FEBRUARY 2006 £3.80 - Index of

KNOW-HOW DC MOTORS
Figure 12.
The distance between
the magnets in a multipole
machine (in this
case with exterior
rotor) should not be too
small. Right, a ten-pole
rotor where one pole is
formed by each group
of three adjacent
magnetic strips.
Figure 13.
Diagram of a
brushless motor with
exterior rotor.
Figure 14.
Brushless motor with
exterior rotor designed
for use in models.
20
Shaft
Rotor
than their diameter. This gives improved efficiency
(greater than 90 %) and power-to-weight ratio.
More poles
Stator
Magnets
Connections
Bearing
Back iron
Winding
050321 - 18
Greater torque is of course desirable. One tried and
tested method is to increase the number of magnet poles:
in conventional technology (using brushes) this was simply
too complicated to achieve. A four-pole motor (Figure
9) has a magnet pole every 90 degrees, alternating
north and south. This functions as if there were two
motors in one enclosure. This halves the speed of the
motor, as one transition from north to south and back corresponds
to only half a revolution. The torque, however,
is multiplied by four. Sometimes we speak of an ‘electrical
2:1 reduction gearing’.
This idea can be taken further: to six, eight or even ten
poles distributed around the rotor, with a corresponding
gearing-down effect. The paths of the magnetic flux are
also shorter, running from a north pole to an adjacent,
rather than an opposite, south pole. The enclosure completing
the magnetic circuit can then be made thinner,
thus reducing the total weight of the motor.
An interesting development in this direction is the ‘Tango’
modellers’ motor from Kontronik. The 6-pole rotor (Figure
10) is surrounded by an iron-free self-supporting coil
as stator. This is enclosed in a thin-walled iron cylinder
which completes the magnetic circuit. The novel feature is
that this cylinder is mechanically linked to the rotor and
turns with it. There is thus no relative motion between the
magnetic field and the iron, minimising speed-dependent
losses. This is a brushless variation on the ironless core
motor which, thanks to the use of six poles, offers formidable
torque (see Figure 11).
External rotors
Of course there are limits to the number of poles that can
be used. As the magnets get smaller the windings also
have to be split into more and more segments. In itself
this does not cause any great problems, but it turns out
that as the poles of the magnets are sited closer and
closer together efficiency falls off. This is because part of
the flux finds its way to a neighbouring pole without passing
through the stator. As a result, the gearing relationship
between speed and torque does not hold for higher
pole counts. Greater spacing is required between the
poles, which implies that they need to be arranged in a
larger circle (see Figure 12). So as not to increase the
size of the enclosure, the design is turned inside-out: thin
permanent magnets on the outside, thick electromagnets
(the coils) on the inside. The result is a so-called ‘external
rotor’ (see Figure 13). One useful side-effect of this
arrangement is the greater leverage that the force produced
between stator and rotor has on the output of the
motor, which increases torque still further. The exterior of
the motor can no longer be held fixed, but there is the
advantage that the magnets, turning along with the exterior
part of the enclosure, are better cooled and therefore
less likely to overheat when the motor is overloaded.
Multi-pole exterior rotor designs, with their exceptional
torque, are pre-eminent among electric motors and strike
terror into the hearts of gearbox manufacturers. If a gearbox
is required, it is essential to ensure that it can withstand
the torque the motor is capable of producing. A
disadvantage of the exterior rotor design is that it is
harder to cool the stator, which now lies in the middle of
the motor. Copper and iron losses have to be managed,
and there is less space for the windings. The main application
area for this type of motor (Figure 14) is therefore
where brief or intermittent bursts of power are
required, such as in hybrid-drive cars and in electric
model aircraft. A special place is occupied by LRK
motors, which fulfil the requirements of modellers for
directly driving as large a propeller as possible. They feature
a very simple and therefore economical construction:
a free-running rotor with normally 14 magnets (ten magnets
is also possible) encloses a 12-part stator. A special
winding technique is used called ‘separated phase sectors’,
or SPS: here each phase is assigned to a separate
sector. This guarantees a very close magnetic coupling
between the two magnetic systems and a high speed
reduction ratio and correspondingly high torque.
Drives of the future
Power electronics and modern magnetic materials have
brought about radical, but practically unnoticed changes
elektor electronics - 2/2006