Electricity: motive power systems – Automatic and/or with time-delay means – Nonresponsive or less responsive for limited periods
Patent
1997-03-10
1999-03-02
Masih, Karen
Electricity: motive power systems
Automatic and/or with time-delay means
Nonresponsive or less responsive for limited periods
318798, 318805, 318808, 318434, 318490, G05B 500
Patent
active
058776044
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention is based on a circuit for operating an electric motor, in which the current flowing through the motor is predetermined, the circuit having a monitoring arrangement for ascertaining an overload state of the motor, the monitoring arrangement comparing the motor voltage to a motor voltage limit value that is predetermined by a limit value generator and emitting an overload signal when the motor voltage limit value is not met.
A method of operating an electric motor in which the motor power can be changed as a function of the presetting of a nominal value is known from DE-A 41 29 086. If the nominal value is below a predeterminable threshold, corresponding to low motor power, the voltage applied to the motor is regulated. If the nominal value is above the threshold, corresponding to a higher motor power, the current flowing through the motor is regulated. The known method is supplemented by safety devices. Examples of safety devices include overcurrent shutoff and short-circuit shutoff. In overcurrent shutoff or overcurrent limitation of the current flowing through the electric motor, the actual current value signal emitted by a current sensor is supplied to an overcurrent limiter, which compares the signal to an internally-predeterminable threshold value. After a threshold is exceeded, the overcurrent limiter transmits a switching signal to a selection circuit to suppress the control signal for an end or drive stage. The shutoff signal can also be fed to the motor-drive circuit or the current regulator.
Depending on the type of application, it may occur that a limitation of the current flowing through the electric motor to a fixed, predetermined maximum value does not suffice to provide the used components with permanent protection against thermal overload.
SUMMARY OF THE INVENTION
It is the object of the invention to provide a circuit for operating a motor that assures particularly reliable protection of an end stage and/or the electric motor against thermal overload.
The object is accomplished by a circuit for operating a motor, of the type described at the outset, which is characterized in that the motor voltage limit value is a function of the motor current.
In accordance with a first embodiment of the circuit of the invention, in which the current flowing through the electric motor is set at a predetermined value, a monitoring arrangement compares the motor voltage to a predetermined motor voltage limit value. If the monitoring arrangement determines that the motor voltage limit value is not met, the arrangement emits an overload signal.
In accordance with another embodiment of the circuit of the invention, in which the motor voltage is set at a predetermined value, a monitoring arrangement compares the current flowing through the electric motor to a predetermined motor current limit value that is dependent on the predetermined motor voltage. If the motor current limit value is exceeded, the monitoring arrangement emits an overload signal.
The key feature of the embodiments of the circuit of the invention is that the electric-motor countervoltage in the motor is rpm-dependent. If the motor or the drive connected to the motor is sluggish, the rpm drops and, consequently, so does the electric-motor countervoltage. The electric-motor countervoltage drops to zero if the electric motor is blocked.
The two embodiments of the circuit of the invention have the advantage that overload or complete blockage of the electric motor can be represented by electrical signals that can be detected with simple means. No further sensors, such as an rpm sensor or a temperature sensor, are necessary.
A particularly advantageous modification of the circuit of the invention, in which the current flowing through the electric motor is set at a predetermined value, provides that the motor voltage limit value present in the monitoring arrangement is set as a function of the predetermined motor current. With this measure, it is possible to adapt the motor voltage limit value to different
REFERENCES:
patent: 4123693 (1978-10-01), Anderson et al.
patent: 4710692 (1987-12-01), Libert et al.
Kunitz Norman N.
Masih Karen
Robert & Bosch GmbH
Wood Allen
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