Introduction to the Generator II

A large number of modern, small and medium capacity alternators employ the totally enclosed air cooling method.

Air cooling has the advantage of minimal capital cost due to the absence of a need for special shaft sealing mechanisms and the constant make-up demands of a hydrogen cooling system.

The internal cooling air circuit is sealed to ensure that airborne dust can be excluded from the cooling system and therefore does not deposit on the windings or in the ducts. If dust is allowed to accumulate on the windings the heat transfer process is hindered and the dust layer can absorb moisture and/or oil leading to failure of the insulation.

A totally enclosed system also ensures that moisture contained in normal ambient air is not allowed to enter the alternator provided that the alternator is initially charged with air of a low dewpoint.

The alternator cooling circuit consists of the following components:

The alternator frame not only acts as the support for the stator assembly but also acts to reduce noise transmission and to provide a passage or passages through which the cooling air circulates. Baffles and partition walls direct the cooling air through a path designed to ensure optimum heat transfer is maintained within the alternator. Typical single and double pass cooling circuits are shown in Figure 1. Shaft mounted fans within the alternator frame are used to ensure that a positive cooling air flow is maintained through the cooling circuit while ever the alternator is in service.

Figure 1:  Axial Fan Mounted on a Generator Rotor Shaft 

In some cases counterweighted dampers are used within the cooling circuit to allow the fan to run up to speed under a no load or minimum load condition. As the fan reaches normal operating speed the discharge pressure of the fan forces the damper closed and the entire cooling circuit is then connected to the fan discharge.

Radial ventilation ducts form part of the stator core and allow the cooling medium to pass through the stator from the air gap to the cooling passages within the outer alternator frame.

Axial ventilation ducts are formed at the bottom of the rotor slots below the windings and radial ventilation holes are formed through the winding bundles to allow a passage for the cooling medium throughout the rotor. Figure 2 shows the flow path through the rotor while Figure 6 shows the general pattern of flow throughout the alternator.

Figure 2:  Section through Rotor Showing the Axial and Radial Ventilation Passages.

Heat exchangers are required to remove the heat from the internal cooling circuit and dissipate it to atmosphere through an external cooling medium. This may be through an air to air or an air to water heat exchanger.

Figure 3: End View of Alternator with Air/Air Heat Exchanger.

As the output of the generator increases the choice of water/air heat exchangers becomes more prominent.

The heat exchangers are located within the alternator frame either side or bottom mounted. The air circuit may be directed through a single pass of the heat exchanger or through multiple passes to maximise heat transfer throughout the alternator.

Figure 4:  Showing Finned Tube Water Cooled Heat Exchanger. Note the Drain and Vent Lines Provided on the Inlet and Outlet Manifolds.

The cooling water is normally filtered water supplied from the turbine auxiliary cooling water system, but may be supplied by an independent cooling water system.

The generator air cooler inlet and outlet water-boxes are fitted with vents and drains to allow any entrained air to be vented from the system and to allow the coolers to be drained for maintenance (See flanged connections in Figure 4).

Liquid detectors are provided in the lower points of the frame to monitor any ingress of water into the generator due to tube leakage from the coolers.

The cooling water inlet and outlet temperature and the air temperature within the frame are normally monitored with an alarm being initiated if high temperatures are detected.

Figure 5:  End View of Alternator Enclosure Showing the Bolted Access Doors to the Water-Cooled Heat Exchangers Mounted Within.

Figure 6:  Typical Single and Air Cooling Circuits Showing Passage of Air through the Rotor and Stator Elements.