Note: Descriptions are shown in the official language in which they were submitted.
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
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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
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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.
