1150hp through Ventilated Motor
Excerpt from a paper written by Dr David Staton with Mawdsley Ltd (Gloucester) for ICEM 2002 conference in Belgium. Click here for a downloadable version of the document
This article describes how Motor-CAD was used to model the air flow and heat transfer in a 1150hp through ventilated induction motor used in an offshore oilfield drilling application. A picture of the machine is shown below:
The motors geometry is input into Motor-CAD as shown in Figs 2 and 3. As air flow and heat transfer are both very 3 dimensional in nature both the radial and axial cross sections of the motor must be described.
It is not practical to model each individual conductor so the slot-liner/copper/insulation layered winding model illustrated in Fig. 4 is used. This enables the prediction of winding hotspots in the centre of the coil and the estimation of the short time constant associated with the winding rather than that of the whole motor. The number of layers and copper to insulation thickness ratio is set by the wire diameter, number of turns and subsequent slot-fill. The winding diagrams are useful for gaining a visual indication of the slot-fill which can be achieved.
Thermal resistance values for all conduction paths within the motor are calculated automatically from motor dimensions and material data. Convection and radiation are also automatically accounted for in the calculation. Radiation is just dependent upon the emissivity of the exposed surfaces. Convection is calculated from proven laminar and turbulent convection correlations.
Local air velocity is required in order to calculate the local convective heat transfer. This is calculated by solving a separate network of flow resistances. A flow resistance depends upon the fluid friction at the wall surface and changes in flow condition - such as expansions and contractions in the flow circuit and restrictions due to obstructions in the flow path. The changes in flow condition typically predominate in electrical machines as the flow path is relatively short. Fig 5 shows the intersection of the fan (blower) characteristic and system flow resistance characteristic for the motor being considered here. The resulting total volume flow rate is calculated to be 3500CFM compared to a measured value of 3300CFM. The flow network also calculates the proportion of the air that passes through the stator ducting, rotor ducting and airgap. The pressure drop increase due to the rotation of the rotor ducts is taken into account in the calculation. Temperature and power flow predictions within the motor are shown in the schematic diagram shown in Fig. 6. Excellent agreement with test was achieved, the measured winding hot spot being 157C and the calculated value being 161C - when cooled by 3300CFM from the external blower.
