Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2000-05-16
2001-10-02
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S06800R, C374S152000, C318S798000, C361S140000
Reexamination Certificate
active
06297573
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a three-phase a.c. motor for operation with a three-phase mains supply.
Three-phase a.c. motors with three single- or multiple-strand windings are customarily powered by a three-phase mains supply in such a way that they can be turned on and off. A known arrangement for this purpose is to dispose, between the three-phase mains and the terminals that connect the motor to the mains, two- or three-phase electromechanical switches, relays or contactors by means of which the electrical connection between the three-phase mains and the motor is made or broken. Instead of electromechanical switching elements, it is becoming increasingly common to use electronic switching elements. With the latter, contact abrasion is avoided. The electronic switching elements allow the amount of current that flows to be controlled by gate terminals. As in the case of electromechanical switching elements, at least two electronic switching elements are required. Ordinarily these consist of triacs, alternistors or a combined system in which each triggering circuit is connected to two thyristors in an antiparallel arrangement.
To reduce line and radio disturbances associated with turning the three-phase motor on and off, and to protect the electronic switches, additional components are added to the circuitry, such as RC elements, recovery diodes and varistors. both the electromechanical switches and the electronic switching elements are customarily separated spatially from the three-phase motor, being disposed in switch cabinets or boxes. This arrangement takes up extra space, in addition to that required for the motor itself. Furthermore, additional material is required to construct the switch cabinet or switch box.
Another known feature is the provision of a mechanical relay (contactor) for each phase in the mains supply line, or also of relays switching onto the neutral point. Such arrangements make the cable construction considerably more elaborate and space-consuming.
When electronic switching elements are used to turn the three-phase motor on and off, the total number of switching and peripheral elements is quite large. As a result, considerable costs are incurred in both manufacture and maintenance of the switching arrangements.
The object of the invention is to disclose a three-phase a.c. motor that is turned on and off by electronic switching elements, such that the overall space required and the number of electronic components are reduced in comparison to the known three-phase a.c. motors and electronic switches, and the reliability of operation is improved.
SUMMARY OF THE INVENTION
A three-phase motor having the characteristics given in claim
1
. Further developments are treated.
The three-phase motor in accordance with the invention, for operation with a three-phase mains supply, comprises three single- or multiple-strand windings in a star configuration, such that the neutral ends of the windings are connected to externally accessible terminals. In addition the three-phase motor comprises an electronic switch that is so constructed and so connected to the neutral-end terminals, by way of switch connectors that when the motor is turned on the neutral-end terminals can be put into electrical contact with one another, and when the motor is turned off they can be separated from one another. To control the electronic switch, which preferably consists of thyristors, a control circuit is connected to the electronic switch.
Preferably both the electronic switch and the control circuit are disposed in a terminal box of the three-phase motor, within which are also situated the neutral-end terminals of the windings. In any case, however, the overall space required for the motor and the associated switching arrangement for turning the motor on and off is less than in the known three-phase motors. Also, because of the star configuration the number of electronic components is reduced. Furthermore, there is no need for multiple-phase leads to connect a three-phase motor with a switch cabinet or box.
The terminal box is made of a material with high heat conductance, preferably with external cooling vanes, so that the heat produced in the electronic switch can be released to the exterior by way of the terminal box. Furthermore, the terminal box is attached to the housing of the three-phase motor by way of a gasket, so that the heat-conduction resistance between housing and terminal box is low. The temperature of the terminal box is therefore somewhat different from that of the motor housing.
The electronic switch is constructed as a hybrid circuit, being combined with the temperature sensor and where appropriate also with the electronic control circuit; the components are seated on a ceramic mounting substrate and mounted directly on a hybrid board made of a material with high heat conductance, in particular of copper, so that the heat given off by the electronic switch is transferred to the hybrid board with an extremely low thermal resistance and from the board to the terminal box practically without thermal resistance. The temperature sensor is mounted in a position relative to the electronic switch such that the temperature of the copper hybrid board corresponds to that of the temperature sensor and a certain residual thermal resistance exists between the electronic switch and the temperature sensor.
Preferably at least one additional temperature sensor is provided, which measures the temperature of the windings and for this purpose is mounted in the immediate vicinity thereof. This sensor is connected to the control circuit in such a way that the electronic switch can be turned on only if the winding temperature does not exceed a preset value. By this means protection for the motor windings is also provided in a simple manner.
Because the temperature of the motor housing also influences the temperature of the terminal box, a very reliable thermal monitoring of the whole three-phase motor is thus ensured in a simple way. With this indirect monitoring of the motor temperature, by way of its housing, in some circumstances the additional temperature sensor for the windings becomes unnecessary.
The control circuit comprises an output that signals the operating state as a binary state. When the temperature of the motor or the temperature of the hybrid board (of the electronic switch) exceeds a critical level, or also when the voltage supply for the electronics is disturbed or absent, a binary state is signalled other than that corresponding to the case in which all operating parameters have the normal values. This signal can also be used for other or additional purposes, such as to trigger an alarm signal.
It is preferable for the control circuit to be connected to a measurement device that is in turn connected to the switch terminals, and which senses the voltage at the neutral ends of the windings, the control circuit being so constructed that the processes of switching on and off are controllable in dependence on the neutral-end voltages. In this way the operating safety of the three-phase motor can be increased, in particular for the electronic components, because the level of the neutral-end voltages can be taken into account in controlling the electronic switch.
In a further elaboration the control circuit is so constructed that in response to an external “on” signal, the electronic switch is turned on if at the same time one of the neutral-end voltages exceeds a first threshold voltage, which is higher than a residual voltage between the switch terminals in the turned-on state of the switch, but considerably lower than the voltage of the three-phase mains supply. In particular, the first threshold voltage is 2-5%, preferably 3% of the voltage of the mains supply.
Preferably the control circuit, in particular to protect against overvoltages, is so constructed that the switch is turned on or off when one of the neutral-end voltages exceeds or falls below, respectively, a second threshold value which is higher than the voltage of the three-phase mains
Roth-Stielow Jorg
Schmidt Josef
Marshall Gerstein & Borun
Mullins Burton S.
Sew-Eurodrive GmbH & Co.
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