CA1126804A

CA1126804A - Device for starting single or multi-phase squirrel-cage induction motors

Info

Publication number: CA1126804A
Application number: CA280,150A
Authority: CA
Inventors: Walter Meintrup; Zvonimir Antolek
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1976-06-10
Filing date: 1977-06-09
Publication date: 1982-06-29
Also published as: AU517321B2; AU2577377A

Abstract

ABSTRACT OF THE DISCLOSURE
For the purpose of achieving smooth starting with a three-phase squirrel-cage induction motor, one winding of an iron-cored choke coil having an air gap and two windings is arranged in at least one of the lines connecting the motor to the mains supply, the second choke winding being connected to an alternating-current or to a three-phase-current control element, such as a thyristor.

Description

This invention relates to a device for starting single or multi-phase squirrel-cage induction motors, using a choke in at least one phase.
The range of application of squirrel-cage induction motors is often limited by the relatively unequal torque pattern cluring the starting phase. Fither the stalling torque is too high or else the starting torque is too low or too high. To a limited extent, it is possible to adapt the starting torque to the desired torque pattern by the choic:e of different classes of rotors~
i.e. series of rotors, w~ich differ in construction and/or in the arrange-ment of the rotor winding. Where this is no longer possible, squirrel-cage induction motors are replaced by slip-ring rotor motors, the torque of which may be adapted to the desired torque pattern by the choice of different resistance steps. The disadvantage of this, however, is that torque dis-continuities appear during switching from one resistance step to another.
However, in areas where fire-damp or explosive gases may occur, the use of squirrel-cage induction motors is avoided if possible, because of the danger of ~xplosions, even if pressure-tight enclosures allow such explosions to take place only in the interior of the motor.
It would be desirable to be able to use squirrel-cage induction motors having a single class of rotor for the widest variety of applications, the desired torque pattern during the starting phase being achieved by controlling the supply voltage by means of thyristors, for example alter-nating-current or three-phase-current controls in partial or full-wave control.
Because of the harmonics arising in the control elements, the latter are unsuitable for continuous r.p.m. control. Furthermore, like the choke in known smooth-starting circuits~ the control element would have to be equipped with a choke for the full output of the motor and adapted to be switched in during starting.
It is the purpose of the invention to provide a device which will control the voltage drop over the whole starting range of a squirrel-cage ~26~4 induction motor in such a manner that any desired torque chara-cteristic may be obtained during starting, it being possible, if necessary, to start a plurality of motors with the said device.
According to a broad aspect of the invention there is provided a device for starting a squirrel-cage induction motor having at least one phase compris:ing first switch means for connecting said at least one phase to a voltage supply network and second and third switch means for connecting said at least one phase in series with one winding of a choke between said voltage supply network and a control voltage busbar, said choke having an iron core with an air gap and a further winding - conductively separated from said one winding, said further winding being connected in a series loop with an alternating current control element, said first switch means being open during start-up of the motor while said second and third switch means are closed and said alternating current control means is non-conductive whereby a maximum voltage drop occurs across said one winding, said alternating current control element then being made conductive to minimize the voltage drop across said one winding after which, for normal operation, the motor is disconnected from said one winding by opening said second and third switch means and is connected to said voltage supply net-work by closing said first switch means.
For the purpose of starting a plurality of motors having different starting currents, it is desirable that the reactance of the choke be variable so that it may be adapted to the starting currents of individual motors.
The use of this device does not mean that the entire starting power of a squirrel-cage induction motor is passed

2 ' ' through thyristors, but only the power corresponding to the desired voltage drop. Since only the smaller portion of the starting power is subject to harmonics, the total proportion of harmonics is relatively small, and better use can therefore be made of starting-heat-capacity of the motor. Because of the quadratic relationship between motor torque and voltage, small voltage drops, and therefore lower thyristor capacity, usually suffices to obtain any desired starting characteristic.
Any necessary changes in torque may be obtained rapidly and smoothly. In the case of synchronous motors which have a starter winding, and which, in the starting phase, behave like squirrel-cage motors, the device may also be used.
An example of an embodiment of the device according to the invention is illustrated in the drawing attached hereto to provide a better explanation of the said invention.
In this exemplary embodiment, three three-phase squirrel-cage motors Ml, M2, M3 and a choke 1 are adapted to be connected to a three-phase network, for example a network having a voltage of 6 KV, by means of power circuit-breakers 2, 3. In addition to this, each motor may be /

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connected, through a disconnect switch, to busbar 5, the voltagc of which controls choke 1. Choke 1 is an iron-cored coil having an air gap and at least two windings 6,7 conductively separated from each other, one winding (6) being connected through a power switch 3 to one phase of the supply net-work and to the busbar, whereas the other (7) is fed from an alternating-current control element 8. The said choke may, of course, also be of three-phase design. The size of the choke air gap is such that when the secondary circuit is open, i.e. when the thyristor is blocked and the motor is connected, the maximal voltage drop required is obtained during the starting (run-up) phase. If the secondary circuit is short-circuited, a current flows therein which compensates the main magnetic flux in the iron, so that the voltage drop at the choke is practically zero. The control element in the secondary circuit therefore makes it possible to adjust the voltage drops in the choke between a maximum and zero smoothly and quickly.
If several motors of different output are to be started with the same device, the choke may be provided with various tappings which may be selected by means of a disconnect switch 9. Instead of tappings, which permit only stepped changes in choke reactance, the said reactance may also be changed by altering the air gap. This provides stepless adaptation to the required power and to the necessary starting currents.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A device for starting a squirrel-cage induction motor having at least one phase comprising first switch means for connecting said at least one phase to a voltage supply network and second and third switch means for connecting said at least one phase in series with one winding of a choke between said voltage supply network and a control voltage busbar, said choke having an iron core with an air gap and a further winding con-ductively separated from said one winding, said further winding being connected in a series loop with an alternating current control element, said first switch means being open during start-up of the motor while said second and third switch means are closed and said alternating current control means is non-conductive whereby a maximum voltage drop occurs across said one winding, said alternating current control element then being made conductive to minimize the voltage drop across said one winding after which, for normal operation, the motor is disconnected from said one winding by opening said second and third switch means and is connected to said voltage supply net-work by closing said first switch means.

2. A device as claimed in claim 1 wherein there are three phases and said first, second and third switch means are ada-pted to connect and disconnect all three phases.

3. A device according to claim 1 or 2, for starting a plurality of motors having differing starting currents, further comprising means for varying the reactance of the choke, so that it may be adpated to the starting currents of individual motors.

4. A device according to claim 1 or 2, for starting a plurality of motors having different starting currents, further comprising means for varying the reactance of the choke, so that it may be adapted to the starting currents of individ-ual motors, the choke being provided with tappings selectable by associated disconnect switches whereby different choke reactances may be selected for each motor.

5. A device according to claim 1 or 2, for starting a plurality of motors having different starting currents, further comprising means for varying the reactance of the choke by varying the air gap, so that it may be adapted to the starting currents of individual motors.