Rotary electric machine and drive system using same

There is provided a rotary electric machine that ensures improving a maximum torque and a rated power factor while reducing an increase in a starting current. In view of this, the rotary electric machine includes a shaft, a rotor, and a stator. The rotor is fixed to an outer periphery of the shaft. The stator is located so as to surround an outer periphery of the rotor. The rotor includes a rotor iron core including a plurality of rotor slots located at predetermined intervals in a circumferential direction and rotor bars inserted into the rotor slots. Rotor slits communicate with outer peripheral sides of the rotor slots. The rotor slits have a width ws in a circumferential direction. The width ws is smaller than a height hs in a radial direction of the rotor slit, and when a rated current is denoted as I1, a turn ratio (primary/secondary) is denoted as Tr, and a magnetic permeability in a vacuum is denoted as μ0, a relationship of ws>μ0×I1×Tr/0.6 is met.

TECHNICAL FIELD

The present invention relates to a rotary electric machine such as a squirrel-cage induction motor and a drive system using the same.

BACKGROUND ART

There has been known PTL1 as a related art regarding a width ws of a rotor slit located on an outer peripheral side of a rotor bar in a rotary electric machine. As illustrated inFIG. 20,FIG. 21, and a similar drawing in the literature, PTL1 is designed such that a slot leakage inductance (hereinafter also referred to as “leakage inductance”) becomes an appropriate magnitude by configuring a slot width b (hereinafter also referred to as “width ws in a circumferential direction of the rotor slit”) larger than a slot depth a (hereinafter also referred to as “height hs in a radial direction of the rotor slit”).

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

With the invention of PTL1, for example, as described in page 46 in the Literature that, “The slot leakage inductance, Lslot, which is one component of the rotor leakage inductance, is inversely proportional to the slot reluctance. With reference to the slot width b and slot depth a as illustrated inFIGS. 18 and 19, the slot leakage inductance is approximately given by the following expression: Lslot≈K1(μ0aL)/b (17) where K1is a factor dependent upon the turns ratio, winding distribution, slot numbers, etc. Both the slot depth a and the width b can be changed—as illustrated inFIG. 20which shows a wider width b and a shallower depth a—to increase the amount of spatial modulation which can be obtained as compared with adjustment of the width only,” the slot leakage inductance is determined by the slot depth a (a height hs in a radial direction of the rotor slit) and the slot width b (the width ws of the rotor slit). However, since an influence from an increase in magnetic field intensity at a starting operation is not reflected to the design of the slot leakage inductance, the invention cannot improve a maximum torque and a rated power factor while reducing an increase in a starting current at the starting operation.

Solution to Problem

An object of the present invention is to provide a rotary electric machine that reduces an influence given to a leakage inductance by intensity of a magnetic field increasing at a starting operation to ensure improvement in a maximum torque and a rated power factor while reducing an increase in a starting current.

One example of a rotary electric machine of the present invention to solve the above-described problem is the rotary electric machine that includes a shaft, a rotor, and a stator. The rotor is fixed to an outer periphery of the shaft. The stator is located so as to surround an outer periphery of the rotor. The rotor includes a rotor iron core including a plurality of rotor slots located at predetermined intervals in a circumferential direction and rotor bars inserted into the rotor slots. Rotor slits communicate with outer peripheral sides of the rotor slots. The rotor slits have a width ws in a circumferential direction. The width ws is smaller than a height hs in a radial direction of the rotor slit, and when a rated current is denoted as I1, a turn ratio (primary/secondary) is denoted as Tr, and a magnetic permeability in a vacuum is denoted as μ0, a relationship of ws>μ0×I1×Tr/0.6 is met.

Advantageous Effects of Invention

A rotary electric machine of the present invention can improve a maximum torque and a rated power factor while reducing an increase in a starting current at a starting operation.

DESCRIPTION OF EMBODIMENTS

The following describes examples of a rotary electric machine of the present invention in detail with an example of a squirrel-cage induction motor with reference to the drawings. In the respective drawings describing the examples, identical names and reference numerals are assigned for identical components and the repeated description is omitted.

First Embodiment

FIG. 1includes partial cross-sectional views of an induction motor of Example 1. In the left drawing ofFIG. 1, reference sign17denotes a shaft serving as a rotation shaft. Reference sign1denotes a rotor fixed to an outer periphery of the shaft17. Reference sign2denotes a stator located so as to surround the outer periphery of the rotor1. Rotation of the rotor1using the shaft17as an axis causes the induction motor to function. The rotor1is constituted by a lamination of a plurality of rotor iron cores12each of which is made of an electromagnetic steel plate and the like. Circular rotor iron core retainers18retain both ends of the rotor iron cores12to integrate the rotor iron cores12. The respective rotor iron cores12include a plurality of rotor slots13located at predetermined intervals in a circumferential direction. Both ends of rotor bars14, which pass through the rotor slots13, are fixed by circular end rings16. Thus, the plurality of rotor bars14are fixed so as to be located parallel to the shaft17. The stator2is constituted by a lamination of a plurality of stator iron cores22each of which is made of an electromagnetic steel plate and the like. Circular stator iron core retainers26retain both ends of the stator iron cores22to integrate the stator iron cores22. The respective stator iron cores22include a plurality of stator slots23located at predetermined intervals in a circumferential direction. A stator winding24is inserted so as to pass through the stator slots23. An energization to this stator winding24rotates the rotor1with the shaft17as the axis.

The right drawing ofFIG. 1is a partial cross-sectional enlarged view taken along A-A in the left drawing ofFIG. 1and illustrates cross-sectional shapes of the rotor1through which the rotor bar14is inserted, the stator2through which the stator winding24is inserted, a clearance3disposed between the two, and a similar component. As illustrated here, in the stator2, the stator winding24is fixed to the stator slots23with a wedge25. In the rotor1, a rotor slit15is disposed on the outer peripheral side of the rotor slots13into which the rotor bars14are inserted. A width ws in the circumferential direction of this rotor slit15has been configured to be smaller than a height hs in a radial direction of the rotor slit15. Furthermore, denoting a rated current flowing through the rotor bars14as I1, a turn ratio (primary/secondary) as Tr, and a magnetic permeability in a vacuum as μ0, the width ws of the rotor slit15was set so as to meet ws>μ0×I1×Tr/0.6 (Formula 6) described later). The following describes the reason that the width ws of the rotor slit15is set like (Formula 6) and effects brought by the setting in detail.

Denoting the width of the rotor slit15as ws, a magnetic field intensity of the rotor slit15as Hs, a magnetic path length of the rotor iron cores12near the rotor slit15as lc, a magnetic field intensity of the rotor iron cores12near the rotor slit15as Hc, and a current flowing through the rotor bars14as I2, considering an influence from magnetic saturation of the rotor iron cores12near the rotor slit15caused by a leakage flux, a relationship of the following formula is generally established by the Ampere's circuital law.
ws×Hs+lc×Hc=I2  (Formula 1)

Here describes the relative magnetic permeability of the rotor iron cores12with reference to the semilogarithmic graph ofFIG. 2. InFIG. 2, the horizontal axis indicates the magnetic field intensity Hc of the rotor iron cores12, and the vertical axis indicates a relative magnetic permeability p.u. indicative of ease of passing of magnetic flux through the rotor iron cores12. As illustrated here, the increase in the magnetic field intensity Hc of the rotor iron cores12decreases the relative magnetic permeability p.u. of the rotor iron cores12. Therefore, as a result of an increase in a level of the magnetic saturation of the rotor iron cores12near the rotor slit15, the magnetic flux illustrated in the right drawing ofFIG. 1becomes less likely to pass through, resulting in a small leakage inductance. Conversely, a decrease in the magnetic field intensity Hc increases the relative magnetic permeability p.u. Therefore, as a result of the decrease in the level of the magnetic saturation of the rotor iron cores12near the rotor slit15, the magnetic flux illustrated in the right drawing ofFIG. 1becomes likely to pass through, resulting in a large leakage inductance.

Based on this point, focusing on the magnetic field intensity Hc of the rotor iron cores12, which is the left side of (Formula 1), a current I2flowing through the rotor bars14at a starting operation is larger than that at a rated operation. Accordingly, the magnetic field intensity Hc of the rotor iron cores12at the starting operation also becomes larger than that at the rated operation, and the relative magnetic permeability p.u. of the rotor iron cores12becomes small. The level of the magnetic saturation of the rotor iron cores12near the rotor slit15becoming large, resulting in the decrease in the leakage inductance. That is, it is seen that the current I2has a relationship of almost inverse proportion to the leakage inductance.

It is seen from the above-described examination that, as long as the reduction in the leakage inductance can be reduced at the starting operation, that is, as long as the level of the magnetic saturation of the rotor iron cores12near the rotor slit15increasing at the starting operation can be decreased, an increase in a starting current Ish flowing through the rotor bars14at the starting operation can be reduced, thereby ensuring improvement in a maximum torque and a rated power factor at the starting operation.

Focusing on the magnetic field intensity Hs of the rotor slit15, since the rotor slit15is a non-magnetic body, even when the magnetic field intensity Hs of the rotor slit15is large, the magnetic saturation does not occur in the rotor slit15. Therefore, the increase in the magnetic field intensity Hs of the rotor slit15does not lower the leakage inductance.

Accordingly, by configuring the magnetic field intensity Hs of the rotor slit15to be sufficiently larger than the magnetic field intensity Hc of the rotor iron cores12, the decrease in the leakage inductance at the starting operation is reduced and the increase in the starting current Ish is reduced. Thus, the leakage inductance at the rated operation can be lowered, and the maximum torque and the rated power factor can be improved.

The table ofFIG. 4shows experimental results of the starting current Ish, a maximum torque Tmax, and a rated power factor p.f. under respective conditions in which the width ws of the rotor slit15ofFIG. 1was changed in seven stages (1.5 to 10.5 mm) and a leakage permeance rate Ps at the rotor slit15with each width ws was changed in four or five stages. Here, the starting current Ish, the maximum torque Tmax, and rated power factor p.f. with the width ws of 7.5 mm and the leakage permeance rate Ps of 5 are defined as 100 points to indicate the experimental results by relative values.

FIG. 3is a graph based on the table ofFIG. 4. The horizontal axis indicates the starting current Ish with respect to the rated current I1, the vertical axis indicates a maximum torque Tmax with respect to a rated torque TL, and the corresponding rated power factor p.f. is also described for some samples. Here also defines all of the three “maximum torque/rated torque,” “starting current/rated current,” and “rated power factor” with the width ws of the rotor slit15of 7.5 mm and the leakage permeance rate Ps of 5 as 100 points.

With the width ws of the rotor slit15of 1.5 mm (the dotted line), while changing the leakage permeance rate Ps from 1.0 to 10.0 allows decreasing the starting current/rated current, the maximum torque/rated torque and the rated power factor decrease substantially. Meanwhile, with the width ws of 6.0 mm (the solid line), changing the leakage permeance rate Ps from 1.0 to 5.0 allows decreasing the starting current/rated current and further reduces the decrease in the maximum torque/rated torque and the rated power factor. Since the induction motor desirably has properties of the small starting current/rated current and the large maximum torque/rated torque and rated power factor, it can be determined that the design with the wide width ws of 6 mm is more desirable from the comparisons of the two. As shown inFIG. 4, since the experimental results are approximately similar in the case where the width ws of 6.0 mm or more,FIG. 3indicates the superimposed experimental results with the width ws of 6.0 to 10.5.

As described above, it is confirmed fromFIG. 3that increasing the width ws of the rotor slit15improves the maximum torque Tmax and the rated power factor p.f. while reducing the increase in the starting current Ish. Especially, like this example, the configuration of the width ws of 6 mm or more ensures maximally obtaining the effects to improve the maximum torque and the rated power factor while reducing the increase in the starting current Ish.

Next, the following describes consumption of a magnetomotive force of the induction motor of Example 1 at the starting operation with reference toFIG. 5. Generally, the consumption of the magnetomotive force is a product of the intensity of the magnetic field and a length of the magnetic path. Therefore, the consumption of the magnetomotive force at the rotor slit15is expressed by (the width ws of the rotor slit15×the magnetic field intensity Hs of the rotor slit15), the consumption at the magnetomotive force of the rotor iron cores12near the rotor slit15is expressed by (the magnetic path length lc of the rotor iron cores12×the magnetic field intensity Hc of the rotor iron cores12), and as indicated by (Formula 1), the sum of the two becomes the current I2flowing through the rotor bars14.

Here, as illustrated inFIG. 5, while increasing the width ws of the rotor slit15increases the consumption of the magnetomotive force (ws×Hs) at the rotor slit15, the consumption of the magnetomotive force (lc×Hc) at the rotor iron cores12decreases. It is seen that, since a magnetic path length lc of the rotor iron cores12is approximately constant, increasing the width ws of the rotor slit15decreases the magnetic field intensity Hc of the rotor iron cores12.

By increasing the width ws of the rotor slit15up to around 6 mm, the magnetic field intensity Hs of the rotor slit15becomes sufficiently large with respect to the magnetic field intensity Hc of the rotor iron cores12. This ensures maximally obtaining the effects to improve the maximum torque and the rated power factor while reducing the increase in the starting current Ish, which become apparent from the comparison of the widths ws between 1.5 mm and 6.0 mm ofFIG. 3.

Accordingly, with the width ws of the rotor slit15of 6 mm or more, the consumption of the magnetomotive force (lc×Hc) at the rotor iron cores12becomes extremely small; therefore, the second term in the left side of (Formula 1) is omitted and the formula is approximated as the following formula.
ws×Hs≈I2  (Formula 2)

Denoting a magnetic-flux density of the rotor slit as Bs and a magnetic permeability in vacuum as μ0, the magnetic field intensity Hs of the rotor slit15is a quotient found by dividing Bs by μ0; therefore, (Formula 2) is expressed as the following formula.
ws×Bs/μ0≈I2  (Formula 3)

Accordingly, the magnetic-flux density Bs is expressed as the following formula from (Formula 3).
Bs≈μ0×I2/ws(Formula 4)

The width ws of the rotor slit15is expressed as the following formula from (Formula 4).
ws≈μ0×I2/Bs(Formula 5)

Denoting the rated current as I1and a turn ratio (primary/secondary) as Tr, the current I2used in the experiments ofFIG. 4becomes around 2×I1×Tr, 5500 A. At this time, the width ws of the rotor slit15with which the effects to improve the maximum torque and the rated power factor are maximally obtained while reducing the increase in the starting current Ish is 6 mm or more from the experimental results, and Bs at the time is obtained as around 1.2 T from (Formula 4).

Accordingly, the width ws of the rotor slit15with which the effects to improve the maximum torque and the rated power factor are maximally obtained while reducing the increase in the starting current Ish is the width ws when Bs in (Formula 5) becomes 1.2 T or less and expressed as the following formula.
ws>μ0×I2/1.2=μ0×I1×Tr/0.6  (Formula 6)

That is, by configuring the width ws of the rotor slit15so as to meet (Formula 6), the effects to improve the maximum torque and the rated power factor can be maximally obtained while reducing the increase in the starting current Ish.

While this example designs the width ws of the rotor slit15so as to have the constant size regardless of the position of the rotor slit15in the radial direction, the width ws does not have to have the constant size. In the case where the width ws does not have the constant size, when a minimum width wsn of the width ws meets (Formula 6), the effects to improve the maximum torque and the rated power factor are maximally obtained while reducing the increase in the starting current Ish.

When (Formula 6) is met, the leakage permeance rate Ps at the rotor slit15is approximated by Ps≈height hs/width ws of the rotor slit15. As also illustrated inFIG. 3, according to the experimental results, the larger Ps increases the effects to improve the maximum torque and the rated power factor while reducing the increase in the starting current Ish by configuring the width ws of the rotor slit15to be large.

The comparison of the widths ws between 1.5 mm and 6.0 mm under conditions of, for example, Ps=1 and Ish=145 points results in the maximum torque of 135 points and the rated power factor of 106.9 with the width ws of 6.0 mm while the maximum torque of 125 points and the rated power factor of 105.9 points with the width ws of 1.5 mm. That is, under the condition of Ps=1, improvements are observed in the maximum torque by 10 points and in the rated power factor by 1.0 point by widening the width ws.

Meanwhile, the comparison of the widths ws between 1.5 mm and 6.0 mm under conditions of Ps=5 and Ish=100 points results in the maximum torque of 100 points and the rated power factor of 100 points with the width ws of 6.0 mm while the maximum torque of 60 points and the rated power factor of 91.5 points with the width ws of 1.5 mm. That is, it is seen that, under the condition of Ps=5, widening the width ws brings remarkable improvements, 40 points in the maximum torque and 8.5 points in the rated power factor; therefore, the larger Ps brings the larger effects.

In contrast to this, with Ps smaller than 1, the improvements are small, less than 10 points in the maximum torque and less than 1.0 point in the rated power factor, and therefore, the improving effects obtained by increasing the width ws of the rotor slit15are small. Therefore, with this example, by increasing Ps to be larger than 1 (configuring the width ws of the rotor slit15to be smaller than the height hs), the effects to improve the maximum torque and the rated power factor are sufficiently obtained while reducing the increase in the starting current Ish.

While this example designs the width ws of the rotor slit15so as to have the constant size regardless of the position of the rotor slit15in the radial direction, the width ws does not have to have the constant size. In the case where the width ws does not have the constant size, when a maximum width of the width ws is configured to be smaller than the height hs of the rotor slit15, Ps becomes at least larger than 1 and the effects to improve the maximum torque and the rated power factor are sufficiently obtained while reducing the increase in the starting current Ish.

Denoting a skin depth of the current flowing through the rotor bars14at the starting operation as d and an average width of the rotor bars14up to the skin depth of the current flowing through the rotor bars14at the starting operation as wd, the leakage permeance rate Ps of the rotor slots13at the starting operation is d/(3×wd) at most, and becomes around 0.85 at most in this example. In this example, Ps is configured to be 1 or more, which is larger than 0.85. This ensures sufficiently obtaining the effects to improve the maximum torque and the rated power factor while reducing the increase in the starting current Ish.

In this example, the width ws of the rotor slit15>the average width wd of the rotor bars14is established. This is because, when this inequality is met while a starting current Ish is small and while the torque at the starting operation, namely, the starting torque is large, the effects to improve the maximum torque and the rated power factor can be maximally obtained while reducing the increase in the starting current Ish. The following describes the reason that this inequality is derived in detail.

First, denoting a secondary resistance as R2, a power frequency as f, and a rated slip as s, the rated torque TL is approximated as the following formula.
TL≈3×I12+R2/(2πfs)  (Formula 7)

Denoting a deep groove effect coefficient of the secondary resistance at the starting operation as Kr, a starting torque Tst is approximated as the following formula.
Tst≈3×Ish2+Kr×R2/(2πf)  (Formula 8)

Therefore, Kr is expressed by the following formula using a quotient found by dividing (Formula 8) by (Formula 7).
Kr≈(Tst/TL)/(s×(Ish/I1)2)  (Formula 9)

Replacing Tst and Ish with a ratio of TL to I1to make it dimensionless (Formula 9) as the following formula.
Kr≈Tst/(s×Ish2)  (Formula 10)

Generally, specifications of the induction motor are configured such that the starting torque Tst becomes the minimum value and the starting current Ish becomes the maximum value; therefore, Kr that can satisfy both specifications of Tst and Ish is expressed as the following formula from (Formula 10).
Kr>Tst/(s×Ish2)  (Formula 11)

Here, denoting a cross-sectional area of the rotor bars14as Sb, Kr is approximated as the following formula.
Kr≈Sb/(d×wd)  (Formula 12)

Denoting a resistivity of the rotor bars14as p, d is approximated as the following formula.
d≈(ρ/(σ×μ0×f))0.5(Formula 13)

wd that can satisfy both specifications of the starting torque Tst and the starting current Ish is expressed as the following formula from (Formula 11) and (Formula 12).
wd<Sb×s×Ish2/(Tst×d)  (Formula 14)

Accordingly, in the case where the starting current Ish is small and in the case where the starting torque Tst is large, that is, in the case where the specifications of Ish and Tst are severe, wd satisfying both specifications of Tst and Ish becomes small.

When the width ws of the rotor slit15with which the effects to improve the maximum torque and the rated power factor are maximally obtained while reducing the increase in the starting current Ish becomes larger than wd satisfying both specifications of Tst and Ish, the relationship as the following formula is established from (Formula 6) and (Formula 14).
K<2×10−3.5/1.2=0.00053  (Formula 15)
K=(f/ρ)0.5×Sb×s×Ish/(I1×Tr×Tst)   (Formula 16)

That is, with a squirrel-cage induction motor establishing the relationship of (Formula 15) where the starting current Ish is small and the starting torque is large, the width ws of the rotor slit15with which the effects to improve the maximum torque and the rated power factor are maximally obtained while reducing the increase in the starting current Ish becomes larger than wd satisfying both specifications of Tst and Ish.

In this example, f is 60 Hz, ρ is 7.5×10−8Ω·m, s is 0.7%, I1×Tr/Sb is 4 A/mm2, and Ish/Tst is 7.5, and K becomes 0.00037 from (Formula 16), thereby establishing the relationship of (Formula 15).

Accordingly, this example has the relationship of ws>wd so as to maximally obtain the effects to increase the maximum torque and the rated power factor while reducing the increase in the starting current Ish.

While this example designs the width ws of the rotor slit15so as to have the constant size regardless of the position of the rotor slit15in the radial direction, the size of the width ws may be changed at the position in the radial direction. When the width ws does not have the constant size, configuring the minimum width wsn of the width ws larger than wd (wsn>wd) allows maximally obtaining the effects to increase the maximum torque and the rated power factor while reducing the increase in the starting current Ish.

When the size of width ws of the rotor slit15is changed at the position in the radial direction, the width ws may be the minimum width at the outer peripheral side of the rotor slit15. Decreasing the width at the outer peripheral side of the rotor slit15lowers harmonic components of an iron loss generated near the inner peripheral surfaces of the stator iron cores22. This improves efficiency at the rated operation and lowers the current at no-load operation, improving the power factor at the rated operation.

Second Embodiment

FIG. 6is a partial cross-sectional view of an induction motor of Example 2. The following omits descriptions of points common to the above-described example.

In this example, denoting an average width of the rotor slots13up to the skin depth d of the current flowing through the rotor bars14at the staring operation as ws′, the relationship of the following formula is met.
ws′>μ0×I2/1.2=μ0×I1×Tr/0.6  (Formula 17)

This example increases the average width ws′ at the outer peripheral side of the rotor slots13in addition to the width ws of the rotor slit15to ensure reducing the low leakage inductance occurred at the staring operation. Accordingly, establishing (Formula 17) ensures obtaining the effects to improve the maximum torque and the rated power factor while reducing the increase in the starting current Ish.

Additionally, in this example, a difference between the average width ws′ of the rotor slots13and the average width wd of the rotor bars14increases; therefore, a void between surfaces in the circumferential direction of the rotor bars14and the rotor iron cores12becomes large. Accordingly, cooling air also blows to the surfaces in the circumferential direction of the rotor bars14, ensuring an effect of improvement in cooling performance as well.

Third Embodiment

FIG. 7is a partial cross-sectional view of an induction motor of Example 3. The following omits descriptions of points common to the above-described examples.

This example forms the rotor slots13into an asymmetrical shape in the circumferential direction and brings only one surface in the circumferential direction of a part of the rotor bars14up to the skin depth of the current flowing through the rotor bars14at the starting operation into contact with the rotor iron cores12in Example 2.

The current concentrates on the rotor bars14up to the skin depth at the starting operation and a loss concentrates on the identical parts. Accordingly, bringing the rotor bars14up to the skin depth into contact with the rotor iron cores12allows transmission of heat generated in the rotor bars14to the rotor iron cores12, thereby ensuring lowering a temperature rise at the rotor bars14.

In addition to this, bringing the rotor bars14into contact with the rotor iron cores12by only one surface allows satisfying the relationship of (Formula 17), thereby ensuring the effects to improve the maximum torque and the rated power factor while reducing the increase in the starting current Ish as well.

Fourth Embodiment

FIG. 8is a partial cross-sectional view of an induction motor of Example 4. The following omits descriptions of points common to the above-described examples.

This example brings the outer peripheral surfaces of the rotor bars14into contact with the rotor iron cores12in Example 3. By this configuration, when a centrifugal force occurs in the rotor bars14toward the outer peripheral side by the rotation of the rotor1, the outer peripheral surfaces of the rotor bars14are brought into contact with the rotor iron cores12to ensure further strongly supporting the rotor bars14by the rotor iron cores12.

Fifth Embodiment

FIG. 9is a partial cross-sectional view of an induction motor of Example 5. The following omits descriptions of points common to the above-described examples.

This example forms the rotor bars14into an asymmetrical shape in a circumferential direction and forms the rotor slots13into an approximately symmetrical shape in the circumferential direction in Example 4. Since this rotor slots13meet (Formula 17) of Example 2, the end portions on the outer peripheral side have a slightly inclined shape. That is, the right side surfaces of the rotor slits15and the right side surfaces of the rotor slots13ofFIG. 9do not constitute the identical plane, having a positional relationship of inclination by a predetermined degree.

In the case where radial ducts are disposed at the squirrel-cage induction motor, duct pieces19are radially located toward the radial direction. Like this example, forming the rotor slots13into a shape almost symmetrical in the circumferential direction allows the duct pieces19to be located near the center in the circumferential direction of rotor iron cores12, facilitating a joining of the duct pieces19with the rotor iron cores12.

Sixth Embodiment

FIG. 10is a partial cross-sectional view of an induction motor of Example 6. The following omits descriptions of points common to the above-described examples.

This example forms the rotor bars14into an asymmetrical shape in the circumferential direction, brings only one surface in the circumferential direction of a part of the rotor bars14up to the skin depth of the current flowing through the rotor bars14at the starting operation into contact with the rotor iron cores12, and brings the outer peripheral surfaces of the rotor bars14into contact with the rotor iron cores12. While Example 5 forms the rotor slots13into the approximately symmetrical (partially asymmetrical) shape, this example forms the rotor slots13into a symmetrical shape for further easy production.

With the induction motor of this example, the current concentrates on the rotor bars14up to the skin depth at the starting operation and a loss concentrates on the identical part. Accordingly, bringing the rotor bars14up to the skin depth into contact with the rotor iron cores12allows transmission of heat generated in the rotor bars14to the rotor iron cores12, thereby ensuring lowering a temperature rise at the rotor bars14.

Additionally, bringing the rotor bars14into contact with the rotor iron cores12by only one surface allows satisfying the relationship of (Formula 17), thereby ensuring the effects to improve the maximum torque and the rated power factor while reducing the increase in the starting current Ish as well.

The rotation of the rotor1causes the rotor bars14to generate the centrifugal force toward the outer peripheral side. Accordingly, bringing the outer peripheral surfaces of the rotor bars14into contact with the rotor iron cores12ensures further strongly supporting the rotor bars14by the rotor iron cores12.

In the case where radial ducts are disposed at the squirrel-cage induction motor, the duct pieces19are radially located toward the radial direction.

Since the rotor slots13have the shape symmetrical in the circumferential direction, the duct pieces19can be located near the center in the circumferential direction of the rotor iron cores12, facilitating the joining of the duct pieces19with the rotor iron cores12.

Seventh Embodiment

FIG. 11is a block diagram of an induction motor system of Example 7. The following omits descriptions of points common to the above-described examples.

This example is a drive system that includes an induction motor100, which is described in any one example of the first to Example 6s, and a load102driven by the induction motor100. In the drive system, the induction motor100is started by a power supply101by full voltage starting.

Since the drive system is the drive system using the induction motor100described in any one example of the first to Example 6s, the effects to improve the maximum torque and the rated power factor are obtained while reducing the increase in the starting current Ish.

REFERENCE SIGNS LIST

1rotor2stator3clearance12rotor iron core13rotor slot14rotor bar15rotor slit16end ring17shaft18rotor iron core retainer19duct piece22stator iron core23stator slot24stator winding25wedge26stator iron core retainer100induction motor101power supply102loadHc magnetic field intensity of the rotor iron core near the rotor slitHs magnetic field intensity of the rotor slitI1rated current flowing through the rotor barI2current flowing through the rotor barIsh starting current flowing through the rotor barPs leakage permeance rate of the rotor slitTr turn ratioTL rated torqueTst starting torqueTmax maximum torquelc magnetic path length of the rotor iron core near the rotor sliths height in a radial direction of the rotor slitws width in a circumferential direction of the rotor slitwd average width of the rotor bars