Electric motor with field-current-generated magnetic-field brake

Electricity: motive power systems – Braking – Dynamic braking

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Details

318123, H02P 300

Patent

active

057898855

DESCRIPTION:

BRIEF SUMMARY
FIELD OF INVENTION

The present invention relates to an electric motor with an electrodynamic brake and with a controller that controls the motor current or motor speed by way of a power controller unit.


BACKGROUND INFORMATION

German Patent No. 3/539/841 C2 has already disclosed a braking means for a series motor, where the polarity of the field coil is reversed in the braking mode and the field coil is connected in series with the armature across a solid-state switching device. In motor operation, however, the power semiconductor device has practically no effect because it forms a separate circuit from the motor. If this is to be a variable speed motor, another power controller unit would be necessary for the controller. The circuitry required for braking and motor operation of the electric motor is relatively complicated because of the two power controllers that must be used.


SUMMARY OF THE INVENTION

The electric motor according to an embodiment of the present invention has the advantage that the same power controller can be used in both motor operation and in the braking mode. It is especially advantageous that this double use of the power controller not only reduces the circuitry but also makes it possible to eliminate an additional heat sink. As a rule, a heat sink takes up a lot of space and therefore is not desirable.
It is also advantageous for the power controller to control the field current that creates the magnetic field in braking. The braking current can be controlled to advantage by the control input of the power controller. This is preferably accomplished by designing the controller 5 as a phase controller, which is provided anyway for the purpose of speed control in motor operation.
Due to the controller that is provided for the braking current, the braking time for braking the motor, which may be connected to a cutting tool or a saw, can be preset. Especially with circular saws, hedge clippers and lawnmowers, it is desirable to brake the running tool very quickly in order to minimize the risk of injury to the operator.
It is also advantageous for the controller to have devices for measuring the braking current for example, a voltmeter to measure the voltage, or an ampmeter to measure the current as is known in the art. An example of such a device is shown schematically as 16 in FIG. 4. With the help of these devices, the field current can be kept constant over the entire braking operation or adapted to a given curve to keeping the braking current constant, preferably over the entire braking operation, is desirable in order to avoid high current peaks when initiating the braking operation. These current peaks cause extreme brush sparking, which in turn causes excessive and irregular wear on the carbon brushes. This results in relatively gentle braking of the electric motor or hand tool.
It is also advantageous for a first change-over switch to be used to direct the motor current through the power controller in motor operation or to reverse the field coil or parts thereof when braking, so the field current can generate the braking magnetic field.
In addition, a current limiting resistor is also provided in the field circuit (2,6,7,3,4) to avoid overloading the field coils. Likewise, a braking resistor can also be provided in the armature circuit for the braking operation. This permits separate adjustment of both the field current and the braking current.
Another application of the electric motor according to the present invention is in an electric hand tool, where the tool should be braked as quickly as possible to prevent the danger of injury. This is desirable with a saw, a grinder, a lawnmower, etc.


BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block schematic of a first embodiment according to the present invention.
FIG. 2 illustrates a second embodiment according to the present invention.
FIG. 3 schematically illustrates a device powered by an electric motor which may be, for example, a hand tool, a grinder, a saw, a lawnmower.
FIG. 4 illustrates a block schematic of

REFERENCES:
patent: 4054821 (1977-10-01), Williamson
patent: 4112341 (1978-09-01), Fath et al.
patent: 4144482 (1979-03-01), Schwab
patent: 4145641 (1979-03-01), Ozaki
patent: 4423363 (1983-12-01), Clark et al.
patent: 4514677 (1985-04-01), Kaufman et al.
patent: 5036234 (1991-07-01), Friedrich et al.
patent: 5099184 (1992-03-01), Hornung et al.
patent: 5294874 (1994-03-01), Hessengerger
patent: 5315221 (1994-05-01), Corey
patent: 5517093 (1996-05-01), Augustyniak et al.

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