Magnetic suspension systems, motor/generators and power electronics for flywheel energy storage
A flywheel energy store comprises a motor/generator driving a flywheel where the noload
losses are reduced so that half of the stored energy remains available after six
hours of coasting. Analysis of the permitted level of losses is followed by discussion of
how the losses can be minimised. The merits of induction and permanent magnet
machines are compared in terms of their coasting losses.
There is discussion of zero-power magnetic bearing systems developed to suspend
vertical-axis energy-storage flywheels. With permanent magnet material in the
magnetic circuit, an electronic control system can hold the airgap at the otherwise
unstable point of equilibrium, where the attractive force due to the magnetic flux
exactly balances the weight of the flywheel. Under steady state conditions the power
dissipated in the control coil is virtually zero.
The torque generation characteristic of an induction machine is explained from first
principles. This is followed by an equivalent circuit analysis that is specific to
operation at a constant flux level with a variable supply frequency and voltage. The
supply voltage for constant flux operation is specified as a function of the rotational
speed and the absolute slip frequency.
An all-digital, three-phase inverter developed for use with an induction motor gives
four-quadrant control of the torque applied to a load. A low cost incremental shaft
encoder and straightforward digital logic allows precise and instantaneous generation of
absolute slip frequency. A modified PWM switching topology minimises switching
losses and maximises the utilisation of the DC link voltage. The inherent speed of the
all-hardware design and its precise control of slip frequency, make it ideal for highspeed
motors or for drives requiring frequent and rapid reversal.
A new design for an ironless stator winding for a permanent magnet motor/generator
minimises electromagnetic drag when coasting. The winding meets the predicted
reduction in drag torque and provides an extremely pure sine wave output with very
low source impedance.