Electrical Machines


Motors and Generators are electrical machines. They can be categorized in types: induction machines, synchronous machines and direct current machines, as well as in rated powers: smallest motors (till 1 kW), small motors (1 kW till 0.75 kW), medium motors (0.75 kW till 375 kW) and large motors. 
Synchromotors either have electrical excitation through the rotor winding or they have permanent excitation with permanent magnets.

Winding

Small motors have a low voltage, the conductor is a lack isolated copper wire with a diameter between 12 mm and a few millimetres. The conductor might be winded per hand or with a winding machine.
Large motors have a medium voltage between 1 kV and 52 kV, the square-cut windings have a cross-sectional area of a few square centimetres and they can only be winded per strong mechanical deformation. The insulation is realized with different layers of materials with a thickness of a few tenth of millimetres. The winding of electric motors in the stator and in the rotor can consist of a few bars on top of each other.

 
 Rotor of a small low-voltage motor with concentated winding and lack-insulated copper wires
Rotor of a small motor with concentrated winding

 Cut stator winding of a large motor with distributed winding Cut stator winding of a large motor with distributed winding

Energy efficiency norms

The IECC European norms on efficiency will include more and more industry segments in the coming years. In order to stay below the temperature limits, the electrical machine companies will need to invest in more efficient cooling concepts. A better cooling design is mostly reserved for newer machines; however, a retrofit usually offers the possibility to make the cooling concept more efficient and thereby increase the power output.  

Heating and life duration

Electrical machines are highly efficient; the electrical losses reach however such high levels that the produced losses must be removed. The winding resistance is temperature-dependent, and accordingly, an improvement of the cooling would bring about a decrease in the copper losses. Due to the high material costs, it is desirable to use this to its limit: through a more efficient cooling process, the current density can be increased and the power output can be increased by maintening the machine dimensions.
The cooling system has to ensure homogeneous temperature distribution during the start-up, as excessive thermal stress provokes cracks. The life expectancy of the winding isolation strongly depends on temperature; an increase of the temperature level by 5 to 8 Kelvin results in a halving of the life expectancy. The insulation materials are therefore categorized in insulation classes and each class corresponds to a maximal allowable temperature by continuous operation of the machine.
All these interrelations explain the main application area of CFD analysis. With the help of the results of a global or a partial analysis, the electrical machines companies can offer cheaper machines without exposing themselves to technical risks.

Erectiion of the Hydrogenerator Rotor of the Unit 18A of Itaipu
Erection of the Rotor of a Hydro Generator © Hydropower Consult


Rotor assembly