work
Brushless DC motors
of having the magnets on the stationary casing and the windings
on the rotor, we could put the magnets on the rotor and
the windings on the stator. That way, we won't need brushes
at all because the winding is stationary. However, now we
need to find a way to switch the current through the windings
at the right moment to ensure the torque on the rotor is
always in the same direction. In a conventional motor, this
happens automatically as the commutator acts as mechanical
switch. With a brushless motor, we need some way to sense
the position of the rotor, and then electronically switch
the current so it's going the right way through the right
motors are found in computer hard drives, CD and DVD players,
and in anything else where efficiency and reliability are
more important than price. As the cost of electronics continues
to come down, perhaps one day all DC motors will be built
• No brushes
• Windings are attached to the casing, and easier
• Requires complex drive electronics
brushes are bad news. True, they're a clever way to ensure
that, as the rotor turns, the current is automatically switched
around the windings to ensure the motor keeps turning. However,
everything else about them is bad: they are noisy, create
friction, generate electrical interference (because of the
sparking) and reduce efficiency (because there will always
be a voltage drop across the brushes). Not only that, but
they eventually wear out. With modern electronics, we can
instead sense the position of the rotor (for example, with
a Hall-effect device), then switch the current with, for
example, a MOSFET transistor.
is a fan that spent most of its life inside a computer keeping
the microprocessor cool. It runs off 12 volts DC and has
a brushless motor, as it thoughtfully explains with large
friendly letters on the label.
the magnets are on the rotor (with fan blades attached)
in a ring around the outside of the hub. By “feeling”
them by using a small compass as a probe, we find that there
are four poles, running N-S-N-S around the ring.
“stator”, in the centre, has four small coils
with shaped pole pieces to create a strong magnetic field
next to the rotor. Depending on which director the current
flows through each coil, it will attract or repel a north
pole. So, all we have to do is to keep switching the direction
of current flow through the coils in synchronisation with
the rotation of the magnets, and we’ll keep exerting
a torque that keeps the fan turning.
we’ve peeled the label off and can see the electronics
that does the switching. It consists of a single integrated
circuit and a few small capacitors, so it’s actually
not all that complex! If we google the part number of the
chip (LB1962M), we find it is a “Fan motor single
phase full-wave driver”, which I guess is reassuring.
how does the motor know the exact moment that the magnet
has passed one pole, and therefore that it’s time
to reverse the current flow? There are three techniques
Hall-effect sensors. This is a neat, non-contact way of
knowing where the magnets are.
• Back EMF. This is even neater. We don’t use
sensors at all, but use the fact that the magnet moving
past the coil will induce a voltage in it, and use this
voltage to tell us where the magnet is.
• Don’t bother. For the ultimate minimalist
approach, just keep switching the coils in sequence and
assume the rotor will keep up. For motors with a small load
that is well defined (eg, a fan), this works pretty well.