GB2450758A

GB2450758A - Device for reducing moisture in a rotating electrical machine

Info

Publication number: GB2450758A
Application number: GB0713194A
Authority: GB
Inventors: William Brian Turner; Philip David Bend
Original assignee: Cummins Generator Technologies Ltd
Current assignee: Cummins Generator Technologies Ltd
Priority date: 2007-07-06
Filing date: 2007-07-06
Publication date: 2009-01-07
Also published as: GB0713194D0

Abstract

A device for reducing moisture in a rotating electrical machine 60 comprises means for defining a channel 62 for air to flow towards the machine, wherein the channel comprises a desiccant. The rotating electrical machine may comprise a housing structure 61, and an electrical device, for example an electrical winding 71. Preferably the channel comprises means for opening and closing the channel, such as louvres, wherein the channel is arranged to be open when the electrical machine is idle. Means for heating 64 the desiccant may be provided along with means for creating turbulent airflow in the channel, such as baffles. A window may be provided so that a portion of the desiccant can be visually inspected whereby the colour of the desiccant can be dependent, in part, on the moisture content. Preferably the device forms a desiccant breather and is arranged so the air that passes into a generator passes through the breather and is dried. The breather may be provided on a wind turbine for keeping the internal components of the turbine dry, especially when alternative means for drying air, such as an anti-condensation heater are unavailable due to a power outage.

Description

1 2450758

AIR SUPPLY TO A ROTATING ELECTRICAL MACHINE

The present invention relates to rotating electrical machines, such as motors and generators. There is a need to keep the internal components of rotating electrical machines dry in all operating conditions. Otherwise electrical components may become damaged and/or may need to be replaced.

There is a natural risk of moisture contamination in ventilated rotating electrical machines as a result of daily temperature cycles which can cause dew to form.

Typically, the air temperature may rise rapidly in the morning, meaning that solid components have the potential to be colder than the atmospheric dew point temperature; this causes dew to form. This is generally not a problem for electrical machines when they are in use, as inefficiencies in the machine tend to increase the temperature of electrical components above the atmospheric dew point. It is a problem, however, when the machine is idle, especially if this is for long periods.

Moisture on live generator or motor windings can cause undesirable flows of current on the insulation that typically covers windings. These flows of current, due to moisture, may produce partially conducting paths as a result of electric leakage on the insulation surface; this process is generally referred to as tracking.

If condensation is allowed to persist then tracking effects can lead to low insulation values and eventual failure. Even before failure on high voltage machines, discharging or corona can cause damage to windings over months and years.

The standard solution for averting the formation of condensation in ventilated rotating electrical machines is to provide an anti-condensation heater. The anti-condensation heater (or heaters) may draw power from an auxiliary power source.

Generally, an anti-condensation heater works to raise the temperature of windings to be around 5 C above the ambient. This is generally sufficient to stop the temperature of the windings falling below the atmospheric dew point temperature.

An alternative solution is to attempt to make a completely enclosed rotating electrical machine. In such a machine, air is contained completely within the machine so that the enclosed air is not prone to atmospheric humidity variations.

While desirable, it is almost impossible to create a completely closed machine without employing expensive sealing methods. For low cost solutions, air exchange between the interior and exterior of a machine is inevitable, even if the sealing appears to be very good. This is driven, in part by pressure differentials which are created during daily temperature cycles, if the air in the device is warmer than the surrounding atmosphere, then there will typically be a positive pressure and air will tend to bleed out of the device through micro-channels. By contrast, when air in the device is cooler than the surrounding atmosphere, air will tend to be drawn into the device. This effect means that, eventually, the air in the device will have approximately the same humidity as the ambient air. Thus, closed rotating electrical machines may be prone to many of the same condensation problems as ventilated rotating electrical machines, and an anti-condensation heater may be required for reliable operation and to avoid the formation of condensation.

A problem exists in rotating electrical machines which are not arranged to generate electricity continuously. In such machines, and where there is no auxiliary power supply, it may not always be possible to provide power to an anti-condensation heater. This may occur, for example, during the prolonged maintenance of a generator, or in a rotating electrical machine that is operated seasonally, for example only in winter for snow making. In such systems, where it is not always possible to operate an anti-condensation heater, there may be moisture formation on the generator windings. As discussed, this may cause damage to the windings, leading to the eventual failure of the machine.

According to an aspect of the present invention there is provided a device for reducing moisture in a rotating electrical machine, comprising: means for defining a channel for air to flow towards the machine, wherein the channel comprises a desiccant.

In this way, atmospheric air may pass through the channel and be dried by the desiccant as it flows towards any electrical devices in the rotating electrical machine. Thus, condensation on the internal electrical components of a rotating electrical machine may be averted, even when no electrical power is available.

According to another aspect of the present invention there is provided a rotating electrical machine comprising: means for defining a channel for air to flow towards the machine, wherein the channel comprises a desiccant.

By providing a desiccant in a rotating electrical machine, it is possible to prevent damage occurring to electrical components during periods of power outage, and to prevent a build up of moisture that could cause damage when a power supply to the rotating electrical machine is restored. The overall effect of the present invention may be to prolong the life of rotating electrical machines; this may be especially advantageous where access to the equipment is difficult and/or expensive.

Preferably the rotating electrical machine comprises a housing structure defining an interior and an exterior. The housing structure may define a portion of the channel, and the channel may also be defined, at least in part, by a device which is attachable to the housing. The housing structure may be a single integrated shell or else may be made up of several different components. For example, some of the internal components of the machine may also form part of the external surface of the machine and, as such, may be considered to be components of the housing structure.

The effectiveness of the desiccant at drying air in the channel depends on a number of factors, including: the relative humidity of atmospheric air, the internal machine air volume, the number of days that the rotating electrical machine is without power, and the typical day to night temperature differential. These factors affect either the volume of air that passes the desiccant andior the amount of absorption required of the desiccant. It may be possible to calculate the length of time that the desiccant will be effective in anticipated operating conditions.

Preferably the desiccant is designed to be effective for a predetermined number of days during which the rotating electrical machine is without power. Thus, an operator may be aware of how long they can safely deprive a rotating electrical machine of electrical power.

Generally speaking a rotating electrical machine may comprise many electrical devices. These may include generator or motor windings, switches, transistors, resistors, capacitors, diodes and so on. However, as previously discussed, it may be most preferable to protect any electrical windings from moisture. Any electrical device may be arranged in the housing interior.

The channel comprising a desiccant may represent the path of least resistance for air passing into and/or out of the machine while the machine is idle; that is to say, while the machine is not operable, as a motor or a generator, for example. The machine may comprise a further channel for an airflow. The further channel may represent the path of least resistance for air passing into and/or out of the machine when the machine is in use. Preferably the further channel does not comprise desiccant. The or each further channel may comprise a valve for opening or closing the channel, as required. The channel comprising desiccant may also comprise a valve, operable for opening or closing the channel.

The machine may comprise an entrance channel and an exit channel for air to flow into and out of the machine respectively, while the machine is in use.

Preferably the entrance channel and the exit channel comprise valves which are arranged to close the channels while the machine is idle, and to open the channels when the machine is in use. A valve in the channel comprising desiccant may be arranged to close the channel when the machine is in use, and to open the channel when the machine is idle. The entrance channel and exit channel may be in the form of sealed louvres.

By having openable and closable entrance and exit channels, the machine may be operable as an open ventilated machine during use, and as a closed machine when idle.

The machine may comprise cooling means operable when the machine is in use.

Possible cooling means include a fan and/or air to water or air to air heat exchangers.

Preferably the housing structure is substantially closed when idle, other than for the channel comprising a desiccant. Thus, the channel may be the path of least resistance through which air can flow into or out of the housing. During daily air cycles in the machine, due to normal temperature variations, air may be forced past desiccant in the channel; in this way, the air that passes into the housing structure can be dried by the desiccant. The housing may be substantially closed without being completely closed, as some micro-channels may exist inevitably and allow limited air seepage into or out of the housing structure.

The rotating electrical machine may further comprise means for heating the desiccant. Over time the desiccant may become saturated with moisture and/or other matter such as salt, from the air passing through the channel. By heating the desiccant, the desiccant's absorptive and/or adsorptive properties may be restored.

As such, the heating means may be considered to be regenerative of the desiccant's properties. Preferably the desiccant is heated by a dedicated electrical heater or heaters that is in or adjacent the channel. However, the desiccant may also be heated by air flowing out of the machine that has been warmed by inefficiencies in the electrical windings of the rotating electrical machine.

Preferably the rotating electrical machine comprises means for creating a turbulent airflow in the channel, in this way, all of the desiccant may be engaged by an airflow. This may lead to more efficient drying of air that passes through the channel.

The rotating electrical machine may comprise at least one baffle for creating a turbulent airflow in the air channel. A sinuous airflow path may be created around baffles in the channel to give maximum interaction between the desiccant and the airflow. A staggered formation of baffles in the channel may provide the optimum air drying conditions. However, the presence of even one baffle may help to create a turbulent airflow, improving the air drying efficiency.

The air channel may be in any suitable shape. The most convenient arrangement may be that of an elongate tube with circular cross-section. However, the cross-section could also be square, rectangular, triangular, and so on. Also, the channel need not be elongate and could be, for example, bent, curved or corrugated.

Preferably the rotating electrical machine comprises an anti-condensation heater, which may be normally powered by an auxiliary power source. The anti-condensation heater may be arranged to operate when the rotating electrical machine is idle in order to raise the temperature of electrical components and avert dew formation. During use of the rotating electrical machine, electrical inefficiencies tend to raise the temperature of electrical components, making operation of the anti-condensation heater unnecessary. When rotating electrical machine is idle and the anti-condensation heater is inoperable, for example when external power is unavailable, the desiccant in the air channel may be used in order to dry air and protect electrical components from moisture and condensation. Preferably the anti-condensation heater is a separate entity to any dedicated regenerative heater for the desiccant which may also be present.

The rotating electrical machine may comprise a window so that a portion of desiccant can be visually inspected. Thus, it may be apparent when the desiccant must be replaced if the desiccant appears visibly saturated. Also, the colour of the desiccant may be dependent, in part, on the salt or moisture content of the desiccant. In this way the colour of the desiccant, possibly as viewed through the air channel window, may be indicative of a need to replenish the desiccant.

According to another aspect of the present invention there may be provided a generator set comprising: a rotating electrical machine as previously defined; and a prime mover for driving the rotating electrical machine. Many types of prime mover may be used, but possibilities include: engines powered by diesel, petrol or gas, wind turbines, wave power actuators, tidal power actuators, hydroelectric turbines and so on.

A particularly advantageous application of the present invention may be with a wind turbine. Wind turbines may be situated in remote locations where repair and replacement of damaged components may be difficult and expensive. Coastal and offshore wind turbines are also common and these suffer from potential salt damage as well as water damage.

Preferably the desiccant is absorptive or adsorptive of salts as well as water. This may be particularly advantageous if the rotating electrical machine is used in marine, coastal and offshore applications.

According to another aspect of the present invention there is provided a method of drying air in a rotating electrical machine, the method comprising the steps of: drawing air into the rotating electrical machine through a channel; and drying air in the channel by providing a desiccant therein.

Any of the apparatus features may be provided with any of the method features and vice-versa.

Preferred features of the invention will now be described purely by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a cross section of a standard ventilated electrical generator; Figure 2 shows an electrical generation device with a dessicant breather, in an embodiment of the present invention; Figure 3 shows a cross section of a dessicant breather, usable in an embodiment of the present invention; Figure 4 shows another cross section of a dessicant breather usable in another embodiment of the present invention; Figure 5 shows a schematic view of a rotating electrical machine in an embodiment of the present invention; and Figure 6 shows another schematic view of a rotating electrical machine in another embodiment of the present invention.

Figure 1 shows a cross section of a standard ventilated electrical generator, which may also be operable as a motor. The generator comprises an inlet air section 2 and an outlet air section 4 having respective sealed louvres 3,5 for an airflow. The louvres 3,5 are angled downwards to avoid the ingress of water to the generator.

The generator comprises a rotor 6 and a stator 8 with respective electrical windings in which currents may flow. A drive shaft 11 is operatively coupled to the rotor 6 in order to rotate the rotor and generate electricity. A fan 10 is also arranged to be driven by the drive shaft 11. The fan 10 creates a one-way airflow through the generator for cooling the rotor 6 and stator 8 windings. The airflow created by the fan 10 draws air from the atmosphere through the Iouvres 3 of the air inlet section 2, past the windings of the rotor 6 and stator 8, past the fan 10, into the air outlet section 4, and out into the atmosphere via the louvres 5.

As discussed, such a generator may be liable to moisture contamination when it is idle, especially in the morning because of dew formation. The normal practice for averting condensation is to provide an anti-condensation heater or air circulator.

However, this is ineffective during periods when there can be no supply of electrical power to the machine.

Figure 2 shows an electrical generator 22 which is a sealed design with an air to air or air to water cooler. The generator 22 is provided with a desiccant breather 20. The desiccant breather 20 in this instance is a channel with a circular cross-section having desiccant distributed evenly throughout. The desiccant breather 20 extends horizontally from the main body of the generator 22. In this embodiment, air is drawn into and out of the generator 22 via the desiccant breather 20. There is no fan for creating an airflow, and an airflow is only created due to night/day pressure differentials. As this is a small effect, only around 0.5m3 per day of air passes through the desiccant breather. However, the volume of air that passes through the desiccant breather may be affected by a number of different parameters including the temperature differential experienced by the machine during the night/day cycle. The load on the machine may also affect the volume of air that passes in and out of the machine, as fully loaded machines operate at a higher temperature and are likely to expel more heated air out of the machine.

The desiccant breather 20 dries the air that passes into the generator 22, and also provides a low resistance path through which air can flow. In the absence of a low resistance path, air will tend to bleed into or out of the generator 22 by passing through oil sealed bearings, such as sleeve bearings as are commonly used with large air cooled machines. Such an effect causes bearing oil losses, which in turn requires monitoring and maintenance. Oil losses may cause low bearing oil levels and, possibly, contamination of the windings with oil. Clearly, the volume of any oil loss is dependent on the volume of airflow into and out of the generator 22, as previously discussed.

The air that is expelled from the generator 22 during normal operation tends to be warm and dry, due to efficiency losses in the generator 22. This tends to promote some drying of the desiccant in the desiccant breather 20.

Figure 3 shows a cross sectional view of a bi-directional desiccant breather 30.

The desiccant breather 30 is generally tubular in shape and comprises internal and external air apertures 34,32 at either end of the tube. The external aperture 32, opens to the atmosphere, and the internal aperture 34 opens to the interior of a rotating electrical machine. The main body of the tubular breather 30 is filled with a desiccant 36. The desiccant 36 is porous to allow air to pass through it and may be of any common composition such as lithium chloride or silica gel.

The breather has heating elements 35 spaced along its length. The heating elements 35 are operative to heat the desiccant and remove water and/or other materials from the desiccant, thereby restoring the absorptive properties of the desiccant. The heating elements 35 may be continuously operable while a power supply is available.

To prevent the ingress of large particles into the breather 30, a 1mm mesh grill 38 is provided in the external air aperture 32. The mesh grill 38 is removable, and this can facilitate the replacement of desiccant in the breather 30. If the desiccant requires replacement the breather is inverted, the grill 38 is removed, and the desiccant can be removed easily.

In order to aid the connection of the breather 30 to a generator, the internal air aperture 34 is provided with a flanged or threaded joining section 40.

It is desirable that there is a turbulent airflow through the breather 30 to ensure that all of the desiccant 36 is engaged by air and to prevent channelling. This will tend to cause a better drying of the air that passes through the breather 30. To this end, there are baffles 42 arranged inside the breather 30. The baffles are staggered in such a way that air passing through the breather 30 has to take a sinuous path in order to get from one end to the other.

Figure 4 shows a cross section of another desiccant breather 50. In Figure 4, a window 52 is provided in the breather 50 so that a portion of the desiccant 54 is visible for inspection. The desiccant 54 is designed to change colour when completely saturated and thus an operator can tell when the desiccant needs to be replaced to maintain its effectiveness, and when accumulated salts need to be disposed of. An alternative to providing a small window 52 in the breather 50 would be to manufacture the breather 50 from transparent material.

Figure 5 shows a schematic view of a rotating electrical machine 60. The machine has a housing structure 61 and a desiccant breather 62 horizontally mounted in a wall of the housing 61. The housing structure 61 is arranged to be substantially closed, so that air can only pass into or out of the machine via the desiccant breather 62.

The breather 62 is cylindrically surrounded by a regenerative heater 64. The desiccant in the breather 62 may become saturated after the absorption of a certain quantity of water and/or other materials. By heating the desiccant the water and/or other materials may be removed from the desiccant to restore the desiccant's absorptive properties. The regenerative heater 64 is a simple low power resistance heater that is arranged to operate continuously while there is a supply of power to or from the rotating electrical machine 60.

The rotating electrical machine 60 has electrical windings 71 on both a rotor 72 and stator 74. The rotor 72 is operatively connected to a drive shaft 70 which horizontally protrudes through the housing structure 61. The drive shaft 70 is arranged either to drive a load, when the rotating electrical machine is arranged as a motor, or to be driven by a prime mover when the rotating electrical machine is arranged as a generator.

An anti-condensation heater 68 is provided within the housing structure 61. The anti-condensation heater 68 may be arranged to heat the air in the housing structure 61 for raising the temperature of the windings above the atmospheric dew point and preventing the formation of dew. The anti-condensation heater is powered by an external power source.

An electrical cable 66 leads away from the housing structure 61 for carrying any generated electricity away. The advantages of the present invention may be most apparent when there is no auxiliary power supply available via the electrical cable 66. In these circumstances moisture formation on the windings 71 may be averted by the drying of the air by the desiccant in the breather 62.

Normal cooling apparatus may be provided to offset the heating emanating from efficiency losses in the windings 71 and to keep the machine 60 operating within its specified temperature ranges. Typical cooling equipment may include air to water heat exchangers or air to air heat exchangers.

Figure 6 shows a schematic view of a rotating electrical machine 80 in another embodiment of the invention. The rotating electrical machine comprises many of the components of the rotating electrical machine 60 described with reference to Figure 5 including: a housing structure 61, windings 71 on a rotor 72 and a stator 74, an anti-condensation heater 68, a desiccant breather 62 with a regenerative heater 64, a drive shaft 70 connected to the rotor 72 and an electrical cable 66 leading away from the housing structure 71. However, the rotating electrical machine 80 also comprises entrance channels 84 and exit channels 85 in the form of sealed louvres, angled downwards to prevent the ingress of water to the machine 80. Both the entrance channels 84 and the exit channels 85 comprise valves that they may be open or closed as required. The exit channels 85 are located in an exit chamber 86, and a fan 82 is also situated in the exit chamber 86, operatively connected to the drive shaft 70.

When the machine is in use, the valves in the entrance and exit channels 84, 85 are open and represent paths of least resistance for air to flow. The drive shaft 70 is rotationally driven and the fan 82 is arranged to create a one-way airflow through the machine 80. The fan 82 draws air through the entrance channels 84 and into the venting chamber 86 to be expelled via the exit channels 85.

The desiccant breather 62 does not represent a path of least resistance during normal use of the machine and so only low volumes of air pass through the breather 62, This is necessary so that the desiccant in the breather does not become needlessly saturated. In some embodiments the breather 62 may comprise a valve to prevent any air from flowing through the breather 62 during normal use of the machine 80.

When the machine 80 is not in use, the valves in the entrance and exit channels 84, 85 are closed, and any valve in the desiccant breather 62 is open. Thus the desiccant breather 62 represents the path of least resistance for air to flow into and out of the machine. During periods when the machine 80 is without an auxiliary source of power (via the electrical cable 66), the anti-condensation heater 68 is inoperable and the desiccant in the breather 62 is arranged to dry any air that passes into the machine 80.

As previously discussed, an exemplary application of such a rotating electrical machine would be as a generator for use in a remote wind turbine.

Claims

Claims 1. A device for reducing moisture in a rotating electrical

machine, comprising: means for defining a channel for air to flow towards the machine, wherein the channel comprises a desiccant.
2. A rotating electrical machine comprising: means for defining a channel for air to flow towards the machine, wherein the channel comprises a desiccant.
3. A rotating electrical machine according to claim 2 further comprising a housing structure defining an interior and an exterior.
4. A rotating electrical machine according to claim 2 or claim 3 further comprising an electrical device.
5. A rotating electrical machine according to claim 4 wherein the electrical device comprises an electrical winding.
6. A rotating electrical machine or a device for reducing moisture in a rotating electrical machine according to any of the preceding claims wherein the channel comprises means for opening and/or closing the channel.
7. A rotating electrical machine or a device for reducing moisture in a rotating electrical machine according to claim 6 wherein the channel is arranged to be open when the electrical machine is idle.
8. A rotating electrical machine according to any of claims 2 to 7 further comprising an entrance air channel and an exit air channel for air to flow into and away from the machine respectively.
9. A rotating electrical machine according to claim 8 wherein the entrance channel and the exit channel comprise respective means for opening or closing the channels.
10. A rotating electrical machine according to claim 8 or claim 9 wherein the entrance channel and the exit channel are arranged to be open when the electrical machine is in use and closed when the electrical machine is idle.
11. A rotating electrical machine according to any of claims 8 to 10 wherein the entrance channel and/or the exit channel are formed of one or more sealed louvres.
12. A rotating electrical machine according to any of claims 2 to 11 wherein the housing structure is substantially closed when the electrical machine is idle other than for the channel comprising desiccant.
13. A rotating electrical machine or a device for reducing moisture in a rotating electrical machine according to any of the preceding claims further comprising means for heating the desiccant.
14. A rotating electrical machine or a device for reducing moisture in a rotating electrical machine according to claim 13 wherein the means for heating the desiccant comprises a dedicated heater.
15. A rotating electrical machine according to claim 13 or claim 14 wherein the means for heating the desiccant comprises an electrical winding for heating air that subsequently flows in the channel.
16. A rotating electrical machine or a device for reducing moisture in a rotating electrical machine according to any of the preceding claims further comprising means for creating a turbulent airflow in the channel.
17. A rotating electrical machine or a device for reducing moisture in a rotating electrical machine according to any of the preceding claims further comprising at least one baffle for creating a turbulent airflow in the channel.
18. A rotating electrical machine or a device for reducing moisture in a rotating electrical machine according to any of the preceding claims further comprising a window so that a portion of desiccant can be visually inspected.
19. A rotating electrical machine or a device for reducing moisture in a rotating electrical machine according to any of the preceding claims wherein the colour of the desiccant is dependent, in part, on the moisture content of the desiccant.
20. A rotating electrical machine according to any of claims 2 to 19 further comprising an anti-condensation heater.
21. A generator set comprising: a rotating electrical machine according to any of claims 2 to 20; and a prime mover for driving the rotating electrical machine.
22. An electrical generator according to claim 21 wherein the prime mover is a wind turbine.
23. A method of drying air in a rotating electrical machine, the method comprising the steps of: drawing air into the rotating electrical machine through a channel; and drying air in the channel by providing a desiccant therein.
24. An apparatus substantially as herein described with reference to and/or as illustrated in the accompanying drawings.
25. A method substantially as herein described with reference to the accompanying drawings.