Wedge is made by preparing a composition of a thermosetting resin system and magnetizable particles and as a result con tains about 70% of a ferrom agnetic. The choice of magnetic wedg es is a. A wound core assembly for an electrical machine comprising a stack of laminations defining a core having a plurality of poles, the wound core assembly further comprising at least one slot wedge. Slot Wedges In Dc Machine Are Made Of, poker sites australia, no limit texas holdem starting hands chart, engel poker.

Synchronous Machine Rotor Windings

the presence of faults in rotor winding insulation can sometimes be indicated by a change in machine performance rather than by the operation of a protective relay. For example, if a coil develops a short circuit, a thermal bend may develop due to an asymmetric heat input into the rotor. This could lead to an increase in shaft vibration with increasing excitation current. This change can be used in some cases to determine if the interturn fault is significant. The location and severity of a fault cannot always be found easily even when the rotor is removed. This is especially true in large turbine-generator rotors whose concentric field windings are embedded in slots in the rotor body and covered by retaining rings at the ends. Many ground and interturn failures disappear at reduced speed or at a standstill. This makes their detection very difficult and emphasizes the need for on-line detection technique. The following tests are used to determine if faults exist in the rotor winding and/or indicate their location. Solid-state devices used in exciters should be shorted out before conducting any test involving the induction or application of external voltages to the rotor winding.

What Are Slot Wedges In A Dc Machine Made Of Gold

Open-Circuit Test for Shorted Turns. An open-circuit test can be used to confirm if shorted turns in rotor field winding exist when there are indirect symptoms such as a change in vibration levels with excitation. The machine should be taken out of service for a short while but does not need to be disassembled.

Figure 16.7 illustrates the open-circuit characteristic of a synchronous machine. It relates the terminal voltage to field current while the machine is running at synchronous speed with its terminals disconnected from the grid. The open-circuit curve can be used to verify shorted turns if an open-circuit test characteristic with healthy turn insulation was done previously. A higher field current will be required to generate the same open- circuit voltage if there are shorted turns in a rotor field winding. If the difference between the two curves in more than 2 percent, the possibility of a turn insulation fault will be confirmed. The difference in characteristics to indicate a shorted turn depends on the number of turns in the field winding and the number of shorted turns. For example, a sin- gle shorted turn cannot be detected by this test if the connected field winding has a large number of turns. This test is done while the machine is running at synchronous speed with its stator winding terminal open-circuited and the field winding energized. Generators can easily be driven at synchronous speed because their drivers are designed to operate at synchronous speed. Motors may need to be driven by ac or dc drive at synchronous speed.

If the test indicates the possibility of shorted turns, further confirmation should be obtained by performing additional tests. This test has these limitations:

● It may not detect shorted turns if the machine has a large number of turns and/or there are parallel circuits in the field winding.

● Differences in the open-circuit curve will also be created when the machine’s magnetic characteristics change, for example, when the rotor wedges are replaced with a different material.

Air Gap Search Coil for Detecting Shorted Turns. Interturn faults in rotors are detected by air gap search coils. Methods have been developed for on-line and off-line testing. This technique is especially useful for detecting faults present at operating speed which disappear on shutdown. The coils and slots having shorted turns as well as the number of turns shorted can be identified by this method. Permanent flux probes have already been installed on some machines. Each rotor slot has local fields around it. This leakage flux is related to the current in the rotor. The magnetic field associated with a coil will be affected if the coil is shorted. The search coil records the high-frequency waveform (known as slot ripple) generated in the air gap. Each rotor slot generates a peak of the waveform in proportion to the leakage flux around it. If an interturn fault occurs, the peaks associated with the two slots containing the faulted coil will be reduced. The recorded data are analyzed to identify the faulted coil and the number of (faults). Shorted turns also generate significant levels of even harmonics (multiples of the frequency) while a fault-free rotor generates only odd harmonics.

The search coil is normally mounted on a stator wedge. A gas-tight gland is required for the leads of the probe. Shorted rotor turns should not be a cause of grave concern if the rotor vibration is not excessive and the required excitation is maintained. A generator can operate adequately for a period of time under this condition. However, these shorted turns are normally caused by serious local degradation of the interturn insulation and possibly major distortion of the conductors. In some cases where static exciters are used, arcing damage and local welding have been found.

It is difficult to interpret the on-load test results from the search coil due to the effects of saturations and magnetic anomalies in the rotor body. More complex and time-consuming detection techniques are required. However, modern on-line monitors have overcome these difficulties. They are designed for use with turbine generators equipped with air gap search coil. The output from the search coil is continuously being processed. An alarm is initiated when a current-carrying shorted turn occurs in the rotor winding.

Impedance Test with Rotor Installed. Shorted turns in a field winding can also be detected by periodic measurement of rotor impedance using an ac power supply. These tests should ideally be performed while the machine is operating at synchronous speed because shorted turns may only exist when centrifugal forces are acting on the turn conductors. When the machine is shut down, there may not be any contact, or the fault resistance may be high. Shorted turns can be detected more accurately by impedance rather than resistance measurements. This is due to the induced backward current in a single shorted turn, which opposes the magnetomotive force (mmf) of the entire coil, resulting in a significant reduction in reactance. This technique is particularly effective in salient-pole rotors, where one short-circuited turn eliminates the reactance of the complete pole. There is a sudden change in impedance when a turn is shorted during run-up or rundown (Fig. 16.8). A sudden change of more than 5 percent is needed to verify shorted turns.

The highest field current used for this test should be significantly lower than the normal current required for rated stator voltage at open circuit. The voltage applied should not exceed the rated no-load stator voltage. A normal winding will exhibit a reduction in

impedance up to 10 percent between standstill and operating conditions due to the effects of eddy currents on the rotor.

This test can only be performed if the field winding is accessible through collector rings because the low-voltage ac power should be applied while the machine is running. A 120-V, 1-phase, 60-Hz ac power is applied. The voltage, current, and shaft speed are measured. The power supply should be ungrounded because the rotor could get damaged if the field winding has a ground fault.

The test includes these steps:

1. Perform an insulation resistance test on the field winding of the machine to be tested to check for ground faults. The impedance test should not be performed if a ground fault is found. The ground fault should be located using a different procedure.

2. Connect an instrumented and ungrounded power supply to the field winding (Fig. 16.9).

The instruments used should be properly calibrated.

3. Take the reading from the local speed indicator to determine the relationship between impedance and speed.

4. Adjust the field winding voltage to give a maximum permissible current of 75 percent of the current required to achieve the rated open-circuit stator voltage.

GENERATOR SURVEILLANCE AND TESTING 16.15

5. Increase and decrease the speed of the machine while the stator windings are disconnected from the power supply. Measure the current, voltage, and speed starting at zero and increasing the speed at 100 rpm intervals until the rated speed is reached. Continuous measurements can also be recorded simultaneously on a multichannel strip chart recorder.

The values of the impedance (Z = V/I ) should be plotted against the speed (Fig. 16.8). A sudden change in impedance of 5 percent or more or a gradual change of more than 10 percent will indicate a strong possibility of shorted turns in the winding. This test is not as sensitive as the previous two described earlier. It is also important to note that solidly shorted turns will not produce an abrupt change in impedance.

Detecting the Location of Shorted Turns with Rotor Removed. The exact location of a shorted turn should be found to minimize the disturbance to the winding when making repairs. One or a combination of the following procedures should be used:

Low-Voltage AC Test. When the field winding of a synchronous machine rotor having shorted turns is connected to a low ac voltage (typically 120 V), the tips of the teeth on either side of the slot(s) having the shorted turns will have significantly different flux induced in them. Figure 16.10 illustrates how the relative magnitudes of tooth fluxes can be measured. The teeth are bridged by a flux survey using a laminated-steel or air-core search coil, which is connected to a voltmeter and wattmeter. The voltage is measured by the voltmeter while the direction of the induced flux is given by the wattmeter. The search coil is moved across all the teeth of the rotor, and voltage and wattage readings are taken. The search coil readings depend on its axial location along the rotor. Therefore, all the read- ings should be taken with the coil located the same axial distance from the end of the rotor. Since the readings vary significantly near the end of the rotor, the coil should not be placed near the end of the rotor. It is important to note that core saturation may occur when a 60-Hz power supply is used. A higher frequency should be used, if possible, to reduce this problem.

The equipment used for the EL-CID test, described later, can be used to detect the shorted field winding turns. This test can be done without removing the end-winding retain- ing rings if the rotor has steel wedges and no damper winding. If the rotor has a separate damper winding or aluminum alloy slot wedges (shorted at the ends) used as a damper winding, they must be open-circuited at the ends before the test can be done. In this case, the retaining rings should be removed. Since many shorts are created by the action of centrifugal forces, they may not appear at standstill.

Figure 16.11 illustrates the flux distribution for a rotor with and without shorted turns. The sharp change in direction of the induced flux indicates the slot containing the shorted turns.

Low-Voltage DC Test (Voltage Drop Test). This method is used to locate the shorts based on dc voltage drop between turns. The end rings should be removed to provide access to the turns. In some cases, the shorts should be induced by applying a radial force to the coils. This is normally done by tapping the wedges with a wooden block or clamping the coils at the corner.

The test is done by applying a dc voltage to the field winding and measuring the drop in voltage across the turns. If a short occurs, the voltage drop across the turn will be lower than normal.

Field Winding Ground Fault Detectors. A large generator rotor operates at 500 V dc and 4000 A normally. If the insulation between the winding and the body is damaged or bridged by conducting materials, there will be a shift of the dc potential of the winding and exciter. The part of the winding where the fault occurred becomes the new zero potential point. In most cases, this will not cause an immediate problem if there is no additional ground fault. A second ground fault in the rotor will be catastrophic.

A rotor ground fault detector is used to enunciate when a fault occurs. Some units are tripped automatically due to possible extensive damage to the rotor body by a dc arc across a separate copper connection. Ground fault detectors have various configurations. The rotor winding is grounded in simple dc schemes on one end through a high ohmic resistance.

However, these schemes become insensitive if the fault occurs close to this end. Ohm’s law determines the magnitude of leakage current from the rotor winding to the ground fault relay. The shaft should also be grounded.

A sophisticated technique was developed to continue operation of a generator having a known ground fault (second ground fault detector). It uses a microprocessor and measur- ing resistors to determine whether the power dissipated by the leakage current exceeds a value that would cause a failure if there were two or more ground faults from the winding. A search coil mounted in the air gap has been used to detect interturn faults and a second ground fault.

If a fault is identified, measurements of slip ring to shaft voltages will give an indication of the location of the fault. (Is the fault at the middle or end of the winding?) After dis- connecting the ground fault detector and while the generator is still on-line, the voltage readings between the brush holders and the shaft are taken. If one ground fault is present, the approximate location of the fault in percent of winding resistance is

During rundown of the unit (when it is unloaded and tripped), an insulation resistance tester is used to test the fault resistance. The brushes are raised or the field circuit breaker is opened to determine if the fault is in the generator rotor, external bus, or exciter. The fault is also monitored as the speed drops. If the fault disappears, it will be impossible to find its location. The operator may decide to put the machine back in service. If the fault reappears when the unit is returned to service, the process should be repeated. If the fault is sustained, a low voltage is applied across the slip rings while the rotor is at standstill. It is usually pro- vided from a 12-V car battery or from a 120-V ac variac. The voltage between the rotor body and each slip ring is measured. If the readings are full voltage with one and zero with the other, there is likely a low-resistance path at the slip rings. It could be caused by carbon dust or insulation failure. It may easily be corrected with a good cleanup. The rotor should be withdrawn if the fault is within the winding. When the rotor is removed, the low-volt- age source is reapplied to the slip rings, and a voltmeter is installed between the rotor body and a long insulated wire. The insulation is removed from the last 5 mm of the wire, and a probe is used to contact the rotor winding metal through ventilation holes and under the retaining rings. This technique will identify the slot, bar, or ventilation hole having the clos- est voltage to the rotor body. The fault is usually located under the wedge near this location. The problem is rectified sometimes by cleaning the ventilation ducts. Otherwise, additional dismantling may be required.

If the ground fault is transient and needs to be found, a failure is forced with a moderate high-potential test and the same technique is used. The hipot test should be used as a last option.

Surge Testing for Rotor Shorted Turns and Ground Faults. This off-line method is used to detect rotor winding faults on stationary and rotating shafts. The location of the fault is identified. This method is very effective in finding ground faults and shorted turns. There is electrical symmetry in a healthy rotor winding. The travel time of an identical electric pulse injected at both slip rings through the winding should be identical. The reflection of the pulse back to the slip rings would also be identical. If there is a short or ground fault, some of the pulse energy will be reflected back to the slip ring due to the drop in imped- ance at the fault. The reflections will change the input pulse waveform depending on the distance to the fault. Therefore, a fault will generate different waveforms at each slip ring unless it is located exactly halfway in the winding.

Recurrent surge oscillography (RSO) is a technique based on the above principle. This test cannot be done on-line because the winding should be isolated from the exciter. Two identical, fast-rising voltage pulses are injected simultaneously at the slip rings. The potential at each injection point is plotted versus time. Identical records should be obtained if there is no fault due to the symmetry in the winding. Differences between the traces are indicative of the winding fault. The fault is located from the time at which irregularity occurred. Ground faults having a resistance less than 500 D will be detected by the RSO method. These faults are also normally detected by the generator protection systems. The RSO technique is used to confirm ground faults. Interturn faults having a resistance of less than 10 D will also be detected by RSO. Faults which have a resistance more than 10 D are more significant during operation and less severe off-load. These faults cannot be detected by RSO.

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The present application generally relates to the design and construction of a wound core assembly for an electrical machine.

Electrical machines in general are constructed from laminations of electrical sheet steel, the resulting structure being used to carry the magnetic flux on which the machine depends for its operation. The structure is laminated to reduce the effect of eddy currents, which flow in the steel due to the time rate of change of the flux. Usually only machines with unvarying flux have unlaminated structures. For example, the field structure of a dc machine can be unlaminated (i.e. made of solid metal), though even in these machines a laminated structure is often adopted in order to improve the transient response when the machine moves to a new operating condition. The degree of lamination is usually based on the frequency of flux variation in the machine. For example, in a machine energised directly from the 50 or 60 Hz mains supply and operating at, say, 1500 or 1800 rev/min, a lamination thickness of 0.50 or 0.65 mm is often adopted. For a machine operating on a 400 Hz supply and running at 12000 rev/min, a lamination thickness of 0.20 mm might be selected.

The laminations are stacked to provide a pack or core of the desired length, the stationary laminations forming the stator core and the moving laminations forming the rotor core. While the wound core assembly disclosed herein will be described for convenience in terms of a rotating machine, the principles of this wound core assembly are equally applicable to a linear machine having a stator in the form of a track and a moving part moving along it. The word “rotor” is used in the art to refer to the movable part of both rotating and linear machines and is to be construed herein in this way. Accordingly, the following descriptions of several examples are made by way of explanation and not for the purposes of limitation to rotating systems.

The laminations forming a core have to be held securely together, not only to facilitate subsequent assembly of the required windings of the machine, but also to minimise vibration when the machine is used. Vibration leads to acoustic noise and degradation of the insulation of the winding. A number of techniques have been evolved to hold the packs together, some of which are more suited to small machines, some to volume production of machines, others to small numbers in a production run.

One common method is shown in FIG. 1, where a bundle of laminations 15 are stacked to form a core 10. Each of the laminations defines a back-iron section 11 and a number of radially protruding teeth or poles 12. The outside of the core 10 is provided with notches or grooves 14, into which axial runs of weld 16 are made. The laminations are thus securely held together. This technique is common, as it can be carried out manually or by automated means. During welding, the pack of laminations is held together in a clamp. This technique is most often used on stator cores, since it is more difficult to access the inside of the bore of the rotor.

One of the difficulties with this technique is that when the welds cool, they contract and the resulting tension in the core tends to make the poles 12 of the laminations near the end of the core splay outward when the clamps are removed. For example, referring to FIG. 1, the laminations at the outside ends of the core would be under tension at an outer edge due to the contracted weld in the grooves 14. This tension causes the laminations to be pulled apart in the region of the pole sections, the separation increasing towards the tip of the pole. This separation of the laminations can make the subsequent insulation of the winding of the wound core assembly more difficult. Further, the separation of the laminations in the region of the poles may compound the problem of vibration of the laminations during operation. This vibration can cause metal fatigue and can also abrade the insulation of the winding arranged around the poles, eventually leading to a short circuit and failure of the machine.

Another method of holding the pack together is by cleating, in which strips of, for example, mild steel, are forced into dovetail-shaped slots around the outer diameter of the core and their ends bent over the ends of the core. While this method avoids the problems of the welds contracting, the laminations may still vibrate in the pole regions due to lack of appropriate support.

To support the tips of the end laminations, it is known to incorporate supporting plates 20 at the ends of the core, as shown in FIG. 2. These plates are typically several times the thickness of the individual laminations and are usually included in the run of weld, so that they are secured to the pack of laminations and support the end laminations. However, if they are made of mild steel, which is generally chosen for ease of machining and cost-effectiveness, they are prone to carry fringing flux and therefore suffer from significant iron losses. These iron losses can reduce the efficiency of the machine. Furthermore, like the laminations themselves, these plates may still splay outwards after welding (for the same reasons) and so can be less effective than expected.

There is therefore a need for a simple way of addressing one or more of the above disadvantages of supporting the laminations in the wound core assembly of an electrical machine.

The present invention is defined in the accompanying independent claims. Further, optional features of embodiments of the invention are recited in the claims respectively dependent thereon.

According to an embodiment of the invention, there is provided a wound core assembly comprising a stack of laminations defining a core having a plurality of poles, the wound core assembly further comprising at least one slot wedge, wherein the slot wedge comprises a first portion arranged between adjacent poles and at least one second portion protruding from the first portion and abutting an end face of the core.

Embodiments of the present invention provide a core having slot wedges secured between adjacent poles which support the laminations of the wound core assembly in the region of the poles. This support can prevent vibration of the laminations in operation of the electrical machine of which the wound core assembly is a part, extending the operating life of the electrical machine. Further, this support can prevent splaying of the laminations, making it easier to insulate the wound core assembly.

The slot wedge may have only one second portion, such that the slot wedge is substantially “L” shaped. The second portion of such a slot wedge may be arranged to engage the end face of one pole.

The slot wedge may have at least two second portions. The at least two second portions may extend from opposite sides of the slot wedge such that the slot wedge is substantially “T” shaped. This slot wedge may be symmetrical about a centre line. The second portions of such a slot wedge may engage the end face in the region of two adjacent poles.

An edge of the first portion may have an interference fit with a side of the pole. An edge of the second portion may apply pressure to the end face in the region of the pole. The edge of the second portion may be arranged at a right angle to the edge of the first portion.

The slot wedge may be a unitary piece of material or the slot wedge may be constructed from more than one piece of material. The slot wedge may comprise a first piece defining the first portion and a second piece defining the second portion, wherein the first piece is attached to the second piece.

The slot wedge may comprise an insulating material. The insulating material may comprise a glass-reinforced epoxy resin.

The junction between the first portion and the second portion may define a recess.

According to an embodiment of the present invention, there is further provided a method for assembling a wound core assembly for an electrical machine, the assembly comprising a stack of laminations defining a core having a plurality of poles, the space between adjacent poles defining a slot, the method comprising inserting a slot wedge into the slot such that the slot wedge is secured between the adjacent poles, wherein the slot wedge has at least one protruding portion arranged to support an end face of the core.

Other aspects and advantages of the apparatus disclosed herein will become apparent upon reading the following detailed description of examples of a wound core assembly and upon reference to the accompanying drawings, in which:

FIG. 1 shows a core of a known machine;

FIG. 2 shows a core with end plates;

FIG. 3 shows a part section of a wound core assembly;

FIG. 4 shows a developed schematic of the wound core assembly of FIG. 3;

FIG. 5 shows a wound core assembly having T-shaped slot wedges;

FIG. 6 shows a developed schematic of the wound core assembly of FIG. 5;

FIG. 7 shows a wound core assembly having L-shaped slot wedges;

FIG. 8 shows a developed schematic of the wound core assembly of FIG. 7;

FIG. 9 shows a T-shaped slot wedge;

FIG. 10 shows an L-shaped slot wedge; and

FIG. 11 shows a further example of a wound core assembly having T-shaped slot wedges.

FIG. 3 shows a part section of a stator core 10 of an electrical machine. The profile of the core 10 defines a circumferential back iron part 11 and a plurality of radially inwardly projecting poles 12. The core is made by stacking a plurality of laminations together. The core comprises a stack of radial laminations of electrically magnetizable steel, each defining the core profile. The centres of the laminations form the axis of the core and the axially outward facing faces of the first and last lamination of the stack each define an end face of the core.

The perimeter of each slot is generally insulated with a slot liner 30 made from a sheet of insulating material. Coils 32 of several turns 33 of insulated wire inserted into the appropriate slots are connected to form phase windings. To hold the windings securely in the slots, slot wedges 34 are inserted at the mouth of the slot between the pole tips. Slot wedges are known by the alternative term “top sticks”. The slot wedge 34 may be one piece, approximating to the axial length of the core, or may be in several axial sections, to aid their installation. The slot wedges are usually retained in position by being placed under the overhanging ends of the poles or, as shown in FIGS. 3 and 4, by fitting into notches 35 in the sides of the poles 12. Conventionally, the entire wound core assembly is coated with an insulating varnish, which helps to stabilise the winding and also bonds together all of the many components of the insulation system.

FIG. 4 shows a schematic of a developed view of the wound core assembly of FIG. 3, looking from the centre of the core bore. The slot wedges 34 in this case are in three sections, but any number could be chosen, depending on the length of the core.

FIG. 5 shows a wound core assembly having T-shaped slot wedges, in accordance with one aspect of the invention. The wound core assembly, including slot liners and coils may be as in FIG. 4, but the slot wedges 60 are a different shape and combine two functions. As seen more clearly in the developed view of the core shown in FIG. 6, the slot wedges 60 at the ends of the core are shaped in the form of a T, with the central portion 62 locating in the notches 35 in the sides of the pole. The lateral width of this central portion 62 is chosen to be an interference fit between the sides of the poles. The length of the central portion 62 is selected according to the length of the core and whether or not intermediate standard slot wedges 34 are used. As shown in FIG. 6, one intermediate standard slot wedge 34 is used and two T-shaped slot wedges 60 are used, one inserted from each end of the core. With this arrangement, the central portion 62 is a little less than one third of the core length. Other arrangements for the length are clearly possible, as will be understood by those skilled in the art.

A cross piece 64 of the T extends across the outward facing face of the end lamination (which defines an end face of the core), as shown in FIGS. 5 and 6, and the slot wedge 60 is pressed into a position such that the cross piece 64 exerts pressure on the end lamination to hold it in its correct position against the rest of the core.

This design of the slot wedge 60 is an improvement on the basic slot wedge because it both retains the winding in the slot and, in addition, holds the outer laminations in place. The cross piece 64 may also be used to at least partly support the end winding of the coil. Once all the windings and wedges are assembled, the wound core assembly can be impregnated with insulating varnish in the conventional way. This bonds all the components together.

A suitable material for the wedge is glass-reinforced epoxy resin to the NEMA G11 standard, for example Pyrotek TS-111, but other similar rigid non-magnetic materials with similar structural strength will be appropriate.

It is possible to make the T-shape of the slot wedge asymmetrical, i.e., the crosspiece 64 need not extend symmetrically either side of the central portion 62.

The sides of a slot are defined by a plurality of laminations 15. Due to manufacturing tolerances, the edges of the laminations 15 will likely not line up perfectly. Typically, there is some variation in position of the side of a lamination 15 along the side of a slot. That is, some laminations will protrude into the slot further than other laminations. Such a variation results in the side of the slot having an irregular, serrated surface. Any such serration assists in securing and retaining the slot wedge in the slot. A well secured slot wedge is beneficial when the slot wedge is used to support one or more splayed end laminations.

A wound core assembly having L-shaped slot wedges is shown in FIGS. 7 and 8. The central portion 82 of the L-shaped slot wedge fulfils the same function as in the previous example. A limb 84 extends to one side only and presses against the face of only one lamination. This L-shaped slot wedge has the advantage that the shape may “nest” or tessellate better when making a number of wedges from a sheet of raw material, thus saving on material cost.

FIGS. 9 and 10 show further forms of T-shaped and L-shaped slot wedges. The T-shaped slot wedge 90 of FIG. 9 has a central portion 92 and a cross piece 94. The junction between the central portion 92 and each limb of the cross piece 94 is formed as a radiussed longitudinally extending recess 96. Similarly, the L-shaped slot wedge 100 of FIG. 10 has a radiussed longitudinally extending recess 106 at the junction between the central portion 102 and limb 104.

The recesses 96, 106 are for accommodating edge burrs and any other manufacturing edge defects in the laminations of the core. For example, if the laminations are manufactured by stamping, then a curved surface may be created on one face of the lamination due to the stamping force. This curved surface could prevent a slot wedge without a recess from seating properly against the top surface of the pole. The recesses 96 and 106 overcome this problem.

The slot wedges can be cut out from a sheet of material by a stamping process such as fine blanking or by routing. However, a sharp internal corner is difficult to produce by a stamping or routing process. Accordingly, the recesses 96 and 106 also allow the slot wedges 90 and 100 to be more readily produced. The skilled person will realise that the recess need not be radial in form but can be any easily produced shape which functionally gives relief at the internal corner of the slot wedge.

A further example of a wound core assembly having T-shaped slot wedges is shown in FIG. 11. In this example, two types of slot wedges are used at the ends of the slots: a first type 34 has a standard shape with no cross piece, and a second type 110 is T-shaped. The T-shaped slot wedge 110 has a central portion 112 and a cross piece 114. The cross piece 114 fulfils the same function as in the previous embodiments. However, in this example T-shaped slot wedges 110 are used in alternate slots with standard shaped slot wedges 34 used in the slots in between. The cross piece 114 is shown as extending substantially all the way across an end face of each adjacent pole 12. In alternatives, the cross piece 114 of the slot wedge 110 may extend all the way or only part way, for example half way or more or less than half way, across the end face of the pole 12. Further, the slot wedge 60 of FIGS. 5 and 6 may be used in an alternating configuration in accordance with this example. This example has the advantage that a greater number of standard slot wedges 34 can be used, reducing manufacturing cost.

In FIG. 11, T-shaped slot wedges 110 are shown used in both ends of a particular slot, with both ends of the adjacent slots having standard slot wedges 34. In an alternative to this arrangement a particular slot has a T-shaped slot-wedge 110 at a first end of the core, with a standard slot wedge 34 at the other, second end of the core. In this arrangement, the slots adjacent the particular slot have T-shaped slot-wedges 110 at the second end of the core, with standard slot wedges 34 at the first end of the core. In this way, each slot has a T-shaped slot wedge 110 at only one end, with adjacent slots having T-shaped slot wedges 110 at an opposite end.

The T-shaped and L-shaped slot wedges are shown in the drawings as comprising a unitary piece of material. Alternatively, the slot wedges may comprise a first and second piece of material fixed together by some mechanical or bonding means. The T-shaped and L-shaped slot wedges have a central portion and a cross piece. One way of fabricating such a slot wedge is to cut the desired shape from a sheet of material. Such a cut may be preformed by way of a stamping process. Another way would be to mould the desired shape as a unitary item. However, yet another way to create the desired shape is to separately create a central portion and a cross piece, a limb, or a pair of limbs and then to attach the two portions together. The portions may be attached by way of a mechanical fixing means such as nuts and bolts or rivets, or by bonding such as welding or using an adhesive. These manufacturing methods may be equally applied to the T-shaped and L-shaped slot wedges disclosed herein.

What Are Slot Wedges In A Dc Machine Made Of Household

The examples of the wound core assembly disclosed herein are shown with either 2 or 3 slot wedges per slot. In alternatives, any number of slot wedges could be used per slot, the choice usually determined by the length of the core.

The slot wedges disclosed herein apply also to inverted machines, i.e., those machines where the rotor revolves around the outside of a stator having radially outwardly extending poles. In this arrangement, the stator core is usually welded or cleated at points around an inner diameter, the outer diameter forming one side of the working airgap of the machine. In such a machine the slot wedges disclosed herein are used to support the ends of the laminations near this outer diameter.

What Are Slot Wedges In A Dc Machine Made Of Dirt

The skilled person will appreciate that variation of the disclosed arrangements is possible without departing from the scope of the claims. Accordingly, the above description of several embodiments is made by way of example and not for the purposes of limitation. It will be clear to the skilled person that minor modifications can be made to the arrangements without significant changes to the operation described above. The present invention is intended to be limited only by the scope of the following claims.