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Cleaner Propulsion For Small Craft

01 November 2003

It is remarkable that at the turn of the 19th century there were as many
as a hundred electrically powered launches operating on the River Thames.
Since that time, electric boats never entirely went away and today,
thanks to modern dc motors and future power source developments, there
are strong environmental reasons for a comeback. Simon Hunt explains

BBrushed dc motors are available with many designs of stator, offering a
range of field excitations, including series, shunt, separately excited,
compound, and permanent magnet. Each of these various field topologies
has its own performance characteristics* and is suited to specific
applications. The inverse exponential torque-speed characteristic of the
series wound motor, for example, makes it ideal for applications
requiring a high starting torque and a diminishing torque demand with
speed. A typical application is the bow thruster, mounted submerged, on
or under the hull of a boat and used for manoeuvring the craft in
confined spaces such as busy harbours. Here the performance of a series
motor is ideal, since a high starting torque is needed to overcome the
inertia of the craft.

When considering main propulsion drives for boats, other design issues
need to be addressed to provide higher power output. Typical inboard
marine propulsion motors operate continuously at speeds in excess of 800
rpm. In this application, a series field motor can be equipped with
compensating windings known as interpoles to suppress the armature
reaction field, which considerably improves the commutation. Dissipation
of heat in the motor housing is a major constraint on maximum power, and
assisted cooling is often needed. For inboard-mounted propulsion motors,
forced air-cooling provides the best solution, while submersible outboard
motors benefit directly from the water's ambient temperature.

Separately excited and shunt motors are favoured for constant speed
applications, such as winches. In a separately excited brushed dc motor
the field current is supplied from a separate power supply, while in a
shunt dc motor the field circuit is directly powered from the armature
terminals. The performance of a shunt motor is identical to that of a
separately excited motor in which the field is supplied by a
constant-voltage source. Both provide a flat torque-speed characteristic,
resulting in a motor speed that is almost independent of load variations.

In these motor types, the armature current is proportional to the square
of the torque, which makes them much more sensitive to overloading than
with series wound types. The flat torque-speed characteristic and the
potential problems with speed runaway make them unsuitable for marine
propulsion, but these windings are commonly used in marine winches, sail
hoists and generating systems.

Permanent magnet dc motors have characteristics similar to separately
excited motors, but with a constant flux field. Ferrite magnets provide
the cheapest option for motors up to 4kW, while more powerful bonded
NdFeB magnets can be used up to 12kW. Permanent magnet motors are usually
used for applications that require a cheaper and more compact design (for
example, bow thrusters in a confined hull space).

The future for electric boats looks buoyant. Advances in technology such
as higher efficiency solar panels can only increase their popularity,
while the arrival of commercial fuel cells is likely to lead to dramatic
increases in use.

Simon Hunt is with Astrosyn International Technology. The company has a
range of motors suitable for marine applications, some of which are
illustrated here.

*Readers wishing to learn the fundamentals of dc motor operation are
subject, which ended in September 2003. All articles are available on
this site.

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