Electricity: motive power systems – Series motors
Reexamination Certificate
2000-03-14
2001-05-22
Dang, Khanh (Department: 2837)
Electricity: motive power systems
Series motors
C318S756000
Reexamination Certificate
active
06236173
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a series wound commutator motor suitable for an electric power tool comprising switching means for switching between a motor operation mode and a braking operation mode, wherein in motor operation, at least a field coil and an armature winding are connected in series in a motor circuit fed with a voltage supply, and wherein in braking operation, the at least one field coil with the armature coil form a closed braking circuit separated from the voltage supply.
Such a series wound motor is disclosed in EP 0 471 038 B1. The known motor is particularly suited for driving inverse-speed electric power tools, such as right-angle grinders or circular saws and employs a multi-pole switching unit for switching between motor and braking operation modes. The switching unit short-circuits the motor in braking operation and the pole of the field coil is reversed. Means are also provided for limiting the braking current through the field coil. In this manner a smooth and rapid short-circuit braking through a separate auto-excitation is achieved. The armature or rotor is connected between the field coil and the commutating winding in motor operation, while in braking operation a current path is provided between the armature and the commutating winding. The current path comprises a Zener diode arrangement for limiting the braking current, such that only a predetermined amount of the braking current flows across the field coil.
In such motors, a reliable braking or retardation of the motor by switching into the braking operation is achieved, however, the use of commutating windings is more complicated and expensive.
Attempts have been made to avoid such problems by providing a capacitor to be charged in motor operation, which is then used for braking by auto-excitation (DE 42 01 023 A1). The known circuitry has the disadvantage that the stored charge can only be used once for introducing braking. If the charge is not sufficient to initiate braking, then braking cannot be expected to occur after capacitor discharge.
An object of the present invention is therefore to provide an improved series wound motor, which avoids the above disadvantages, in particular which is simple and inexpensive in structure and which guarantees a reliable braking when switching to braking operation.
SUMMARY OF THE INVENTION
According to the present invention, a series wound motor of the type mentioned is provided comprising a mains-powered transformer whose secondary winding is connected in the braking circuit parallel to the at least one field coil and a transistor circuit for controlling the current flowing in the braking circuit across the armature winding and the at least one field coil. The object of the present invention is solved completely in this manner.
A current is introduced in the braking circuit by means of the secondary winding of the transformer, which is sufficient to achieve a reliable initiation of braking. According to the present invention, a braking initiation when switching to the braking mode is guaranteed even in the most unfavourable situations.
The transistor circuit allows the braking current to be regulated, so that a large braking moment is available even in advanced stages of the braking process and provides a short braking time. In addition, the use of a diode arrangement connected in parallel to the field coil becomes superfluous.
Furthermore, the present invention allows complete elimination of commutating windings, which leads to a distinct simplification of the configuration and a cost reduction.
In an advantageous embodiment of the present invention, a field-effect transistor is provided, whose source and drain are connected in parallel to a load resistor in the braking circuit between the armature winding and the at least one field coil and which is triggered by a control voltage between the source and gate depending on the braking current.
With this, a sufficient voltage stability of the transistor is guaranteed, also in unfavourable circumstances, without the necessity of taking additional measures. By controlling the field-effect transistor between the source and gate depending on the braking current, it is achieved that the full braking current flows over the load resistor at the beginning of the retardation process and that the field-effect transistor is triggered at an advanced stage of braking, such that a high braking moment is generated even at the end.
In another preferred embodiment of the present invention, a voltage stabiliser is connected to the secondary winding of the transformer. The stabiliser is connected with a current-sensing resistor connected in series with the load resistor in the braking circuit to generate a pre-voltage between the source and gate of the field-effect transistor which is dependent on the braking current and is opposed to the voltage of the voltage stabiliser. The desired triggering of the field-effect transistor depending on the braking current is thus achieved in a particularly simple and reliable manner.
A Zener diode is provided as the voltage stabiliser in a preferred embodiment which is connected to the at least one field coil, one end of the secondary winding and the drop resistor at its anode and connected to the gate of the field-effect transistor and to the other end of the secondary winding at its cathode. The desired triggering of the field-effect transistor is therefore achieved with particularly simple means.
In another embodiment, a capacitor is connected in parallel to the Zener diode and the cathode of the Zener diode is connected to a diode at the secondary winding of the transformer. With this, the control voltage of the field-effect transistor is smoothed, which leads to a uniform braking or retardation effect.
According to a further embodiment of the present invention, the field-effect transistor and the drop resistor are connected to the cathode of a diode, the anode of which is connected to the armature winding in the braking circuit. Protection is provided against the theoretical possibility that an auto-excitation in the opposite direction could be initiated in an extremely unfavourable circumstance, which could lead to over-voltage between the drain and gate and thus destroy the field-effect transistor.
According to a further aspect of the present invention, the secondary winding of the transformer is connected via a rectifier to one pole of the armature winding. This ensures the initiation of the braking process in the desired direction, since the current across the secondary winding can only flow in one direction.
In a further embodiment of the present invention, a protective diode is provided to guard against arcing at the switching means between one pole of the armature winding and the other end of the secondary winding or the at least one field coil. This prevents damage of the transformer and the rectifier connected in series with the secondary winding should an arc develop on the switching unit during the switching process.
According to a further aspect of the present invention, the load resistor between the source and drain is provided as a commutating winding. This has advantages in some cases, especially for larger machines with a motor rating of 2000 Watt or more. Heat problems are avoided which could arise through the strong loading of the resistor. Such a measure however is not absolutely necessary for high performance motors. It is advantageous only in individual cases, since the production complexity and production costs would increase.
In an alternative embodiment of the present invention, the transistor circuit comprises a field-effect transistor which is connected with its source and drain to the at least one field coil via a diode in parallel and which regulates the current through the at least one field coil as a function of the current flowing through the armature winding. In this embodiment, the use of commutating poles can be completed eliminated.
A particular advantage is that the braking characteristic can be adjusted such that a slow con
C. & E. Fein GmbH & Co.
Cummings & Lockwood
Dang Khanh
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