WO2007075130A1

WO2007075130A1 - Device and method for cooling a power device

Info

Publication number: WO2007075130A1
Application number: PCT/SE2005/002050
Authority: WO
Inventors: Peter Isberg; Torbjörn PERSSON; Tobias WIKSTRÖM
Original assignee: Abb Ab
Priority date: 2005-12-27
Filing date: 2005-12-27
Publication date: 2007-07-05

Abstract

The invention relates to an apparatus and method for cooling a power device. The apparatus comprises a heat sink (12) , in thermal contact with the electric and/or magnetic power device and comprises a closed cavity .(16) containing a phase- change material (18) . The apparatus is, for instance, used during a start or stop process for power devices.

Description

Device and method for cooling a power device

TECHNICAL FIELD

The present invention is concerned with an apparatus and method for cooling power devices. In particular it is concerned with cooling of power devices used in a start-up process or a braking process of electrical machines.

BACKGROUND ART

In this document a power device means an electric and/or magnetical device. In certain processes during use of power devices a high amount of thermal energy is created in the device during a short time period. For instance during start or stop process for power devices such as electric controllers comprising soft- starters, or in some applications when using transformers.

Soft-starters are used, for instance, to start and stop an electrical machine, such as a motor or a generator, in a desired manner, which means, for instance, to eliminate electrical surges in the electrical supply and/or overheating in the electrical machine. A soft-starter typically measures the input current and/or voltage of the electrical machine and regulates the input current and/or voltage received by the electrical machine to achieve a desired start or stop performance. When doing so the soft-starter is exposed to a high starting current and/or a high starting voltage, and thereby a large amount of thermal energy is created. This may cause a thermal overload. Especially if the power device comprises sensitive components, such as semi-conductor components, the thermal overload may cause an abnormal performance and damage to the power device. Protection against overheating of the power devices is therefore necessary.

To solve this problem the power devices are supplied with cooling apparatuses comprising heat sinks in thermal contact with the power devices, especially the sensitive components. However, heat sinks have only a limited thermal load and heat-dissipating capacity. To get rid of the surplus energy, fans are often used. Further, heat exchanger systems may be used, which heat exchanger systems comprise several cooling subsystems. A heat exchanger system comprises, for instance, a first subsystem comprising the heat sink in thermal contact with the power device that needs cooling and a second subsystem, for instance cooled by internal channels comprising flowing water, wherein the second subsystem cools the first subsystem. However, fans and heat exchange systems comprising several subsystems are voluminous components which need continuous service, such as exchanging fan as well as components of the fan such as fan belts, or controlling the flow of water.

In industry it is a desire to accomplish a less space-demanding, simpler and more effective cooling apparatus for power devices.

SUMMARY OF THE INVENTION

The aim of the invention is to provide an improved apparatus for cooling a power device.

In a first aspect of the invention this aim is obtained by an apparatus as defined by claim 1.

Such an apparatus comprises a heat sink, in thermal contact with the power device. The heat sink comprises a closed cavity containing a phase-change material (PCM) suitable for absorbing heat from the device as said phase- change material changes phase.

The cavity comprising the phase-change material is thereby used as an energy reservoir. When in use the power device will produce thermal energy and expose the cooling apparatus and thereby the heat sink to the thermal energy. By using a heat sink comprising a closed cavity containing a phase-change material, the phase-change material is absorbing the thermal energy. During the time the material is changing phase, the temperature is essentially constantly close to the phase-change temperature, for instance, the melting temperature of the material, and accordingly also constant on the surface on the heat sink and thereby the temperature is defined and controlled by using the phase-change material. Thereby the cooling is improved. When the power device is turned off or used in normal operation the cooling apparatus is exposed to less thermal energy, than during the start and stop processes, and will cool down and slowly emit the absorbed thermal energy to the environment by self-convective cooling, such as self-convective ambient air cooling. The heat sink is, for instance, used in applications wherein the heat release is limited, and wherein the heat release is allowed to be a slow process.

Because the cooling-down process is done by a phase-change in the phase- changing material, a fan is no longer needed. A fan has a physical extension and needs a security range around it. Therefore it is possible to mount a cooling apparatus according to the invention more compactly, for instance, closer to other apparatuses in a facility. Further, since the apparatus has no moving parts and the cooler volume is reduced, less material is needed to produce the apparatus. Because the apparatus needs less material, it is simple and inexpensive to produce. Further, because the apparatus has no moving parts that may brake down, the apparatus is also more reliable than if it would comprise moving components.

In a further embodiment of the apparatus according to the invention, the phase- change material is suitable to accumulate the thermal energy produced by the apparatus during a fixed period of time. This is an advantage when using the apparatus for cooling processes that are not performed substantially continuously. That means that the heat emission is increased during a short time period during temporary overloads. Because the heat emission is increased during a fixed period of time, it is possible to adapt the size of a heat sink body surrounding the phase-change material to convectively cool the power device during normal operation. This done instead of adapting the size of the body of the heat sink to convectively cool the maximum thermal energy produced during the use of the power device as in the prior art. Thereby it is possible to make the surface of the cooling apparatus in thermal contact with the power device smaller, thus using less material when producing the cooling apparatus. Experiments have shown that it is possible to reduce 60-80% of the cooler. Because a smaller device can be used, the cooling apparatus is also cost-saving. In a further embodiment of the apparatus according to the invention, the phase- change material has a phase-change temperature in the range of 20-200 ⁰C. This is an advantage because many applications comprising power devices are operated in this temperature interval.

In a further embodiment of the apparatus according to the invention, the phase- change material is changing between solid phase and liquid phase in said temperature range. This is an advantage because a solid phase material is easy to handle when the cooling apparatus is produced.

In a further embodiment of the apparatus according to the invention, said material has a heat of fusion in the range of 200-500 kJ/dm ³ . This is an advantage because the higher heat of fusion, the more thermal energy is absorbed during a phase change in a certain amount of material. Further, in this range the heat of fusion is adjusted to applications performed in the range of 20-200 ⁰C, that is a typical temperature interval in industry applications. It is possible to reduce the volume of a heat sink if it comprises a phase-change material.

In a further embodiment of the apparatus according to the invention, said phase-change material is a eutectic salt. Eutectic salt means a salt comprising a mixture of substances that has a melting point lower than that of any mixture of the same substances in other proportions. This is an advantage because a eutectic salt involves no environmental hazard.

In another further embodiment of the apparatus according to the invention, said phase-change material is a metal or metal alloy with low-melting point, such as a melting temperature suitable for the application.

It is an advantage to use a phase change material such as, for instance, a low temperature melting metal/metal alloy or eutectic salt in liquid, gel or solid form to absorb the excess thermal energy. Since the heat absorption occurs during a phase change, the phase-change material will not be active before reaching an activation temperature, a phase-change temperature such as a melting temperature. At the activation temperature the material absorbs a large amount of thermal energy. The components of a eutectic salt and low-melting metals or metal alloys can be chosen so that the eutectic salt and low-melting metals or metal alloys actively absorb heat in the temperature range of 20-200 C. Further, these phase-change materials can be easily introduced into a cavity in a heat sink, when producing the cooling apparatus.

In a further embodiment of the apparatus according to the invention, the cooling power device is a motor controller comprising a soft-starter. This is an advantageous embodiment because soft-starters works at full power intermittent, i.e. mainly during the upstart and stop procedures during the maximum thermal load during the operational cycle of an attached electrical machine controlled by the motor controller.

In a more preferred embodiment the motor controller comprises a soft-starter that is a by-pass soft-starter, comprising a by-pass contactor. This is an advantage because the cooling apparatus is disconnected during the operative phase of the electrical machine, and therefore the heat dissipating during the cooling down of the heat sink is more effective. Therefore it is possible to reduce the heat sink volume even more.

In a further embodiment of the apparatus according to the invention, the soft- starter is a by-pass soft-starter where the by-pass soft-starter works in parallel with a contactor. This is an advantage because the soft-starter only works for a short time and the contactor during normal operation takes the current load. Other advantages are that the over-load capability is increased, and a better cooling efficiency is attained.

In a further embodiment of the apparatus according to the invention, said power device is a transformer.

In a second aspect of the invention this aim is obtained by a method as defined by claim 11.

Further, the invention relates to a method for cooling a power device with a cooling apparatus, wherein said apparatus comprises a heat sink, in thermal contact with the power device, wherein said heat sink is provided with a closed cavity containing a phase-change material, said method comprising the following steps:

- conducting thermal energy from the device to the cavity,

- absorbing the thermal energy by changing phase of the phase-changing material.

Further, advantageous embodiments of the method according to the invention are defined by claims 12-14.

Use of an apparatus according to any of claims 1-9 in a start up process or a braking process of an electrical machine. This is an advantage because electrical machines produce more thermal energy during the start and stop processes than during normal operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in more detail in connection with the enclosed schematic drawings.

figure 1A shows schematically an embodiment of a power device and a cooling apparatus according to the invention,

figure 1 B shows a cross section A-A of the power device and the cooling apparatus shown in figure 1A,

figure 2 shows a motor controller comprising cooling apparatuses according to an embodiment of the invention, and

Figure 3 shows a transformer comprising a cooling apparatuses according to an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1A shows schematically a power device 10 mounted on an embodiment of a cooling apparatus 11 comprising a heat sink 12. Figure 1B shows a cross section of the power device 10 and the heat sink 12 shown in figure 1A. The heat sink 12 comprises at least one outer surface 14 adapted to be in thermal contact with a corresponding outer surface 15 of a power device 10 that needs cooling. The surfaces 14, 15 are in this case even surfaces. As long as the outer surfaces 14, 15 are adapted to be in thermal contact with each other, the surfaces 14, 15 can have other shapes, for instance, as folded planes. In the figures 1A-1B the power device 10 is mounted on the cooling apparatus 11 and thereby in thermal contact with the heat sink 12. The heat sink 12 further comprises a cavity 16. The cavity 16 is adapted to contain a phase-change material 18. When the cooling apparatus 11 is produced the cavity 16 is sealed after the cavity 16 has been charged with the phase-change material 18. The cavity 16 is thereby totally enclosed.

By comprising the phase-change material 18 the heat sink 12 is adapted to absorb a temporary increase of heat in a device that needs cooling, which device is mounted in thermal contact with the heat sink 12. The device is, for instance, a power device 10 that is exposed to a higher current during a limited period of time. For instance, when exposed to start or stop currents during start and stop processes, which start and stop currents are significantly higher than the currents during the normal process between the start and stop processes.

The thermal energy created in the power device 10 during the temporary increase of thermal energy is conducted to the heat sink 12 and then to the phase-change material 18 in the heat sink material. The ambient temperature compared with the maximum heat power produced by the heat sink 12 has to be considered when deciding the size of the heat sink 12 and the cavity 16.

The material in the heat sink 12 is, for instance, aluminium (Al) or another metal with a high capacity of heat transmission. The power device 10 is an electrical or magnetic device which, for instance, comprises a semi-conductor device, such as a chip or a thyristor. The phase-change material 18 is, for instance, a eutectic salt or metal/metal alloy with a low fusing point, for instance 57Bi/26ln/17Sn. The phase change material 18 has in this case a fusing point in a proper temperature interval, preferably in the range of 20-200 ⁰C.

A phase change material has the characteristics that when the material goes through a phase change, i.e. solid to liquid or liquid to vapor, the process demands a large amount of energy (Q). By choosing which components the eutectic salts and low-melting metals/metal alloys comprise, it is possible to create eutectic salts and low-temperature melting metals/metal alloys with desirable characteristics for a chosen application, for instance eutectic salts 5 and low-melting metals/metal alloys that actively absorb heat in the temperature range of 20-200 ⁰C.

During the temporary increase of heat power in the heat sink and thereby in the phase-change material, the phase-change material absorbs the excess energy.

10 When the temperature in the phase-change material arrives at the phase- change temperature, such as the melting temperature, of the phase-change material, the temperature is maintained almost constant in the material until all the available phase-change material has changed phase. Thereby the temperature in the heat sink and accordingly also on the outer surface on the

15 heat sink is maintained almost constant. The phase-change material is used as a means to increase the cooling efficiency, thus lowering the temperature of the power device. The closer to the power device the phase-change material is placed in the heat-sink, the higher the cooling efficiency of the cooling apparatus.

20

Assuming a thermally insulated (adiabatic) system without phase-change material enclosed in a heat sink, the thermal storing capacity in the heat sink is defined in the equation below;

J o.Dc n Sl -- c ^P(Cu) * AT¹ *V ^V Cu + ^ C ^ P(Al) * ¹A^T¹ *V ^V Al M \ ' lJ

However, if the heat sink comprises a phase-change material, and if the final temperature is equal or larger than the phase-change temperature of the phase-change material, the equation the thermal storing capacity in the heat 30 sink is defined in the equation below;

Q H --

* ^ AT¹ * V ^V Cu + ⁺ C ^" P(Al) * ^a ΛT^l * V ^V Al + ⁺ C ^U p(PCM) * Δ ¹^T¹ * V ^V PCM + ^ΥV ^V PCM * C ¹^m(PCM) ( \^2)l

Q is the total amount of energy needed. ΔT is the temperature rise in the power

35 device. The ambient temperature also has to be considered. Vcu is the main volume of the materials in a power device with excellent heat conductivity, for instance, copper (Cu). VAI is the volume of the material in the heat sink. Cp is the heat capacity (Cp) per volume. The heat capacity (Cp) per volume is important, where the metals despite the difference in density have roughly equal thermal heat capacity. Another important parameter is the heat of fusion (Cm), which is the thermal energy consumed to melt a certain amount of material. Typical corresponding values for the thermal heat capacity and the heat of fusion are a follows:

Thermal heat capacity (Cp): [J/dm³K] Aluminum 2430

Copper 3450

57Bi/26ln/17Sn 2500

Water 4.18

Example of eutectic salt (density: 1450 kg/ dm³) 6,06

Heat of fusion (C_m): [J/dm³]

57Bi26ln17Sn 408 465

Example of eutectic salt 245 232

After the start process, during the normal process, the absorbed energy in the phase-change material is dissipated by a cooling process comprising convection. Then the cooling apparatus 11 is ready to use again, for instance for a stop process.

Figure 2 shows a motor controller 20 comprising a soft-starter 21 comprising cooling apparatuses according to an embodiment of the invention.

Many electrical machines, such as motors or generators, used in industrial and commercial processes and buildings are controlled by motor controllers comprising soft-starters. Soft-starters are used to start and stop an electrical machine in a desired manner, such as to eliminate electrical surges in the electrical supply and/or overheating in the electrical machine. A soft-starter typically measures the input current and/or voltage of the electrical machine and regulates the input current and/or voltage received by the electrical machine to achieve a desired start or stop performance. Soft-starters works with maximum power during 5-15 seconds during starting and stopping of the electrical machine. Thereafter, they are turned off when using a by-pass soft- starter, or work at lower power equal to the nominal power of the electrical machine. After the start of the electrical machine during normal process, the heat sink will cool down and slowly emit the heat to the environment by convective or forced air cooling. A soft-starter is often used in an application as a motor-brake to rapidly stop an electrical machine.

The soft-starter generally requires configuration to suit the circumstances of each application. During configuration the parameters influencing in particular the starting current, and also factors such as the speed, start up time or maximum input current etc. have to be selected or set up in the soft-starter according to what kind of start or stop that is desired for the electrical machine and/or the equipment it is driving. Although AC machines are used all over the world, the AC supply frequency may be either 50Hz or 60Hz and the range of operating currents and operating voltages supplied is extensive.

The soft-starter 21 shown in figure 2 is a by-pass soft-starter, which means that the soft-starter works in parallel with a contactor, so that it is possible to disconnect the soft-starter during the normal process.

The motor controller 20 is connected between an electrical machine 22, in this case a motor, and a power source 24. The electrical power from the power source 24 is transmitted to the electrical machine 22 through the soft-starter 21. The soft-starter 21 comprises a selection unit 25 comprising two semi- conductor devices 26A, 26B, in an anti-parallel connection 28, which anti- parallel connection is coupled in parallel with a contactor 30. Each semiconductor device 26A, 26B is mounted on and in thermal contact with a cooler apparatus 32A, 32B according to an embodiment of the invention. Each cooling apparatus has a heat sink with a closed cavity comprising a phase-change material, not shown in the figure. The phase-change material is adapted to an appropriate phase-change temperature.

The semi-conductor devices 26A, 26B will be producing heat during the start or stop process and will thereby be warm, and the heat will then be absorbed by the phase-change material, thereby cooling the semi-conductor devices 26A,

26B. As the semi-conductors 26A, 26B will be warm when in use, especially if they are thyristors, the soft-starter 21 is in this case bypassed when it is not in active use for starting or stopping. The selection unit 25 is used to connect and disconnect the soft-starter. During starting and stopping of the electrical machine 22, the contactor 30 is open, thereby connecting the soft-starter 21. During normal process operation of the electrical machine 22, the soft-starter 21 is turned off by closing the contactor 30 and thereby the anti-parallel connection 28 is by-passed. The thermal energy absorbed in the heat sink is then released during the normal process. The use of such a bypass coupling avoids developing too much heat power in the power devices. This method of protecting the power devices against an excess temperature is in addition to a current overload protection, if any.

The phase change material absorbs the excess thermal energy during the maximum thermal heat power production during the operational cycle of a soft- starter. The phase-change material is for instance a low-temperature melting metal/metal alloy or eutectic salts in liquid, gel or solid form that can be easily introduced into the cooling apparatus.

Typical values for a motor controller connected to a three-phase current are: a voltage in the range of 400-700V and with an estimated voltage drop over the semi-conductor devices assumed to be 1V for a 100A, a 500V motor, the current to the motor changes, for instance, from 300A, during start, to 3OA, when the semi-conductor is bypassed after the start. A typical motor controller adapted to the mentioned figures is a semi-conductor based AC motor controller and motor starter according to IEC:1999+A1 :2001 and UL Standard

UL 508.

Figure 3 shows a transformer 40 comprising a cooling apparatus 42 according to an embodiment of the invention. The cooling apparatus 42 comprises a heat sink that has a closed cavity comprising a phase-change material, not shown in the figure. The transformer transforms a first current J1 into a second current J2.

In this application the transformer is a step-up transformer used during a short time period. The step-up transformer is then exposed to a high current, creating a considerable amount of heat power. The cooling apparatus functions as the cooling apparatus described in the text to figures 1 A-1B.

A cooling apparatus according to an embodiment of the invention may also be used in other applications with power devices, such as motor controllers and transformers exposed to a high current during a short period of time.

Claims

1. An apparatus (11) for cooling a power device (10), wherein said apparatus comprises a heat sink (12), in thermal contact with the power device, characterised in that said heat sink comprises a closed cavity (16) containing a phase-change material (18) suitable for absorbing heat from the device as said phase-change material changes phase.

2. An apparatus (11) according to claim 1 , characterised in that said phase-change material (18) is suitable to accumulate the thermal energy produced by the device (10) during a fixed period of time.

3. An apparatus (11) according to any of claims 1 and 2, characterised in that said phase-change material (18) has a thermal heat of fusion (Cm) in the range of 200-500 kJ/dm ³ .

4. An apparatus (11) according to any of claims 1-3, characterised in that said phase-change material (18) has a phase-change temperature in the range of 20-200 ⁰C.

5. An apparatus (11) according to claim 4, characterised in that said phase-change material (18) is changing between solid phase and liquid phase in said temperature range.

6. An apparatus (11) according to any of claims 1-5, characterised in that said phase-change material (18) is a eutectic salt.

7. An apparatus (11) according to claim 4, characterised in that said phase-change material (18) is a metal or metal alloy with a melting point in the range of said temperature interval.

8. An apparatus (11) according to any of claims 1-7, characterised in that said power device is a motor controller (20) comprising a soft-starter (21).

9. An apparatus (11) according to claim 8, characterised in that the soft- starter (21) is a by-pass soft-starter.

10. An apparatus (11) according to any of claims 1-7, characterised in that said power device is a transformer (40).

11. A method for cooling a power device (10) with a cooling apparatus (11), wherein said apparatus comprises a heat sink (12), in thermal contact with the power device, wherein said heat sink is provided with a closed cavity (16) containing a phase-change material (18), said method comprising the steps of: - conducting thermal energy from the device to the cavity, and - absorbing the thermal energy by changing the phase of the phase-changing material.

12. A method according to claim 11 , wherein said phase-change material (18) is suitable to accumulate the thermal energy produced by the power device (10) during a fixed period of time.

13. A method according to any of claims 11 and 12, wherein said phase- change material (18) has a thermal heat of fusion (Cm) in the range of 200-255 kJ/dm ³ .

14. A method according to any of claims 11 -13, wherein said phase-change material (18) has a phase-change temperature in the range of 20-200 ⁰C.

15. Use of an apparatus (11) according to any of claims 1-8 in a start-up process or a braking process of an electrical machine (22).