1.1.3 Classification by Cooling

Another important means of classifying machines is by the type of cooling medium they use: water, air, and/or hydrogen gas. One of the main heat sources in electrical machines is the DC or AC current flowing through the stator and rotor windings. These are usually called I2R losses, since the heat generated is proportional to the current squared times the resistance of the conductors (almost always copper in stator windings, but sometimes aluminum in SCI rotors). There are other sources of heat: magnetic core losses, windage losses, and eddy current losses. All these losses cause the temperature of the windings to rise. Unless this heat is removed, the winding insulation deteriorates and the machine fails due to a short circuit.

Indirect Air Cooling. Motors and modern generators rated less than about 100 MVA are almost always cooled by air flowing over the rotor and stator. This is called indirect cooling since the winding conductors are not directly in contact with the cooling air due to the presence of electrical insulation on the windings. The air itself may be continuously drawn in from the environment, that is, not recirculated. Such machines are termed open-ventilated, although there may be some effort to prevent particulates (sand, coal dust, pollution, etc.) and/or moisture from entering the machine using filtering and indirect paths for drawing in the air. These open-ventilated machines are referred to as weather-protected or WP.
       A second means of obtaining cool air is to totally enclose the machine and recirculate air via a heat exchanger. This is often needed for motors that are exposed to the elements. The recirculated air is most often cooled by an air-to-water heat exchanger in large machines, or cooled by the outside air via radiating metal fins in small motors or a tube-type cooler in large ones. Either a separate blower motor or a fan mounted on the motor shaft circulates the air. IEC and NEMA standards describe the various types of cooling methods in detail [1.4, 1.5].
       Although old, small generators may be open-ventilated, the vast majority of hydrogenerators and turbogenerators (rated less than about 50 MVA) have recirculated air flowing through the machine. Virtually all hydrogenerators use recirculated air, with the air often cooled by air-to-water heat exchangers. For turbogenerators rated up to a few hundred megawatts, recirculated air is now the most common form of cooling.

Indirect Hydrogen Cooling. Almost all large turbogenerators use recirculated hydrogen as the cooling gas. This is because the smaller and lighter hydrogen molecule results in a lower windage loss and better heat transfer than air. It is then cost effective to use hydrogen in spite of the extra expense involved, due to the few percent gain in efficiency. The dividing line for when to use hydrogen cooling is constantly changing. In the 1990s, there was a definite trend to reserve hydrogen cooling for machines rated more than 300 MVA, whereas in the past, hydrogen cooling was sometimes used on steam and gas turbine generators as small
as 50 MVA [1.6, 1.7].

Directly Cooled Windings. Generators are referred to as being indirectly or conventionally cooled if the windings are cooled by flowing air or hydrogen over the surface of the windings and through the core, where the heat created within the conductors must first pass through the insulation. Large generator stator and rotor windings are frequently “directly” cooled. In direct-cooled windings, water or hydrogen is passed internally through the conductors or through ducts immediately adjacent to the conductors. Direct water-cooled stator
windings pass very pure water through hollow copper conductors strands, or through stainless steel tubes immediately adjacent to the copper conductors. Since the cooling medium is directly in contact with the conductors, this very efficiently removes the heat developed by I2R losses. With indirectly cooled machines, the heat from the I2R losses must first be transmitted through the electrical insulation covering the conductors, which forms a significant thermal barrier. Although not quite as effective in removing heat, in direct hydrogen-cooled windings the hydrogen is allowed to flow within hollow copper tubes or stainless steel tubes,
just as in the water-cooled design. In both cases, special provisions must be taken to ensure that the direct water or hydrogen cooling does not introduce electrical insulation problems.
See Sections 1.4 and 8.13.
       Direct water cooling of hydrogenerator stator windings is applied to machines larger than about 500 MW. There are no direct hydrogen-cooled hydrogenerators. In the 1950s, turbogenerators as small as 100–150 MVA had direct hydrogen or direct water stator cooling. Modern turbogenerators normally only use direct cooling if they are larger than about 200 MVA.
       Direct cooling of rotor windings in turbogenerators is common whenever hydrogen is present, or in air-cooled turbogenerators rated more than about 50 MVA. With the exception of machines made by ASEA, only the very largest turbo and hydrogenerators use direct water cooling of the rotor.