Advanced Electric Machine Design with Electromagnetic and CFD Simulations

You may have read a quick blog post at Desktop Engineering about ANSYS’s electric machine simulation capabilities. Here we dive into the technical aspects and implications of thermal simulation for electric machines.

Electric machine geometry with cooling and integrated power electronics.

Electric machine geometry with cooling and integrated power electronics.

Modern electric machines are designed to meet a wide range of applications, all facing a variety of different technical challenges. They are designed to be compact with high power densities, to have integrated power electronics, to be high-speed for higher power density, and to handle harsh environments.

These challenges all have thermal implications that affect the lifetime and performance of the electric machine and power electronics, and must be balanced with cost goals. ANSYS simulation tools, Fluent and Maxwell, can be used to predict the thermal and electromagnetic performance of these systems, and can therefore be used to optimize design choices for both thermal and cost considerations while meeting all application objectives.

There are many loss mechanisms in an electric machine, and good estimates of loss quantities are necessary to predict performance and temperature. ANSYS Maxwell can be used to calculate electric and magnetic losses. These losses include ohmic losses in the stator windings, core losses in the steel, solid losses in the permanent magnets and solid rotor bars. Several models are available for core losses, including a dynamic transient model, a frequency-domain loss model, and user-defined core loss capabilities.

Using the ANSYS Workbench integrated simulation environment, these electric and magnetic losses can be mapped directly from Maxwell to Fluent. Further mechanical losses such as windage and bearing loss can be added to the thermal simulation according to the operating speed.

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ANSYS Workbench environment connecting Electromagnetic and CFD simulations.

Time-averaged transient core losses calculated in Maxwell and mapped directly into Fluent.

Time-averaged transient core losses calculated in Maxwell and mapped directly into Fluent.

Electric machine cooling is a prime topic for CFD simulations where the design of air-flow or fluid-flow is of importance. There are many electric machine cooling solutions, including our partner Motor-CAD, and CFD is not always the answer from a simulation efficiency perspective. However, there is no better tool to determine the complex flow-paths of blowers or turbulent heat exchangers than ANSYS Fluent. In many applications an integrated fan is used to cool the electric machine and even integrated power electronics, and the thermal implication is to keep the critical electronics components (e.g. capacitors and varistors) under acceptable temperature limits.

For electric machines, the thermal implications are often directed at the winding insulation, where increased temperature results in shortened operating lifetimes or increased material cost, and so tremendous efforts are afforded to cool the windings including spray cooling and internal forced-air ducts. Liquid cooling may be required when high power-densities are the goal, and heat-transfer can be improved by proper design of turbulent flow within the liquid channel, and these flows can be studied and optimized through simulation with ANSYS Fluent.

Electric machine temperature and cooling simulation results.

Electric machine temperature and cooling simulation results.

The electric machine lifetime and performance can both be affected by temperature increases. Lifetime is decreased by thermal cycling and fatigue due to degradation. The winding insulation is particularly susceptible, and although high-temperature insulation exists, is a more costly option. Also thermal expansion of steel parts may cause separation of bonded surfaces, which can further degrade thermal performance. The torque output of the machine will also be dependent on temperature — this is true for all types of machines, although with a variety of implications.

Stator winding resistance has a nearly linear dependence on temperature through the copper’s electrical conductivity temperature coefficient. Induction machines have a similar temperature dependence of rotor bar resistance, which affects their operating performance. Permanent magnet machines have a temperature dependence which will affect the maximum operating torque. Within the simulation, the Fluent temperatures can be mapped back into Maxwell to provide a precise spatial variation of temperatures, which has a dramatic effect on permanent magnet fields, including possible irreversible demagnetization. These temperature effects have a direct implication on the maximum peak and continuous operating power for the electric machine.

ANSYS Maxwell is used by design engineers to evaluate the magnetic performance effects of temperature for peak and continuous operating power conditions. For more information check out Comprehensive Multiphysics Design for Electric Machines.