IL45360A - Devices for starting a c motors - Google Patents

Description

Devices for starting A.C. motors pai n uTT yua nyjnn D'j nn The present Invention relates to devices for starting A.C. motors having wound col ls and 1n particular it relates to devices fqfl^^ starting wound-rotor motors by means of Iron-losses element or elements.

It 1s known that during the starti ng period of a motor, the variation of the current and electromagnetic torque depend on the motor' s parameters while being Independent of the load torque. As high starting current has adverse effects on the motor and on Its load , 1t 1s desired to limit, as far as possible, the starting current of the motor, without affecting Its torque-speed curve.

Therefore , 1n order to obtain the desired torque-speed curve with a relatively low starting current, variable resistances are coupled to the motor to serve as external Impedances of the rotor. The nstantaneous overal l Impedance 1s manual ly or automatically regulated during the starting period of the motor by means of a control unit composed of resistors , and a plural ity of contacts . However, this method for achieving the desired low starting current has many disadvantages: the resulting torque-speed 1s not smooth but rather Increases In a step-Hke fashion; the mul ti pl icity of elements such as contacts resistors etc. 1n the control unit greatly detracts from the unit's workable safety and rel iability; and final ly the necessity of such a mul tielement control unit makes the device cumbersome and costly to build and maintain.

It was therefore suggested to util ize the Iron-losses element as an external impedance In the rotor by capital izing on the facts that Iron losses 1n a motor are dependent on frequency of operation and on the magnetic Induction.

In such known motor starting devices , for a given Iron core element, the ratio of the active to the Inductive power , 1s frequency dependent [PBF(f) , Q»F(f)] and can not be changed. Thus the range of use of such devices 1s very limited.

The broad object of this nvention 1s therefore, to provide which overcomes the above mentioned disadvantages and 1n which the ratio of the active to the Inductive power k= and their frequency dependency, can be controlled. Furthermore, 1t 1s also an object of the Invention to provide a device for the above described purpose having Improved heat-exchange characteristics and a smaller size.

In accordance with the above objects, there 1s provided a device for controlling an A.C. motor having an electromagnetic field inducting coll wound around a conductive tubular element wherein said tubular element is provided along its surface with one or more throughgoing longitudinal slots extending along at least a part of the length of said surface, wherein the width a_ of at least a part of said slot is equal or greater than δ where 6 is the wave penetrating depth of an instantaneous operating frequency of the Induced electromagnetic energy, and further characterized in that said coil Is wound so as to surround said open slot and said element, whereby the exposed surface area of said element 1s increased for Increasing the absorption of the Induced energy.

This simple yet effective construction of the device can also be provided with one or more additional internal coaxial inserts, and similarly to said coll wound tubular element, additional element or elements each having at least one throughgoing longitudinal slot, can be provided.

Experiments carried out with motors having power outputs of 200 kw and of 1000 kw, respectively, have shown that a motor-starting device, constructed 1n accordance with the Invention can be made in a size which 1s 3.5 to 4 times smaller than a similar device of the known type, I.e. a device with a complete tubular element or elements.

The reason for this size reduction feature of such a device is believed to be due to the Increased surface area which is exposed for the absorption of energy. This opening allows energy of electromagnetic waves to enter the tubular element itself and thus a larger surface area on both sides of said tubular element is exposed to the electromagnetic energy. The increased exposure of the tubular element to the electromagnetic waves increases iron losses therein and consequently there 1s an Increase in the resistance in series with the winding of a rotor to which said device is attached. It should also be mentioned that while the slot in the tubular element allows the electromagnetic energy to enter therethrough, said slot also provides additional opening through which the generated heat can be dissipated by aeration.

While the invention will now be described 1n connection with certain preferred embodiments, it will be understood that it is not Intended to limit the invention to these particular embodiments. On the contrary, 1t 1s Intended to cover all alternatives , modifications and equivalent arrangements as may be Included within the scope of the In^Ktlon, as defined by the appended claims. Nevertheless , 1t Is believed that the embodiments of the Invention will be more ful ly understood from a consideratio of the following I llustrative description read 1n conjunction with the accompanying drawings , 1n which: F1g. 1 1s a cross-sectional view of the construction of a known three- phase device for starting a motor; F1g. 2 shows cross-sectional and top views of the shape of the cores of the known device shown 1n F1g. 1 ; F1g. 3 shows cross- sectional and top views of a core 1n accordance with the Invention; Fig. 4 1s a diagram showing the penetration of an electromagnetic wave in an Iron body, at two frequencies ; Figs. 5 to 7 are cross-sectional views of tubular elements having slots of different shapes ; · F1g. 8 Is a cross-sectional view of the device having means for regulating the effective size of a slot; Fig. 9 Is a top sectional view of a different embodiment of the device having means for regulating the effective size of a slot; F1g. 10. 1s a top sectional view of a sti ll different embodiment of the device having means for regulating the effective size of a slot, and F1g. 11 1s a cross-sectional view of the device of the Invention having dlsplaceable inner core elements .

Referring first to F1gs . 1 and 2 1n which the constructional detai ls of the known motor-starting device 1s shown, It can be seen that essentially the device consists of a tubular core element 2 fitted on Its outer surface wi th an Insulating spacer 4 on which a coll 6 1s wound. For a three phase motor, three of such assemblies are coupled together by This known device works as follows: Upon the starting of the motor the frequency of the vol tage applied to the rotor is stepwlsely ^ varied from 50Hz to 2-3Hz. This variation will cause a stepwise change 1n the penetrating depth of the electromagnetic wave In the core and 1n turn resul t 1 n a corresponding stepwise starting of the motor.

The device according to the invention 1s shown 1n F1g. 3. It consists of a tubular core element 12 1n which there is provided a through-going longitudinal slot 14 along its entire length. The width a_ of the slot and its shape need not be constant along the entire length of the slot but could be varied, as will be explained hereinafter. The basic requirement, however, of the width a_ of said slot for al lowing electromagnetic waves to pass therethrough is that a^ 2^ where fi Is the wave penetrating depth of the electromagnetic energy. Optionally, the core element 12 may be provided with one or more additional tubular core elements 16 , made with or wi thout slots. Also , it should be realized that each of sai d core elements could have more than one slot and wherein each of the slots could be shaped differently.

The Influence of the shape of a slot on the starting process will now be described i n connection to Figs . 4 to 7.

The electromagnetic energy that 1s introduced in an iron surface can be calculated according to Maxwel l 's equation and Poynting's vector.

For a plane electromagnetic wave . P = dEx _ dHy_ rot E = -ar -u -dt where H is the magnetic field intensity; E is the electric field Intensity; is the conductivity of iron; and is the permeability of iron.

\ If H ■ H O = Fun (S1n cot) and E » E 0 Fc (Sin ωΐ), then from o Maxwell's equation It can be obtained that: where, the wave penetration depth 6 SV ω μ γ At a distance x = δ , the amp rlitude of the field 1s H y = H oe"1 * ΗΛ o i e Thus, H decreases to ^ of Its initial value, while the wave penetrates y e to a distance δ.

Since ω = 2Hf , the wave penetration 1n a conductive medium can be expressed by and calculated from the following formula: For example, as shown in F1g. 4, in the operating frequency of 50Hz, the maximum penetrating depth of a plane electromagnetic wave in an iron body having a magnetic field intensity Hme ,1s 0.4 cm, while at the operating frequency of 2-3 Hz, the penetrating depth is 1.4 cm. - 5 a - With reference, now, to Figs. 5 to 7, the following three general cases may prevail In a slotted Iron tubular element for a given ^ frequency: 1. When the slot's width a. > 26Fe, then the electromagnetic wave enters the slot and 1s absorbed by the inner surface of tubular element 12; wherein an additional Inner core 16 exists , said electromagnetic wave is also absorbed by Its outer surface (see F1g. 6) ; 2. When the width of the slot 6Fe < a_ < 26Fe , the electromagnetic wave only partly enters said slot1, and 3. When the width of the slot a_ < 6Fe, it is assumed that the electromagnetic wave does not enter the slot.

In this latter case, the effect of the slot 1s merely to increase the length of the path for the Foucault currents which in turn, i creases the inductive part in the impedance of the tubular element.

The variation in the slot's width a_ along its length , for example, a slot having a general trapezoidal shape as in F1gs . 5 and 6 or a wedge shape as in F1g, 7 , could be analyzed in accordance with the above described three general cases. The tubular element can be considered to be an assembly of superposed annul uses each having the height of one winding of a coll . Thus , the sections of a tubular element having a variable width slot where the width of the slot a > 26 wil l albw an Incoming electromagnetic wave to enter said slot.

With respect to the figures , considering now the frequency of 50Hzj In this particular frequency 26 = 26^ and thus , 1n the sections of the slots h-h-j where a> 26^ the electromagnetic wave wil l enter the slots, while 1n the sections h-j where the slot' s width a_ 26- , the wave will not enter.

Upon starting the motor, the frequency decreases from said 50Hz when the motor speed n=0, to 2-3H* and 26 * 26g, when the motor reaches the nominal speed n=n.. Hence while the starting speed and frequency change, the length of the slot' s sections through which a wave can enter simultaneously change from (h-h^ ) to (h-hg) where the ratio jj-' depends on the^ shape of the slot. Therefore, the amount of energy which 1s entered Into the slot , or conversely, the amount of energy which 1s absorbed by the tubul ar element depends also on the shape of the sl ot. It 1s thus clear that changes Introduced 1n the spae of a slot I .e. , 1n the width a_ of said slot, facil itate the control l ing of the device 1n the various p frequencies , by the controll ing of the ratio ^ namely, the ratio between the active to Inductive power of the device.

In order to widen the range of the effective control during the operation of such devices beyond the predetermined range prescribed by the particular shape of the sl ot, 1t Is suggested to provide means for varying the size of the opening of the slot during said operation. One possible embodiment of such a device 1s shown 1n F1g . 8. It consists of the assembly for starting a three-phase motor shown 1n F1g. 1 housed Inside an enclosure 18. In addition , each tubular element 12 1s made with a slot 14 and 1s provided wi th a metal plate 20 of a shape substantial ly Identical to the shape of the slot. The plates 20 are adapted to be reciprocal ly displaced 1n the slots along their main axis by means of an electromagnetic drive means 22.

On the basis of the preceding explanation 1t is clear that any part of the slot which 1s blocked by the plates 20 does not allow the entrance of an Incoming electromagnetic wave. Thus , the amount of energy absorbed by the device 1s control led by the instantaneous size of the opening 1n the sl ot and consequently, the speed of the inductive motor 1s correspondingly Influenced. The reciprocating plates provide, therefore, additional means , beside the predetermined shapes of the slots and the Instantaneous operating frequencies , by which the starting operation of a motor can be controlled.

It is understood that the slots and plates need not necessarily be of the same shape and that various obvious variations are possible. Also could be adjusted to provide the desired sensitivity In the controlling ... procedure. ^ Moreover, the described reciprocating action of the plates , could automatical ly be governed by obtaining a feedback signal from the motor and applying it to the electromagnetic drive means 22.

Another arrangement for regulating the effective size of a slot 14 1 s shown in Fig. 9. As seen, a plate 24 of any shape 1s plvotal ly situated 1n the slot 14 and adapted to act as a rota table valve as Indicated by arrow W in order to control the size of a slot' s opening. This dynamic arrangement 1s characterized by Its relatively high operating speed , thus providing a fast-reacting and sensitive means for control l ing the amount of electromagnetic energy which 1s desired to enter tubular element 12 , at any particular Instant during the starting operation.

Still a different construction for a dynamic control of the size and shape of a slot Is illustrated 1n Fig. 10. According to this embodiment, the Interior core element 16 1s provided with a tongue 26 along ¾t least part of Its exterior length, which tongue projects across the space between the core element 16 and the slotted tubular element 12. The con? element 16 1s adapted to rotate about Its axis 28 as Indicated by arrow W, causing by Its rotation the displacement of the tongue 26 from behind slot 14, thus al lowing the control led entrance of electromagnetic waves for the energization of the Interior surfaces of the device.

In F1g. 11 there 1s shown a further way by which dynamic regulation of the amount of absorption of energy by the device Is achieved.

While 1n the different embodiments described hereinbefore the amount of energy absorbed by the device Is mainly controlled by the shape and size of the slot, according to this embodiment the energy 1s regulated by means of the interior core element or elements.

As mentioned above , the Interior of a slotted tubular element 12 could be provided with several similarly slotted tubular elements , each one of which slots would be Individuall sha ed and control led b the described Instead or in addition to the described slot-regulating systems , any interior core element 30 (see F1g. 11 ) can be adapted to be reclproJWly displaced, by means of an electromagnetic drive , Inside slotted tubular element 12 to provide sti l l a greater range and variety for the regulation of such devices so as to provide a greater number of operating modes and greater flexibil ity.

It wi l l be evident to those ski lled 1n the art that the Invention 1s not limi ted to the detail s of the foregoing I llustrative embodiments / and that the present Invention may be embodied 1n other specific forms ( without departing from the spi rit or essential attri butes thereof , and 1t 1s therefore desired that the present embodiments be considered 1n all respects as Il lustrative and not restrictive , reference being made to the appended claims , rather than to the foregoing description, 1n which ! 1t Is Intended to claim all modification coming wi thin the scope and spirit of the Invention.

Claims (14)

45360/3 CLAIMS: _

1. A device for controlling an A.C. motor having an electro- ^ magnetic field Inducing coll wound around a conductive tubular element wherein said tubular element 1s provided along Its surface with one or more throughgolng longitudi nal slots extending along at least a part of the length of said surface, wherein the width of at least a part of said slot 1s equal or greater than 6 where δ 1s the wave penetrating depth of an Instantaneous operating frequency of the Induced electromagnetic energy, and further characteri zed in that said il 1s wound so as to surround said open slot and said element, whereby the exposed surface area of said element 1s increased for increasing the absorption of the induced energy.

2. The device as claimed 1n claim 1 further characterized by a tubular core coaxlally situated within said tubular element.

3. The device as claimed 1n claim 1 wherein the width of at least a part of said slot is defined by the expression a_ ^_ 2δ where δ 1s the wave penetration depth at an Instantaneous operating frequency of an induced electromagnetic energy.

4. The device as claimed 1n claim 1 wherein said slot is characterized by a constant width along i ts entire length.

5. The device as claimed 1n claim 1 wherein said slot 1s characterized by a variable width along its length.

6. The device as claimed 1n claim 1 wherein said slot 1s provided with means adapted to vary the width and/or the shape of the slot during the operation of the device.

7. The device as claimed in claim 1 wherein said slot 1s provided wi th a longitudinal plate-Uke element having substantial ly the shape of the slot and rotatably mounted in the slot so as to al low the blocking of at least part of the slot when said element 1s swung about · , Its axis.

8. The device as claimed 1n claim 2 wherein said tubular core 1s 45360/2 Its exterior surface and projecting aacross the space between the core and the tubular element and wherein said core and tongue are adapted to be rotated about the core's axis whereby the effective opening width and/or shape of the slot 1s controlled.

9. The device as claimed 1n claim 2 wherein said tubular core 1s adapted to be reciprocally displaced Inside said tubular element from a first position wherein it is fully inserted therein to a second position wherein 1t 1s fully retracted therefrom.

10. The device as claimed 1n claim 2 wherein said tubular core 1s a hollow tube and 1s provided with a throughgolng longitudinal slot along at least a part of its length.

11. The device as claimed 1n claim 1 wherein a longitudinally extending ferromagnetic Insert 1s adapted to be displaced Inside said tubular element from a first fully-Inserted position to a second fully-retracted position.

12. A device as claimed 1n any of the preceding claims or combination of claims comprising at least two of said devices arranged 1n circuit for starting a multi-phase A.C. motor.

13. The device according to claim 9, 10, 11 or 12 wherein the displacement of said tubular core or longitudinal Insert is accomplished by means of an electromagnetic drive.

14. A device for starting an A.C. motor having a coll wound on a tubular element substantially as hereinbefore described and with reference to the accompanying drawings. For the Applicants Wolff, Bregman and Goller