Submersible Motor

Motor-CAD was used to calculate the thermal transient performance of a submersible motor used to manoeuvre an under water camera.

Motor-CAD was used to calculate the thermal transient performance of a set of electric motors used to drive propellers on the remotely operated submersible craft. The Motor-CAD model for the brushless permanent magnet motor is shown below.

In this case there are 18 slots and 6 poles. The end windings are potted to help protect again the water and to aid heat transfer. A stator sleeve is also used to help protect the winding from contact with the water. The rotor magnets have a retaining can. The thermal resistance network for the motor is shown below. The colours used the schematic are the same colours as used in the cross section editors. Horizontal resistances represent axial heat transfer and vertical resistances radial heat transfer.

When the craft is moving there is very good cooling of the motor housing and it is held close to the temperature of the water. The internal winding heats up. When the submersible operator has finished he brings the craft to the surface and turns off the motors. The motor losses are then zero so the winding starts to cool down. At the same time the operator picks the craft out of the water causing the housing cooling to revert to natural air convection. Natural air convection is much less efficient than water cooling so the housing starts to heat up. The analysis was carried out to make sure that the housing does not get to such a high temperature that it will burn the operators hands and he will drop the craft back into the water. In the graph below we plot the variation in winding and housing temperature with time.

The mathematics used for calculating convection in Motor-CAD is based on dimensionless analysis correlations. This has a major advantage that all the convection formulations can be used with any fluid, e.g. gas such as air or liquids such as water and oil etc. All that is important in the calculation is the properties of the fluid, i.e. thermal conductivity, density, specific heat capacity and viscosity.