Electricity: motive power systems – Braking – Dynamic braking
Patent
1995-05-26
2000-01-11
Ro, Bentsu
Electricity: motive power systems
Braking
Dynamic braking
318245, 318246, 318375, H02P 308
Patent
active
060139935
DESCRIPTION:
BRIEF SUMMARY
This application in a National Stage of International Application, PCT/EP94/03261, under 35 USC .sctn.371, filed Sep. 7, 1994.
The invention relates to a braking circuit for a universal motor in which, for the purpose of braking, one or both field windings are switched with reverse polarity into the armature circuit. Such a polarity reversal is disclosed, for example, in DE 38 02 419 A1. The polarity reversal is performed in this case by means of anti-parallel-connected diodes for both field windings. Furthermore, a switch is provided, by means of which the two field winding terminals facing the armature are short circuited for braking purposes. This short circuit is produced immediately by actuating the switch, with the result that, because of the remanence in the field coils, a strong braking current is set up, which is accompanied by undesirable brush sparking. This brush sparking causes premature wear at the collector of the motor. In addition, in the known circuit, the braking current is not controllable, with the result that, in the case of a high rotational speed accompanied by self-induction, a strong braking current is initially set up, which become weaker with decreasing rotational speed. The braking effect is thereby very irregular over the braking time.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a braking circuit for universal motor, which avoids the disadvantages of the prior art.
In keeping with these objects and with others which will become apparent hereinafter, one feature of the present invention resides, briefly stated, in a braking circuit for a universal motor, which has a switching device reversing a polarity of at least one field winding of the motor for braking operation to be connected in parallel with an armature of the universal motor, the switching device has a controllable semiconductor switch, and the semi-conductor switch is connected in a power path to a terminal side facing the armature, of the first and second field windings.
The braking circuit according to the invention has, in contrast, the advantage that a controllable semiconductor switch is switched instead of the mechanical switch. This results in the advantage that no con tact erosion can occur at the switch, since the semiconductor switch operates without wear. However, it is particularly advantageous that the braking characteristic can be precisely controlled by closing the semiconductor switch in a temporally defined fashion.
It is particularly advantageous that the semiconductor switch can be switched intermittently by a control circuit, since the braking current can thereby be arbitrarily regulated. If, for example, the semiconductor switch is conducting, self-excitation occurs. In this case, kinetic energy of the rotor is converted into magnetic energy during braking. When the current has reached a specific upper limiting value, the switch can be reopened, with the result that the current still flows only in the field windings. There, the magnetic energy is converted into heat in this circuit until the current has once again reached a lower limiting value, at which the semiconductor switch is reclosed. A particular advantage yielded by the intermittent operation is a limitation of the braking current, and thus not only lower wear of the carbon brushes, but also soft braking, which is desired, for example, in the case of circular saws or angle grinders. It is particularly advantageous that the kinetic energy and magnetic energy are converted into heat chiefly in the motor and not in the semiconductor component. As a result, it is also possible to use a smaller heat sink.
It is, furthermore, advantageous that the control circuit for driving the semiconductor switch has a further transistor. The semiconductor switch can be driven in low-loss fashion by means of this transistor.
In order to reduce the brush sparking it is, furthermore, advantageous when the closing of the semiconductor for braking the rotating armature is performed with a time delay. If, for examp
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Ro Bentsu
Robert & Bosch GmbH
Striker Michael J.
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