ROTOR RADIAL VENT PUMPING IN LARGE SYNCHRONOUS ELECTRICAL MACHINES

Darren Camilleri (Author)

Cummins Generators Technologies

Fountain Court, Peterborough, United Kingdom, PE2 6FZ

441733395520

darren.camilleri@cummins.com

Dr. Rob Rolston (Author)

Cummins Generators Technologies

Fountain Court, Peterborough, United Kingdom, PE2 6FZ

441733395439

robert.rolston@cummins.com

ABSTRACT

Fan power is a contributor to loss of efficiency in large electrical machines. Reducing fan power will also inadvertently result in a loss of pressure head. Conversely, requirements for optimal power density need sufficient pressure heads to maintain machine hotspots within insulation class limits. The pressure head and airflow provided by the rotor radial-vent pumping was thus investigated.

Vent rotation in the rotor creates a positive pressure head between vent entry and exit, the ‘pumping effect’ that sustains radial outward flow. Rotor vents make use of the pumping effect to increase airflow through the active material thus reducing hotspots. If vent pressure losses are minimised, total machine pressure head can be increased, resulting in improved overall machine cooling.

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Flow in radial venting has a complicated nature due to multiple flow phenomena. The radial vent geometry, axial through-flow velocity, rotational speed and rotor surface temperature can all have an impact. Numerical and experimental methods were found in literature describing ways to understand, analyse and optimise airflow through rotating radial venting.

A parametric computational airflow model was utilised for the investigation of key performance characteristics of the radial rotor vent: vent static pressure head ΔPstatic press. and flow-rate Qvent.

The control volume was reduced to a periodic model of a single rotor radial vent. This simplified model approach was insufficient to compute absolute significances of parameters because it did not consider the non-periodic and non-symmetrical effects that occur in a complete-360° model. However, it enabled efficient computation of a robust DOE containing multiple variables.

ANSYS Fluent 16.0 was used for the CFD simulations. The rotor radial vent baseline geometry was constructed and parameterised using ANSYS DesignModeler. The k-ε turbulence model with Enhanced Wall Treatment and a Moving Reference Frame (MRF) was chosen. For the MRF to be created, the rotor-stator gap was divided from the middle. The rotor domain (inner) was defined as rotating and the stator domain (outer) as stationary. A mesh interface was defined between the two domains. The inlet boundary parameters also operated as variable parameters. For the baseline model, inlet values were defined using the design requirements for the machine.

A 2-level DOE factorial design was selected because of its efficiency at calculating both the effects and possible interactions of multiple variables. Minitab was used to construct the DOE with a total of 32 design points. Through variable parameterization in DesignModeler, the ‘Parameters Set’ table in Workbench was enabled. All the input and output parameters in the CFD model, including all the independent and response variables, were automatically included in this table. The design points were manually entered.

A Regression Analysis on the CFD data was produced using Minitab. A One-Way Analysis of Variance (ANOVA) was performed showing the significance of all independent variables in relation to each response

variable. The p-value statistic was used to determine whether an independent parameter was significant. The α specified for this study was 0.05. The p-value analysis proved inconclusive in determining the levels of significance of the variables in this case. Therefore, the F-statistic was used to further investigate the relative variable significance.

Radial vent length was the most significant variable for ΔPstatic press. This complies with the theoretical maximum dynamic pressure recovery to static pressure. Qvent appears to be significantly dependent on the vent cross-sectional area. Increasing the vent cross-sectional area will increase the rotor surface area in contact with airflow. Increasing radial vent length will also have the same effect.

SUGGESTED THEMES

Electrical machines, radial vent pumping, cooling, CFD, numerical, regression analysis, DOE, parametrization