Electricity: motor control systems – Closed loop speed control system for dc motor with commutator
Reexamination Certificate
2000-05-12
2002-09-24
Fletcher, Marlon T. (Department: 2837)
Electricity: motor control systems
Closed loop speed control system for dc motor with commutator
C388S801000, C318S248000, C318S252000
Reexamination Certificate
active
06456784
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention resides in a circuit arrangement for starting a DC motor, which includes at least one field coil, particularly for switching on a DC motor driving a hydraulic pump for the operation of hydraulic lift and/or tilt actuators of loading platform systems of vehicles.
DC motors are used in almost all industrial fields, particularly in areas where a low voltage DC power supply is present such as in motor vehicles which are not connected to a power supply net. Such vehicles are ships, airplanes and land vehicles, particularly trucks.
It is known that trucks may be provided with so-called loading platform systems in order to be able to lift for example a load from ground level to the level of a loading bed of a truck and vice versa. Certain loading platform systems are not only capable of lifting a loading platform from ground level to the level of the loading bed of a truck, but they are also capable of tilting the loading platform from a horizontal to a vertical position so as to form an end wall of the loading bed of a truck when the truck is moving. The lifting and lowering and the tilting of the loading platform from a vertical to a horizontal position and vice versa is achieved by so-called lift actuators and, respectively, tilt actuators, which always comprise piston-cylinder arrangements, in which a pressurized hydraulic medium causes the movement of the piston or the pistons. The pressure of this hydraulic medium is generated by one or several hydraulic pumps which are driven by DC motors and which are energized by the electrical power supply system of the truck, for example, a 12 volt or 24 volt power supply system. In order to be able to perform the lifting and the tilting functions, particularly the lifting functions, under a maximum load, those DC motors need to be relatively powerful so that they can drive the respective hydraulic pumps and generate the needed pressure of the hydraulic medium.
Typically, these DC motors have a normal power output of 3 kW. If such a DC motor is operated from a 12-volt vehicle power supply at the given nominal power output, the current flow is about 250 Ampere. Switches, which are capable of switching such currents and which need to be operative over a long period, must be relatively large and therefore cannot be integrated into an electromechanical and/or electronic control unit used for the control and supervision of the functions of a loading platform system. Therefore, a relay is generally provided, which is operated by the control unit as described before, and which is capable to switch the high currents by way of high power switches.
The relay for switching the DC motor is mounted near, or on, the DC motor and is connected electrically to the motor as well as to the control unit.
With the rough environment to which the loading platform systems are exposed, such as rain, snow, dust, exhaust gases and also rough treatment by the operators, the relays for switching the power supply to the DC motor operating the hydraulic fluid pump has been found to be a weak link which often results in malfunctions or even total failure of the system. The relay needs to be encapsulated for protection of the rough environment if operational reliability is to be ensured. Furthermore, the separate mounting of the relay on the DC motor or its arrangement near the motor and the wiring with the motor and the control unit involve substantial expenses as the equipment requires special handling during manufacture and installation of the loading platform systems.
It is therefore the object of the present invention to provide a circuit arrangement for the switching of a DC motor of the type described above with which a separate mounting of a relay for the energization of the DC motor is not necessary and the operational reliability of the motor is essentially independent of weather and environmental conditions. As a result, a DC motor with its motor control arrangement can be realized at relatively low costs.
SUMMARY OF THE INVENTION
In a circuit arrangement for starting a DC motor comprising at least one field winding, particularly for switching on a DC motor for driving a hydraulic fluid pump for operating hydraulic actuators of a vehicle loading platform system, means are provided for switching on at least one field winding and means which can be activated by the switching on of the at least one field winding for fully activating and starting the DC motor.
The advantage of the arrangement according to the invention essentially resides in the fact that the switching means itself can be connected with one pole firmly to the power supply of the vehicle by way of a suitably dimensioned electrical conductor. No relay is needed, no relay needs to be mounted to the DC motor or nearby and no installation work is required for establishing the electrical connections between the relay, the DC motor, the vehicle power supply and the control unit.
DC motors can be designed so that the field windings or coils are arranged in series with the armature of the DC motor. Such a motor is called a series-wound motor. But it is also possible to arrange the field windings in parallel with the armature. Such a motor is called a shunt-wound electric motor. Series-wound motors and shunt-wound motors have certain properties that make a particular DC motor type more suitable for certain drive applications. It is therefore advantageous for certain applications to provide the field winding with at least one series exciter winding which causes the starting of the DC motor according to the invention. However, it may also be advantageous to provide the field windings as at least one shunt winding which then causes the switching on of the DC motor in accordance with the invention.
From the state of the art, there are in addition to series-exciter wound DC motors and shunt-wound DC motors, also so-called compound-wound motors, which include at least one series exciter winding and at least one shunt exciter winding. For certain applications, these motors have advantages over pure series-wound motors or pure shunt-wound motors. Also, compound-wound motors can be advantageously operated with the circuit arrangement according to the invention. This is done in a way that the shunt winding is energized independently of the series exciter winding wherein, by energization of the shunt winding, the series exciter winding may be switched on.
The advantage of this arrangement resides essentially in the fact that the shunt exciter winding, which is arranged parallel to the armature of the DC motor, takes a lower current than the series exciter winding, which is arranged in series with the armature. The current flow through the shunt winding is essentially constant independently of the motor speed. As a result, the shunt exciter winding can be directly switched on and off by the control unit for the loading platform system referred to earlier, without the need for a relay. When the shunt exciter winding is switched on by the control unit when needed for switching on the DC motor, the series exciter winding is subsequently switched on by the shunt winding by way of suitable means.: That is, also the series exciter winding is energized after the control unit energizes the shunt exciter winding so that then the DC motor becomes normally operative.
However, it may also be advantageous if the series exciter winding can be switched on independently of the shunt winding such that the shunt winding is switched on by the energization of the series exciter winding. This last-mentioned variation of the circuit arrangement may be advantageous for certain applications, for example, for design reasons depending on the arrangement of the series exciter winding or, respectively, the shunt winding in the motor housing.
In another advantageous embodiment of the circuit arrangement, the magnetic field generated by the energization of the field windings may be used for the switching on of the DC motor. To this end, the DC motor may include switch means or other effective means, which a
Bach Klaus J.
Fletcher Marlon T.
Sörensen Hydraulik, Zweigniederlassung, Ulfborg
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