Engine & Turbine Technology
straightforward to calculate the harmonic levels for a particular design.
Te behaviour of the design in the event of a short
circuit was another important consideration. Short circuits may be caused by mechanical failure in the generator, insulation breakdown or power converter malfunction. Engineers studied the magnetic field generated in each area of the permanent magnet in a short circuit, expecting to ensure that the magnet had the right properties to avoid any damage. Generally when designing a PMG, Indar engineers consider a wide range of factors, including the contribution of magnet temperature, rotational speed, switching frequency and short-circuit (two-phase and three- phase) performance to achieve ideal magnet behaviour for the entire lifetime of the generator circuit. In the full-load simulation, engineers looked at the input required by Ingeteam frequency converters for the available switching frequencies to achieve nominal torque, high current and low losses. Te team examined induction levels in the stator because of their important effect on efficiency. Tough high induction levels make it possible to reduce the size of the generator, they also increase iron losses. Maxwell results showed the distribution of losses over the geometry of the stator, providing guidance for design changes to improve efficiency. Indar engineers continually modified the design, attempting to reduce losses in the stator copper, mechanical losses, and losses created due to switching frequency when working with frequency converters – all while achieving the other design requirements. Because of the magnet field’s high strength even when the PMG is not rotating, Indar engineers simulated the process of assembling and balancing the rotor, ensuring it could be safely accomplished. Tey determined the level of magnetic forces generated while inserting the rotor, which made it possible to specify assembly tools that could withstand these forces. Te generator rotor is balanced by placing it on pedestals instrumented with accelerators that detect forces generated by imbalance in the rotor. Te electromagnetic field generated by the rotor during this process was simulated using Maxwell to ensure it did not interfere with the cables carrying the accelerometer signals. Indar engineers simultaneously studied the
generator’s cooling system because of the interaction between electrical and thermal performance. Te temperature of the magnet plays an important role in its ability to resist demagnetisation, so improvements in cooling performance can increase the magnet’s ability to handle a short circuit. Optimisation of the cooling circuit helps to
improve efficiency by reducing mechanical and cooling losses. To optimise the cooling circuit, engineers used ANSYS Fluent fluid dynamics software to perform a detailed study of fluid flow and heat transfer in and around the generator. Meshing was a challenge because of the difference in scale between the small 5 mm to
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10 mm air gap between the rotor and stator, where accuracy was critical, and the large overall 1 meter length of the generator and cooling system. To minimise computational time, 3D steady-state analysis was used during the majority of the design process, and the model size was reduced by using axial symmetry and periodic conditions. Fluid dynamics results included the local heat transfer coefficients, air flow velocity at every point in the machine circuit, pressure drop of the air circuit through the generator, generator temperature and thermal profile, and magnet working temperature. Engineers used these results as a guide in decreasing temperature hotspots by reducing variations in cooling over the length of the generator. using software from ANSYS helped Indar to easily explore multiple automated parametric design variations. Te electromagnetic and fluid flow simulations
provided far more diagnostic information than was available from physical testing. Simulation provides results for any output at any point in the computational domain, while physical testing provides results only at locations where it is practical to locate sensors. Engineers were able to iterate to a design that
Fig. 2. (Above) Magnetic behaviour in a three-phase short circuit as predicted by Maxwell.
Fig. 3. (Below) Electromagnetic simulation of the balancing operation and inserting the rotor in a stator with a crane.
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