Electrical Machines

Classification of Motors and Generators

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.

Insulation of Wire Winding and Harpin 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 and a high power density. The windings are made of quadrangular conductors with a cross-sectional area of a few square centimetres, also called Hairpin windings. They can be winded only per strong mechanical deformation. The insulation is realized with different layers of materials with a thickness of a few tenths 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 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.
The winding resistance is temperature-dependent, and accordingly, an improvement of the cooling would bring about a decrease in the copper losses. Material costs are high; therefore, materials should be used slightly below their thermal and mechanical limits. Through a more efficient cooling process, the current density can be increased, and the power output can be increased by maintaining the machine dimensions.
The cooling system must ensure homogeneous temperature distribution during the start-up, as excessive thermal stress provokes cracks.
All these interrelations explain the use of the CFD analysis. With the help of the results of a global or a partial analysis, electrical machines companies can offer cheaper machines without exposing themselves to technical risks.

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

Assembly of the turbo rotor of a large synchron generator  Rotor assembly