System for heating an electrical machine

In an electrical machine comprising an electrical winding 1204 which includes a fluid channel for coolant circulation within the winding, a system is provided for selective heating the coolant fluid to increase the temperature of the windings above the ambient temperature by means of a temperature controlled fluid source in fluid communication with the fluid channel. A heat sink 1220 comprising a fan assisted heat exchange and a heat source 1218 comprising an electrically heated chamber are provided in parallel in the fluid circuit and valves 1214,1216 and the heating element 1222 are controlled in response to thermal sensors 1232,1234 by controller 1230. Alternatively the arrangement may comprise three fluid circuits where the heat sink and heat source heat exchangers 1258b,1258a are arranged in series in the fluid circuit connected to the windings 1204. The heat source exchanger is heated by fluid from a gas fired water heater 1260 and the heat sink may be cooled with sea water or another environmental source. Alternatively a single heat exchange may be associated with the cooling or heating fluids. The winding circuit and the heat source fluid may be deionised water. The system prevents a wind turbine generator freezing when not operating.