Electricity: motive power systems – Braking – Dynamic braking
Reexamination Certificate
2000-04-28
2001-09-18
Ip, Paul (Department: 2831)
Electricity: motive power systems
Braking
Dynamic braking
C318S370000, C318S366000, C318S132000
Reexamination Certificate
active
06291952
ABSTRACT:
The invention relates to a method and a circuit arrangement for the operation of an electromagnetically actuated mechanical brake of an electric motor, in particular a geared motor.
BACKGROUND OF THE INVENTION
Electric machines, in particular electric motors, can be very well controlled with respect to their rotational speed. One known means of improving this basic controllability is to provide mechanical brakes, which can be released or raised by means of an electromagnetic apparatus. While the electric motor is in operation, the current powering the electric motor is also supplied to an excitation coil of the electromagnetically actuated mechanical brake. When no current is thus supplied, the electric motor is immobilized by application of a spring force to the brake.
Furthermore, a known means of powering electric motors is by a mains supply of alternating or polyphasic current. In this case, if the motor incorporates an excitation coil operated by direct current, converters or rectifiers are employed. Because the operating voltages of a.c. or polyphasic mains vary in different parts of the world, this equipment is manufactured and marketed in a number of variants, with excitation coils for different operating voltages. For the same reason, diverse rectifier circuit arrangements are produced. In order to reduce costs by increasing the production output, efforts are already being made to introduce a rectifier circuit arrangement that can be operated in several voltage categories that are used globally. However, because the amplitude of the unipolar voltage generated by the circuit arrangement depends on the a.c. voltage amplitude, the excitation coil of the electromagnetically actuated mechanical brake must be selected to be entirely compatible with the mains voltage available at the site where the brake is to be used.
The object of the invention is to develop a method and a circuit arrangement for the operation of an electromagnetically actuated mechanical brake of an electric motor, in particular a geared motor, in such a way that the same type of excitation coil can be used with a.c. or three-phase mains supplies of different voltages.
SUMMARY OF THE INVENTION
The invention thus is concerned with a method for operating an electromagnetically actuated mechanical brake for an electric motor, in particular a geared motor, with an excitation coil to actuate the brake, in particular to raise it, and with an alternating-current mains supply, i.e. one that delivers a mains voltage. the amplitude of which varies cyclically, wherein the excitation coil has a time constant corresponding to a ratio of a coil inductivity and an ohmic coil resistance.
So that the brake can be operated with a broad range of mains voltages, a unipolar voltage that changes with a specified period is produced by a voltage source.
The unipolar voltage in a first time segment of its period is positive and different from zero and in this first time segment has a maximal value, and in a second time segment of its period is zero or at least approaches zero.
A current path for an excitation current from the voltage source to the excitation coil is opened substantially at the beginning of the first time segment.
The current path is subsequently blocked, at a particular moment within the first time segment which is specified to be such that the excitation current does not exceed a predetermined maximal level.
During a time span in which the excitation coil is or should be permanently excited, the current path is opened and blocked repeatedly, once in each voltage cycle.
Preferably the unipolar voltage is produced by a rectifier unit in the voltage source that acts on a single-phase mains input from the a.c. mains supply, in such a way that the first time segment is equal to almost the entire period of the a.c. cycle, so that the second segment is extremely short, and the voltage period corresponds to half of the a.c. period.
Alternatively, the first and the second segment can each correspond to half of the voltage period, and half of the period of the a.c. cycle.
Preferably the current path is controlled by way of the main current electrodes of an electronic one-way valve that can be turned on and off by means of a controlled element, the current path being opened when the one-way valve is turned on and being blocked when the one-way valve is turned off. While the one-way valve is off, a discharge current from the excitation coil passes through a free path parallel to the excitation coil, with a decay time course corresponding substantially to the time constant of the excitation coil.
The current path is opened when the unipolar voltage, rising from zero, first reaches a positive threshold value which is considerably lower than the maximal value.
In a first embodiment of the invention the excitation current is measured and the current path is blocked when the excitation current, as it increases from lower values after the current path has been opened, first reaches a predetermined first threshold maximum.
In a second embodiment of the invention the unipolar voltage is measured and an integral thereof is obtained as the unipolar voltage rises from zero during the first time segment, after the current path has been opened; the current path is blocked when this gradually increasing integral value first exceeds a predetermined second threshold maximum, which is so specified that the excitation current does not exceed a predetermined maximal level.
In a third embodiment of the invention the unipolar voltage is measured and, by way of a first-order delay element, a first weighted measure of the unipolar voltage is obtained; here the current path is blocked when this first weighted measure, as it increases from lower values after the current path has been opened, for the first time exceeds a predetermined third threshold maximum. In this case a time constant of the delay element is adjusted to substantially correspond to the duration of the first time segment and the third threshold maximum is set such that the excitation current does not exceed a predetermined maximal value.
In a fourth embodiment of the invention a voltage that exists along the excitation coil is measured and by way of a firstorder delay element a weighted measure of this voltage is obtained, the current path being blocked when the second weighted measure, as it increases from lower values after the current path has been opened, for the first time exceeds a predetermined fourth threshold maximum. Here a time constant of the first-order delay element is adjusted to substantially correspond to the time constant of the excitation coil, preferably being twice as long as the period of the unipolar voltage, and the fourth threshold maximum is specified such that the excitation current does not exceed a predetermined maximal value. In this way an “image”, so to speak, of the current flowing through the excitation coil is obtained, so that no direct current measurement is needed. This makes the circuit less elaborate.
In all of these embodiments the current flowing through the excitation coil is adjusted to a value that corresponds to the requirements of the brake.
Preferably in the first to fourth embodiments the preetermined threshold maximum is changed depending on the operating state of the brake, in particular depending on whether the brake is being raised or is being kept in the raised state, so that the operating state of the brake (being raised or held up) is taken into account. Preferably when the current begins to flow for the first time in order to raise the brake, for a certain time span (the elevation interval) the predetemined threshold maximum is readjusted to a level higher that than for holding the brake up, so that the raising process is accelerated. This elevation interval is preferably longer than or equal to the time constant of the excitation coil or at least twice as long as the period of the unipolar voltage.
When the one-way valve is used, to provide protection from transient overvoltages, regardless of other control signals the one-
Roth-Stielow Jorg
Schmidt Josef
Ip Paul
Marshall O'Toole Gerstein Murray & Borun
Sew-Eurodrive GmbH & Co.
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