Reduction of cogging forces in a double sided tubular linear permanent magnet generator used for ocean wave energy conversion
Date
2013-01-30
Authors
Schmulian, Rael
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Abstract
An increased awareness of the environment and the growing energy needs of countries
like China and India have accelerated the search for sustainable alternatives to fossil
fuel energy sources. Generating electricity from ocean waves is a viable alternative.
This is because energy densities are sufficient to supplement the grid and thus make
it economically viable. The seasonal supply profile also matches the demand profile.
There are currently many different devices operating on different principles, the most
promising of these is the oscillating body device using a linear generator.
Linear generators are able to convert linear motion into electrical energy without
the need for intermediate gears, screws or crank shafts. This increases the overall
efficiency and makes the device more reliable. Static force simulations were done on
a novel linear generator design. The generator combines the duel air-gap of a double
sided topology with the encapsulation of the magnetic field in a tubular topology.
This results in an increased force density, but also large cogging forces. The cogging
forces consist of end-effects cogging forces that are caused by the interaction of the
Permanent Magnets (PMs) with the stator ends, as well as cogging forces which are
due to the interaction of the PMs and stator slots. The topic of this thesis is to find
suitable methods of reducing the cogging forces.
The end-effect forces were reduced by changing the stator length and altering the
stator ends shape. Creating a quadratic b’ezier curve end shape and optimising
the stator length reduced the end-effect cogging force from 630 N to 7 N. The slot
cogging forces were reduced by dividing the PMs into two sections and to shift the
outer PMs by one slot pitch relative to the inner PMs. This resulted in a decrease
from 663 N to 25 N. However, a compromise needs to be made between reducing
the cogging forces and not reducing the machine performance. Increasing the stator
length increases the machine volume and dividing the magnets reduces the thrust
force. This needs to be taken into account when designing a new machine.