Electricity: motive power systems – Induction motor systems – Braking
Patent
1992-09-04
1994-06-28
Shoop, Jr., William M.
Electricity: motive power systems
Induction motor systems
Braking
318778, 318430, 318501, 318519, H02P 126
Patent
active
053250338
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a circuit arrangement for influencing the start-up and/or braking behavior of three-phase a.c. non-synchronous motors having phase windings which in a first embodiment are connected in star for continuous operation and have terminals for the phase conductors and for a neutral conductor whereas in a second embodiment the phase windings have terminals for the phase conductor and for a neutral conductor.
The invention particularly relates to circuit arrangements for gear-brake motors for machines, production plants, conveyor plants and lifting equipment. Change-pole motors are often used for such purposes. It is known to connect non-synchronous machines in series with various choke circuits for influencing the start-up and/or braking behavior. In special cases one and the same motor may be provided with different choke circuits for start-up and braking. Circuit arrangements of the present kind are required to avoid sudden accelerations and decelerations of the driven parts and knocks in the drive train, which might occur if there is a backlash in the drive train and the drive train in stressed for braking and/or for driving. Known choke circuits for the phases windings may comprise, e.g., resistors that can be connected in circuit or disconnected. Other choke and control circuits are known for non-synchronous machines having rotor windings which can be connected by slip rings to a control circuit. Such a design is more expensive than machines having a short-circuit rotor. In order to provide desirable dynamic characteristics during start-up and braking, additional flywheel masses, such as fans having a high mass, are often used although they involve other disadvantages, particularly a poorer acceleration and deceleration of the drive train.
It is an object of the invention to avoid the disadvantages which have been pointed out and to provide circuit arrangements which in both embodiments mentioned hereinbefore permit an effective influence on the start-up and/or braking behavior to be exerted with simple means.
In the first embodiment the object set forth is accomplished by the invention in that the phase terminal of one phase winding is connected by a change-over switch to the neutral conductor for start-up and/or braking. In the second embodiment the object set forth is accomplished in that the phase windings are connected for continuous operation in star at a star point which is disconnected from only one phase winding, which is permanently connected to the associated phase conductor and the neutral conductor, and the phase terminal of one of the two other phase windings connected at the star point is connected by a change-over switch to the neutral conductor for start-up and/or braking.
Both embodiments are particularly intended for use with machines having a short-circuit rotor. In spite of the connection to only two phase conductors or to only one phase conductor, the mutual influence of the windings and the influence exerted via the rotor cause a rotating field to be generated, which results in a lower start-up torque or during an oversynchronous braking results in a lower braking torque compared to the normal operation. In the first embodiment the voltages in the two phase windings connected to the phase conductors are displaced in phase and the voltage in the third phase winding is not displaced in phase but is much lower. Calculations have shown that the voltages in the two phase windings connected to phase conductors are about 88.2% of the voltages obtained in a normal star connection and the voltage in the third phase winding is 33.3% of the normal phase voltage. At the beginning of the start-up operation the currents in the several phase windings exhibit similar changes. As a result, the rotating field which is generated is elliptical rather than circular and throughout the speed range the torque is one-third of the torque obtained with a normal star connection.
During an oversynchronous operation of change-pole motors the torque may further be reduced in that one or two
REFERENCES:
patent: 2959721 (1960-11-01), Butler et al.
patent: 4311948 (1982-01-01), Brown et al.
patent: 4843292 (1989-06-01), Ono et al.
patent: 4990837 (1991-02-01), Ishitobi
patent: 5138242 (1992-08-01), Kim et al.
Patent Abstract of Japan vol. 8, No. 119 (JP, A59 32372).
Shoop Jr. William M.
Sircus Brian
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