GB1587919A

GB1587919A - Electrical motor windings

Info

Publication number: GB1587919A
Application number: GB43376/77A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1976-11-16
Filing date: 1977-10-18
Publication date: 1981-04-15
Also published as: NO141531B; BE860860A; FR2371088A1; DE2652217C3; NO773810L; FR2371088B1; DE2652217B2; DE2652217A1; NO141531C

Description

(54) ELECTRICAL MOTOR WINDINGS (71) We, SIEMENS AKTIENGESELLSCHAFT a German company of Berlin and Munich, Germany (fed rep), do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to electrical motor windings.

According to the present invention there is provided a winding for an electric motor, the winding.comprising a plurality of single phases in each of which there is: a first winding portion having an end for connection to a respective phase of an electrical supply, the first winding portions being interconnected; and a second winding portion having an end connected to said end of the first winding portion and an opposite end connection to a neutral conductor; each second winding portion having in series with it a contrpllable device operable to vary the current in the second winding portion, there being control means operable to control the controllable devices such that the currents in the second winding portions vary in such a manner as to control in stepless manner the torque produced by the winding.

In asynchronous motors having squirrel-cage rotors for driving heavy loads, more particularly conveyor belts, it is advantageous for the dimensional proportions of the first and second winding portions to be so chosen that the torque can be kept constant during runningup to speed in the range from substantially rated torque to substantially 1.3 times the rated torque, and for the last-mentioned torque value not to be exceeded even during passage through the pull-out torque.

For this purpose, the first and second winding portions may be divided, for example in the case of a six-pole asynchronous machine, in the ratios of coil numbers 3:3, 4:2 or 5:1.

With first and second winding portions of equal sizes, the stator impedance has, for example during starting, twice the value which it has in normal operation at rated torque. The utilisation of the asychronous machine in rated operation with such stator windings remains substantially unchanged.

By stepless parallel operation of the second winding portions a stepless control of the torque is possible. Particúlarly, but not exclusively, where anti-parallel semi-conductor rectifier elements of semi-conductor rectifier means are provided, two modes of control may be provided: a. a mode of control in which the controllable devices conduct variable portions of current half-cylces; and b. a mode of control in which the controllable devices conduct periodically only predeter mined numbers of successive cycles of current, these periods of conduction being interspersed with periods of non-conduction.

One or each of the two above modes of control may be applied in respect of positive or negative current half cycles only.

Different modes of control may be applied simultaneously to different one of the controllable devices.

For a better understanding of the invention and to show how it may be carried into effect reference will now be made, by way of example, to the accompanying drawing in which: Figure 1 illustrates a motor winding according to the invention having commonly controlled semi-conductor rectifier arrangements controlling the second winding parts; Figure 2 illustrates a motor winding according to the invention having separately controlled semi-conductor rectifier arrangements, and Figure 3 illustrates the variation of the torque with speed for variation of the current through the second winding parts.

In each of the embodiments first winding parts A of a three phase motor winding are on the one hand connected to a three phase supply system N, and on the other hand fixedly connected together in star. Second winding parts B are connected on the one hand also to the supply system N, but at their other ends they are connected to a neutral conductor Mp through respective semi-conductor rectifier arrangements V1, V2 and V3. These consist of anti-parallel-connected semi-conductor rectifier elements such as diodes, thyristors, transistors or the like, which are controllable at least in one branch. They can be brought steplessly from the non-conducting state to the fully conducting state in the same manner by a common control device ST for all three phases.Since the semi-conductor rectifier elements are always at one side at the potential of the neutral conductor M during operation, they need be designed only to withstand the internal voltage drops of the motor, which are substantially one power of ten lower than the terminal voltage. Since in addition only the current of the winding parts B flows through them, their loading is often only about 5% of the motor loading and they can be designed accordingly.

By corresponding common control of the semi-conductor rectifier arrangements V1, V2 and V3, the torque according to the characteristic curve I of Figure 3 can be obtained with the winding parts B non-conducting, or the higher torque according to the characteristic curve II can be obtained with the winding parts B in the fully conducting condition.

With semi-conductor rectifier arrangements which are phase-angle-controlled in the partially conducting condition, any intermediate value of torque in the hatched region can be obtained in dependence upon the phase angle. The difference in torque between the characteristic curves II and I depends upon the coil number ratio of the winding parts A: B.

In the embodiment according to Figure 2, in which parts identical with parts of Figure 1 are denoted by the same references as in the latter figure, each semi-conductor rectifier arrangement V1, V2 and V3 is separately controllable by a separate control device STI, ST2 or ST3.

Thus, the braking torque indicated by a chain line in Figure 3 can be obtained by a control in accordance with the aforementioned possibility, i.e. by all the positive half-cycles being blocked and all the negative-half-cycles being fully conducted (for example by means of an uncontrolled diode) or partially conducted. Due to this unsymmetrical control, pulsating direct currents flow in the winding parts B. If the direct-current components of all the phases are of equal value and of like direction (zero system), a relatively weak braking torque of triple pole number is set up.

On the other hand, if all the direct-current components do not have the same direction, substantially stronger braking torques of single pole numbers are set up.

More particularly in the case of the symmetrically controlled arrangements, the connection indicated by a dash-dotted line in Figure 2 may be provided to form a common star point of the winding parts A and B with the neutral conductor Mp. Then each winding part B is connected in parallel with its accociated winding part A.

The embodiments make it possible for the torque of an asynchronous motor having a squirrel-cage motor to be controlled in stepless manner so as to render possible both a gentle start with constant torque which is above the rated torque, and a stepless torque adjustment during continuous operation, while at the same time ensuring that the dimensional loading resulting from the current and voltage loading of the switching means amounts only to a fraction of the motor loading.

WHAT WE CLAIM IS: 1. A winding for an electric motor, the winding comprising a plurality of single phases in each of which there is: a first winding portion having an end for connection to a respective phase of an electrical supply, the first winding portions being interconnected; and a second winding portion having an end connected to said end of the first winding portion and an opposite end connected to a neutral conductor; each second winding portion having in series with it a controllable device operable to vary the current in the second winding portion, there being control means operable to control the controllable devices such that the currents in the second winding portions vary in such a manner as to control in stepless manner the torque produced by the winding.

2. A winding according to Claim 1, wherein each of said controllabe devices is a controllable semi-conductor rectifier arrangement.

3. A winding according to Claim 2, wherein each of said controllable devices is an arrange.

ment of controllable semiconductor rectifier elements connected in anti-parallel.

4. A winding according to Claim 1, 2 or 3, wherein each of said controllable devices is connected between the associated second winding portion and the neutral conductor.

5. A winding according to any one of the preceding claims, wherein said controllable devices have sufficient current carrying capacity to withstand substantially no greater than the maximum current carried by said second winding portions when the stator winding is in operation.

6. A winding according to any one of the preceding claims, wherein each controllable device has associated with it its own respective control device in said control means.

7. A winding according to any one of the preceding claims, wherein the interconnection of said first winding portions is connected to said neutral conductor.

8. A winding substantially as hereinbefore described with reference to Figure 1 or 2.

9. A winding according to any one of the preceding claims wherein the control means is operable to apply to the controllable devices a mode of control such that they conduct variable portions of current half-cycles.

10. A winding according to any one of the preceding claims, wherein the control means is operable to apply to the controllable devices a mode of control such that they conduct periodically only predetermined numbers of successive cycles of current, these periods of conduction being interspersed with periods of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. phases. Since the semi-conductor rectifier elements are always at one side at the potential of the neutral conductor M during operation, they need be designed only to withstand the internal voltage drops of the motor, which are substantially one power of ten lower than the terminal voltage. Since in addition only the current of the winding parts B flows through them, their loading is often only about 5% of the motor loading and they can be designed accordingly. By corresponding common control of the semi-conductor rectifier arrangements V1, V2 and V3, the torque according to the characteristic curve I of Figure 3 can be obtained with the winding parts B non-conducting, or the higher torque according to the characteristic curve II can be obtained with the winding parts B in the fully conducting condition. With semi-conductor rectifier arrangements which are phase-angle-controlled in the partially conducting condition, any intermediate value of torque in the hatched region can be obtained in dependence upon the phase angle. The difference in torque between the characteristic curves II and I depends upon the coil number ratio of the winding parts A: B. In the embodiment according to Figure 2, in which parts identical with parts of Figure 1 are denoted by the same references as in the latter figure, each semi-conductor rectifier arrangement V1, V2 and V3 is separately controllable by a separate control device STI, ST2 or ST3. Thus, the braking torque indicated by a chain line in Figure 3 can be obtained by a control in accordance with the aforementioned possibility, i.e. by all the positive half-cycles being blocked and all the negative-half-cycles being fully conducted (for example by means of an uncontrolled diode) or partially conducted. Due to this unsymmetrical control, pulsating direct currents flow in the winding parts B. If the direct-current components of all the phases are of equal value and of like direction (zero system), a relatively weak braking torque of triple pole number is set up. On the other hand, if all the direct-current components do not have the same direction, substantially stronger braking torques of single pole numbers are set up. More particularly in the case of the symmetrically controlled arrangements, the connection indicated by a dash-dotted line in Figure 2 may be provided to form a common star point of the winding parts A and B with the neutral conductor Mp. Then each winding part B is connected in parallel with its accociated winding part A. The embodiments make it possible for the torque of an asynchronous motor having a squirrel-cage motor to be controlled in stepless manner so as to render possible both a gentle start with constant torque which is above the rated torque, and a stepless torque adjustment during continuous operation, while at the same time ensuring that the dimensional loading resulting from the current and voltage loading of the switching means amounts only to a fraction of the motor loading. WHAT WE CLAIM IS:

1. A winding for an electric motor, the winding comprising a plurality of single phases in each of which there is: a first winding portion having an end for connection to a respective phase of an electrical supply, the first winding portions being interconnected; and a second winding portion having an end connected to said end of the first winding portion and an opposite end connected to a neutral conductor; each second winding portion having in series with it a controllable device operable to vary the current in the second winding portion, there being control means operable to control the controllable devices such that the currents in the second winding portions vary in such a manner as to control in stepless manner the torque produced by the winding.

non-conduction.

11. A winding according to Claim 9 or 10, wherein the control means is operable to apply the or each said mode of control in respect of positive or negative current half-cylces only.

12. A winding according to Claim 9, 10 or 11, wherein the control means is operable to apply different modes of control simultaneously to different ones of said controllable devices.