GB2075271A - Motor control devices - Google Patents

Motor control devices Download PDF

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Publication number
GB2075271A
GB2075271A GB8103544A GB8103544A GB2075271A GB 2075271 A GB2075271 A GB 2075271A GB 8103544 A GB8103544 A GB 8103544A GB 8103544 A GB8103544 A GB 8103544A GB 2075271 A GB2075271 A GB 2075271A
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Prior art keywords
coils
rotor
coil
motor
sets
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GB2075271B (en
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ADWEL Ltd
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ADWEL Ltd
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    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01FMAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F37/00Fixed inductances not covered by group H01F17/00

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Iron Core Of Rotating Electric Machines (AREA)

Abstract

A device for controlling (eg at start-up) the rotor current of a wound rotor induction motor comprises at least one coil linking a magnetic circuit. Such devices have been difficult to adjust and need redesigning for different motors or conditions. Now, a section (16, 34) of the magnetic circuit (14) comprises at least one lamination (18) having a thickness transverse to the magnetic flux path therein substantially in the range of from 2 to 4 times the flux penetration depth therein (as defined). In this range there appears to be an automatic control feature. Coils (12) (one or more for each phase) have taps (20) for adjustment to different motors or conditions. The laminations 18 may be welded together, and may be flat, rods, tubes or split tubes. A plurality of the devices may control the rotors of respective motors. The device may be located on the rotor. In one example a 30 Kw motor operating from 50 Hz had laminations 6.3 mm thick. <IMAGE>

Description

SPECIFICATION Motor control devices This invention relates to devices for controlling the rotor current of a wound rotor induction motor. Such devices are known from British Patent Numbers 16898 of 1915, 133751 and 1484523, to which reference may be made for the general background to the present invention. In specification 16898 of 1915 there is 'disclosed a device which requires a non-magneticcircuitto be introduced in order to obtain adjustment to the unit's reluctance. This in turn limits the use of that device to one machine or motor type and load so that in practice such a unit has to be tailor-made. Specification 133751 disclosed a device being more in the nature of a transformer than a choke and having a short-circuited secondary. Additional secondaries had to be provided to alter the effect of the unit.Specification 1484523 disclosed a device whose core included at least one inwardly directed opening with a gap of a size at least equal to the wave penetration depth (as defined therein) and stated that this provided a reduction in the mass of core needed for the device to control a motor of given power output. The wide gaps were stated to aid heat dissipation by airflow and were stated to increase the surface area available to the magnetic energy because this size of gap allowed the electromagnetic wave to pass through the gap to interior portions of the core. The specification also disclosed methods of varying the gap to vary the control. A theory is given and defines the gap width as inversely proportional to the square root of the frequency.Since the latter may vary from zero to 50 or 60 herz, the required gap width may vary greatly when the device is applied to different motors and in practice it has been found necessary to 'change the design for each type of motor or application to enable correct regulation and starting characteristics.
After considerable experimentation, the present inventors have come to the conclusion that the effective surface area of the core can be increased (or, looked at another way, fhe mass of the core reduced) by laminating the core regardless of the gap between laminations and that the criterion for energy penetration into the core to make use of this increased surface area isthat the thickness of each lamination shall be not less than about twice the flux penetrating depth. The latter is defined for present purposes as the depth at which the flux reduces to lie of its value at the surface when there is flux saturation at the surface. Reference may be had to a paper by Dr E Rosenberg entitled "Eddy Currents In Iron masses" in The Electrician - August 24, 1923 pages 188 - 191.Dr Rosenberg makes some simplifying assumptions and in his equation (3) states that the penetration depth a is: a = 6700A/(pN/fB) where a = depth of penetration in centimetres p = specific resistance in ohm-centimetres N = ampere-turns per centimetre length (r.m.s. value) f = frequency in herz B = flux density in lines per square centimetre.
For present purposes, the specific resistance is known for the core material, the ampere-turns is that produced by the initial rotor current, the frequency is the initial frequency of the rotor current and the flux density can be read off a curve supplied by the manufacturer of the core material for any given number of ampere-turns. The value of a given by Dr Rosenburg is, for practical purposes, close enough to the definition of penetrating depth given above but his formula is to be taken as a guide rather than a definition. If the lamina is reduced to a rod of circular cross-section, the diameter of the rod will correspond to a lamina if said diameter is V2 times said thickness, according to the discussion preceding Dr Rosenberg's equation (10).
The present inventors have also found that if the thickness of the lamina is made substantially within the range of from 2 to 4 times the penetration depth for a particular motor and the initial frequency of its rotor current, the penetrating depth will increase as the rotor speeds up and the frequency of the induced current therein reduces, and thus an automatic control characteristic is built into the device provided the penetration depth can become larger than half the thickness of the lamina at some point in the starting up cycle of the motor. By experiment, it has been found that the optimum is to make the lamina of a thickness substantially in the range of from 2 to 4 times the flux penetration depth at the initial frequency, with a best value of around 3 times the depth.
According to the invention there is provided a device for controlling the rotor current of a wound rotor induction motor, comprising at least one coil linking a magnetic circuit a section of which comprises at least one laminar portion having a thickness transverse to the magnetic flux path therein substantially in the range of from two to four times the flux penetration depth therein at the frequency of, and at any point in the range of sizes of, initial rotor current of motors the device is intended to control.
The device is intended to be used mainly as a starter but can have any of the other uses mentioned in the background material recited above.
The optimum according to experiments is when said thickness is substantially three times said depth.
To accommodate different motors or starting conditions for different uses of the same motor, said coil may be provided with at least one tap to connect part of the coil instead of the whole of the coil to the rotor.
To increase the efficiency of the device in terms of weight of copper in the coil, there is used a plurality of said portions arranged face to face and electrically insulated from each other throughout most of their adjacent surfaces. They may conveniently be welded together at the ends. Provided there is electrical insulation between them, they may be arranged in close juxtaposition, eg separated by a few thousandths of a centimetre. The usual form will be with at least one said portion in the form of a flat plate for convenience of manufacture, though there may be cases in which it is advantageous to have at least one said portion in the form of a tube. In one embodiment, a plurality of such tubes may be arranged alongside each other though electrically insulated from each other throughout most of their adjacent surfaces.In another embodiment, it may be advantageous to have a split tube, though the split again may be only of the order of a few thousandths of a centimetre wide and will- have electrical insulation substantially throughout the facing surfaces of the split. In a particular embodiment, there may be provided a plurality of such split tubes arranged one within another and electrically insulated from each other throughout most of their adjacent surfaces. Again, at least one of the portions may be subdivided into rods extending along the flux path and insulated from each other throughout most of their adjacent surfaces, which can apply to flat-plate shaped portions, eg producing a bundle of rods, orto tubular portions.
A substantial advantage is found when the said section is at least partly within said coil, eg as the whole of the core of the coil, and there is a separate advantage when such a section is at least partly outside said coil, eg forming part or all of the yoke of said coil.
The usual embodiment will comprise a plurality of said coils linking respective branches of said magnetic circuit and will usually be interconnected to form a set for controlling respective phases of a rotor. In a particular embodiment, the device may comprise a plurality of such sets for controlling the rotors of respective motors, the sets linking the same magnetic circuit. This embodiment has the particular advantage that the automatic control feature, produced by the thickness being substantially within the range of 2 to 4 times the penetration depth, allows for considerable variation between the instantaneous loading characteristics of a plurality of motors which are required to be controlled in tandem.A plurality of motors required to operate together in this way may be found in various forms of conveyor or transporter, eg belt conveyors, gantry cranes, lifts or vehicles with independently powered road axles. The coils of said sets may be located between first and second yoke members common to all these coils, in one embodiment the coils being arranged in turn, one from each set, around a common centre and in another embodiment the sets of coils being arranged around respective centres so that adjacent sets overlap.
Due to the automatic control feature mentioned, the device is particularly suitable for use in other situations in which the motor may be subject to varying modes of operation. For example, the device may be located on the rotor of a motor to be controlled thereby. In this case, the device will preferably have the aforementioned taps to enable it to be adjusted to suit different operating conditions of that motor and will either be open to view or will be covered by means allowing access in order to change the tappings, eg an end cap of the motor. In this case, the device must be dynamically balanced about the rotor shaft and the coils may be split, eg so as to provide six coil elements for a three-phase motor.In contrast to cases in which the device is located statically and means are provided to short out the coils when the motor is practically up to speed in order to avoid remanent dissipation, such shorting can be omitted in the case of the device being located on a rotor because of the usually much greater advantages of this case. Alternatively, there can be provided any usual form of switch on a rotor to be actuated by a non-rotor member, eg a sliding collar on the rotor co-operating with a rocking arm on the stator.
Reference will now be made by way of example to the accompanying drawings in which: Figure 1 is a perspective view of a laminated steel core pack for use in an embodiment of the invention; Figure 2 is a side elevation, partly in section, of a device embodying the invention and comprising a core pack as shown in Figure1; Figure 3 is a plan view of the device shown in Figure 2 with the top yoke plate removed; and Figure 4 is a diagramatic representation of some features of particular embodiments.
Referring to the drawings, Figure 2 shows a device 10 for controlling the rotor current of a wound rotor induction motor (not shown), comprising three coils 12 linking a magnetic circuit 14 a section 16 of which comprises a plurality of laminar portions 18 having a thickness transverse to the magnetic flux path therein substantially in the range of from 2 to 4 times the flux penetration depth therein at the frequency of, and at any point in the range of sizes of, initial rotor current of motors the device is intended to control. In this embodiment, the thickness is substantially three times said depth.
There is given below a theoretical treatment believed to be correct and based on experiments which will serve as a guide to choosing the thickness of lamination for particular applications, though some degree of experiment will be required to ensure that the device produced accords best with the specified conditions.
The embodiment shown has taps 20 to accommodate different specified conditions. The section 16 comprises a plurality of the portions 18 arranged face to face and electrically insulated from each other throughout most of their adjacent surfaces, apart from the welds 22 near the ends of the section (on the rear face, notvisible in Figure 1, as well as on the front face shown in Figure 1), serving to hold the portions 18 together. The portions are separated by an electrical insulation sheet of thickness about 1-hundredth of a millimetre. The portions 18 are in the form of flat plates. Alternatively there could be provided a single tube or a plurality of such tubes 24 (Figure 4A) arranged alongside each other and electrically insulated from each other and welded as in. the Figure 1 embodiment. Alternatively, the portion could be in the form of a single split tube or a plurality of such split tubes 26 (Figure 4 B) one within another and electrically insulated from one another and welded together as in the Figure 1 embodiment, with similar separation and welding between the two adjacent ends at the split of each tube. Again, at least one of said portions may be subdivided into rods 28 (Figure 4 C) extending along the flux path and insulated from each other and welded as with the Figure 1 embodiment.
As shown in Figure 2, the section 16 is almost completely within the coil 12 and forms a core for the latter.
As shown in dashed lines in Figure 2, the yokes 30 and 32 may comprise a said section 34 which is outside the coils 12. The advantages for dissipation at control are increased by having a said section at both 34 and 16.
Figure 2 shows two of the three coils 12 linking respective branches 16 of the magnetic circuit. The three coils are interconnected to form a set for controlling respective phases of a rotor, though Figure 2 shows only the connections from one of the coils 12. There may be provided a plurality of such sets for controlling the rotors of respective motors and linking the same magnetic circuit, eg the cores 16 being individual to the coils 12 and the yokes 30, 32 being common. As shown in Figure 2, in respect of one such set of coils, the coils are located between first and second yoke members 30,32 common to all these coils. As shown in Figure 4 D, the coils may be arranged in turn, one from each set 38,40, around a common centre 42.
Alternatively, as in Figure 4 E, the sets 38,40 are arranged around respective centres 44,46 so that adjacent sets overlap, in this case due to one coil of each set being within the polygon formed by the coils of the other set.
In any embodiment with a set of coils, the coils may be connected in star formation or delta formation or star-delta formation.
Agains, the device 10 may be located on the rotor 48 of a motor 50 to be controlled thereby and the device may be provided with changeable taps 52 available through access means 54, as described above and illustrated by way of example in Figure 4 F.
The device illustrated in Figures 1 to 3 is for use in the starting of slipring type wound rotor A.C. electric induction motors and is connected in the rotor circuit to reduce the current and increase the torque during the starting period of the motor. The unit is basically a three phase choke, having laminated mild steel cores and solid mild steel top and bottom yokes. The three tapped phase coils on the laminated cores are connected in star or delta or a variation in series with the rotor winding. On starting a slip ring motor with the choke connected it dissipates maximum energy when receiving maximum frequency at the instant of starting, and offers practically zero resistance when the frequency is low when the motor is at rated operational speed.The variation in frequency from standstill to operational speed produces the automatic and graded reduction in the external rotor impedance. The device comprises three laminated mild steel cores 16. The construction of the cores is important in that the thickness width and length will vary, as will the number of laminations, this aspect being determined by the impedance required and the amount of energy to be dissipated. The laminations are insulated from each other on abutting faces. After the assembly of a group of laminations into a core pack a light weld is made close to the edge of the pack at the ends of both faces. The weld holds the core pack together but more importantly completes the electrical and magnetic circuits at the core extremities.After assembly of the core it is insulated for the winding of the coil, which is wound directly onto the insulated core. The coil will be round or strip copper or aluminium suitably insulated, the winding length of the coil being the maximum obtainable in order to minimize the leakage magnetic flux and keep the reactance also to a minimum. The three completed core/coil assemblies are now securely bolted to the top and bottom solid steel yokes. At this stage the assembly is vacuum impregnated in a suitable varnish and baked. The coil leads are now made offto a terminal board. The arrangement for starting a slipring motor with this device is to connect it in series with the wound rotor through the sliprings with a normally open contactor connected across the terminals.Following starting of the motor through the stator supply contactor the motor accelerates to the rated speed at which point a pre-set timer operates to close the contactor in the rotor circuit and short-circuit both the wound rotor and the impedance device. The preferred insulation thickness within the cores 16 is 0.3mm. The main bolts in Figure 2 are tack welded. This device is appropriate to motors of size 40 kw. Figures 2 and 3 are drawn to the scale shown thereon.
Theory (see above reservation).
As a starter the device is chosen to enable limited starting current, maximum starting torque or a compromise between the two. For maximum torque the current at starting will typically be three times the maximum normal running current. The minimum starting current will be equal to the normal running current. To enable the optimum design, the said thickness is made three times the penetration depth at line frequency (equal to the starting frequency) for a starting current of three times the normal maximum running current.
The optimising factors in efficiently starting a wound motor using the device are as follows: 1. The power factor at the point of switching on and during a substantial portion of the acceleration period should be as close to unity as possible.
2. The magnitude of the starting impedance should reduce as nearly linearly as possible with frequency, as the motor accelerates.
To achieve condition 1, there should be a minimum interaction between the flux penetrating from one surface and that penetrating from the opposite surface of a lamination. It has been found in practice that as long as the penetration depth is less than about one-third of the thickness of the conductive laminations, this condition is satisfied. To achieve condition 2, the skin depth should have risen to at least the thickness of the lamination by the time the motor has accelerated to 95% of full speed. The initial depth is determined by the applied ampere-turns and the line frequency at the point of switch-on. The method for determining the lamination thickness on a particular starter is as follows: The applied ampere-turns for starting the motor at 3 times full load current is calculated and the penetration depth at line frequency is computed.This figure is trebled and used for the core element and yoke plate thickness.
Because of the increase in penetration depth due to fall in frequency it is found that condition 2 is still satisfied for motors having a starting current which is only equal to the full load current (ie one-third of the rated starting current) and for motors of one-third ofthe maximum design size. This condition is obtainable by using taps on the coil, to provide a minimum length of coil for maximum initial current condition.
This enables motors of different sizes and with different rotor winding characteristics to be started using a single unit.
Thus the coil impedance on start can be adjusted as required. The result is a standard design which can accommodate a variety of motor sizes, and without any alterations to the magnetic circuit.
The core is made up from thick laminations, and (following Dr Rosenberg) current flows in layers on the opposing faces of the lamination. As long as the thickness (W) of the lamination is greater than about doubie the penetration depth d (= a of Dr Rosenberg) the impedance reflected in the coil due to this current is related solely to the penetration depth. The latter is related to both the applied ampere-turns N and the frequency f. When 2d < W then approximately N d a a Z, the reflected impedance f1'2 The phase angle, and consequently the power factor, of the reflected impedance is constant and determined by the physical characteristics of the magnetic material.
As the motor accelerates the frequency falls resulting in an increase in penetration depth. As soon as the currents start to interact at the centre of the lamination it becomes possible for non-dissipating magnetic flux to exist in the centre of the lamination. When 2d > W it can be shown that the impedance is predominantly inductive and directly proportional to frequency. A starter built with a plurality of such laminations and subject to a falling frequency would have an impedance which would reduce gradually at the higher frequency and then more rapidly at the lower frequencies. The powerfactorwould start at a high level then fall off as the machine accelerated.The laminations can be in any shape, eg flat laminations stacked, as in Figure 1, tubes with an insulated slot and clamped transversely to form C cores, and U cores. The primary factor of design would be maintaining a high power factor for the early half of the acceleration period. As full speed is approached, the power factor becomes of less importance but a drop in impedance is essential to allow the motor to accelerate close to full speed against a load. The point where the penetration depth is equal to double the lamination thickness can occur anywhere between 5 and 20 Hz. A starter can be designed to start a load subject to different applied ampere-turns. Thus a standard design of starter could be used to start a motor of only one-third of the design maximum rotor size or load current.By winding the coils with several taps it is possible to alter the starting characteristics for which the starter is adapted without altering the magnetic circuit of the starter.
For example, a 100 Kw motor starter embodying the invention can be used for a 30 Kw motor by making use of taps.
By way of example, the following experimental results are given. For a starter designed for a 30 Kw motor, for which the design range was from 15 Kw to 40 Kw, the core plates were 6.3 mm thick while the initial frequency was 50 Hz (with a final frequency of 11/2 - 2 Hz), B was 2.05 x 104 gauss, N was 160 ampere-turns per centimetre and the specific resistance was 9.5 x 10-6 ohm-centimetre, which gave a value of d of 2.29 mm and for a 15 Kw motor d was 1.9 mm. For these two cases, the ratio of thicknessld was thus 3.3 and 2.75 respectively. On the basis of experiments, both of these were considered to be substantially at the optimum of 3 mentioned above.

Claims (21)

1. A device for controlling the rotor current of a wound rotor induction motor, comprising at least one coil linking a magnetic circuit a section of which comprises at least one laminar portion having a thickness transverse to the magnetic flux path therein substantially in the range of from two to four times the flux penetration depth therein at the frequency of, and at any point in the range of sizes of, initial rotor current of motors the device is intended to control.
2. A device as claimed in claim 1, in which said thickness is substantially three times said depth.
3. - A device as claimed in claim 1 or 2, in which said coil is provided with at least one tap.
4. A device as claimed in any preceding claim, in which a said section comprises a plurality of said portions arranged face to face and electrically insulated from each other throughout most of their adjacent surfaces.
5. A device as claimed in any preceding claim, in which at least one said portion is in the form of a flat plate.
6. A device as claimed in any preceding claim, in which at least one said portion is in the form of a tube.
7. A device as claimed in claim 6, in which a said section comprises a plurality of said tubes arranged alongside each other and electrically insulated from each other.
8. A device as claimed in any preceding claim, in which at least one said portion is in the form of a split tube.
9. A device as claimed in claim 8, in which a said section comprises a plurality of said split tubes arranged one within another and electrically insulated from each other throughout most of their adjacent surfaces.
10. A device as claimed in any preceding claim, in which at least one said portion is subdivided into rods 'extending along the flux path and insulated from each other throughout most of their adjacent surfaces.
11. A device as claimed in any preceding claim, in which a said section is at least partly within said coil.
12. A device as claimed in any preceding claim, in which a said section is at least partly outside said coil.
13. A device as claimed in any preceding claim, comprising a plurality of said coils linking respective branches of said magnetic circuit.
14. A device as claimed in claim 13, in which a plurality of said coils are interconnected to form a set for controlling respective phases of a rotor.
15. A device as claimed in claim 14, comprising a plurality of said sets for controlling the rotors of respective motors and linking the same said magnetic circuit.
16. A device as claimed in claim 15, in which the coils of said sets are located between first and second yoke members common to all these coils, the coils being arranged in turn, one from each set, around a common centre.
17. A device as claimed in claim 15, in which the coils of said sets are located between first and second yoke members common to all these coils, the sets of coils being arranged around respective centres so that adjacent sets overlap.
18. A device as claimed in any one of claims 1 to 13, the device being located on the rotor of a motor to be controlled thereby.
19. A device as claimed in claim 1 and substantially according to any embodiment hereinbefore described.
20. A device for controlling the rotor current of a wound rotor induction motor, substantially according to any embodiment hereinbefore described with reference to and illustrated in the accompanying drawings.
21. A combination of a device as claimed in any preceding claim and a wound rotor induction motor controlled thereby.
GB8103544A 1980-02-05 1981-02-05 Motor control devices Expired GB2075271B (en)

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GB8103544A GB2075271B (en) 1980-02-05 1981-02-05 Motor control devices

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GB8003850 1980-02-05
GB8103544A GB2075271B (en) 1980-02-05 1981-02-05 Motor control devices

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GB2075271B GB2075271B (en) 1983-04-07

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0136809A1 (en) * 1983-09-07 1985-04-10 Ben-Gurion University Of The Negev Research And Development Authority Polyphase assembly for controlling A.C. devices

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0136809A1 (en) * 1983-09-07 1985-04-10 Ben-Gurion University Of The Negev Research And Development Authority Polyphase assembly for controlling A.C. devices
US4626815A (en) * 1983-09-07 1986-12-02 Ben-Gurion University Of The Negev Research And Development Authority Polyphase assembly

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PCNP Patent ceased through non-payment of renewal fee

Effective date: 19970205