FEBRUARY 2006 £3.80 - Index of

KNOW-HOW DC MOTORS
Figure 3.
Large ferrite magnets
are being superseded
by small neodymiumbased
magnets.
Figure 4.
Diagram of a conventional
iron-cored motor.
Figure 5.
The brush system in a
mechanically-commutated
DC motor is complex,
lossy and prone
to wear.
Figure 6.
Diagram of a mechanically-commutatedironless
core motor.
Figure 7.
Diagram of a
brushless motor with
interior rotor.
18
Magnets
Shaft
Bearing
Back
iron
Shaft
Bearing
Stator
Stator
Rotor
Winding
Brushes
Back iron
completing
magnetic circuit
Connections
Commutator
050321 - 12
Rotor
Armature
(potted coil)
Magnets
Brushes
Connections
Commutator
050321 - 13
Connections
Winding Back iron
Bearing
Rotor
Magnets
Shaft
Stator
Back iron
050321 - 14
A powerful magnet is only half the battle. Like an electric
circuit, the magnetic circuit must be short and of
adequately large cross section (low ‘magnetic impedance’).
This is best achieved using a large amount of
iron, which is also, unfortunately, heavy and responsible
for further losses. When an iron core (which in conventional
motors also carries the windings) rotates in a
magnetic field there are iron losses which comprise
losses due to hysteresis and losses due to eddy currents,
and which are speed dependent. It is not possible to
prevent these losses, and so the efficiency of the motor
decreases with increasing speed. To reduce the losses
as far as possible, motor manufacturers use high-quality
iron (a ‘soft magnet’) and a large number of segments.
Iron cores are never solid, but divided lengthwise into
as many layers as possible, isolated from one another
and thus cutting the path of eddy currents. Eddy currents
arise not only in the core but also in all other electrically-conducting
parts of the motor, including the
magnet and the windings. These are reduced by the
rather involved technique of segmenting the magnet
and making the windings from a number of fine parallel
wires or a braid rather than a single solid conductor.
The desire to avoid iron core losses has prompted the
invention of iron-free motors, but these do not exactly
offer high levels of torque. The ideal motor, then, has
yet to be invented: we must be content with selecting a
suitable motor depending on the requirements of each
application.
Classical designs
In numerical terms the dominant technology is the iron
core motor (Figure 4). The rotor has at least three coil
segments, which suffice for most low-voltage applications.
High operating speeds require a balanced epoxypotted
rotor. Around it,
separated by an air
gap of a few tenths
of a millimetre, is
the stator comprising
a system
of permanent
magnets. The
motor housing
completes the
magnetic circuit
(like a return
wire in an electrical
circuit). The
coil connections run to the
individual commutator segments,
from where the current flows via sliding
brushes (generally made of copperloaded
graphite, also called
‘carbon’) to the motor connections.
The brush
arrangement is the part
most prone to wear
and failure. Its
size reflects
the power
of the
elektor electronics - 2/2006