Power Supply System for Linear Motor

This invention relates to a power supply system for linear motors, and in particular to such a system capable of compensating for reactive power which varies as a running body travels.

There are two systems in the case where a running body is driven by a linear motor. In one system the armature coils (commonly called driving coils) of the linear motor are disposed along the track for the running body and alternatively in the other system they are mounted on the running body. It is usual, however, to dispose the driving coils along the track for the convenience of feeding power. In this case, a plurality of driving coils which have proper length are disposed along the track in respect of the power factor and the power supply capacity. These driving coils are respectively connected through switches associated thereto to a power converter device. The individual switches are actuated in accordance with the movement of the running body. The thus constructed power supply system may selectively feed only some of the driving coils located in the vicinity of the running body, and therefore it is more advantageous for the power factor and power supply capacity than a power supply system which feeds simultaneously all the driving coils throughout the track of the running body.

A particular example of a power supply system of this kind is disclosed, for example, in U.S. Pat. No. 4,068,152.

Any electrical equipment involves reactive power. Since a low level of reactive power is of course more desirable, there are some pieces of electrical equipment in which the reactive power may be compensated for, as described, for example, in Japanese Patent Application Laid-Open (Kokai) No. Sho 51-33850. That is, such a reactive-power compensation system is known in which the compensation quantity is adjusted in accordance with the power factor of a load.

Also in a power supply system for linear motors the compensation for reactive power is required. In the case of a linear motor, however, such a reactive-power compensation system as previously described in connection with general electrical equipment cannot be utilized as it is, because the movement of the running body must be taken into consideration.

From the view point mentioned above, it has been desired to provide such a power supply system for a linear motor which is capable of compensating for the reactive power peculiar to linear motors.

It is therefore an object of the invention to provide a power supply system for a linear motor which is capable of compensating for reactive power.

According to the present invention, there is provided a power supply system which comprises a reactive-power controller for controlling the compensation quantity in accordance with the movement of a running body so as to compensate for the reactive power which varies in accordance with the movement of the running body.

The foregoing object and other objects as well as the characteristic features of the invention will become more apparent and more readily understandable by the following description and the appended claims when read in conjunction with the accompanying drawings in which:

FIG. 1 shows a schematic diagram of an embodiment of a power supply system for a linear motor according to the present invention;

FIG. 2 shows waveforms illustrating the operation of the system shown in FIG. 1;

FIG. 3 shows a circuit diagram illustrating the control for a second reactive power controller;

FIG. 4 shows an example of an electric circuit diagram of a reactive power controller;

FIG. 5 shows characteristics of a linear motor; and

FIGS. 6 and 7 show block diagrams of a reactive power setting device and a reactive power controller, respectively, in the case where the reactive power controller is controlled in accordance with the operating condition of the linear motor.

In FIG. 1 showing a schematic diagram of an embodiment of a power supply for a linear motor according to the present invention, linear motor driving coils LM.sub.1, LM.sub.2, LM.sub.3, LM.sub.4 . . . are alternately connected to a feeder F.sub.1 and a feeder F.sub.2 through switches SW.sub.1, SW.sub.2, SW.sub.3, SW.sub.4 . . . , respectively. Power converters CC.sub.1 and CC.sub.2 are connected to the feeders F.sub.1 and F.sub.2, respectively, and a first and a second reactive power controller RPC.sub.1 and RPC.sub.2 are connected to the primary winding of a transformer Tr which is in turn connected to an a.c. source SS.

The operation of this power supply system will now be described. When a running body labelled TRAIN is located over both of the two adjacent driving coils LM.sub.2 and LM.sub.3 as shown in FIG. 1, both the switches SW.sub.2 and SW.sub.3 associated respectively to these coils LM.sub.2 and LM.sub.3 are closed to allow the power converters CC.sub.1 and CC.sub.2 to feed these two driving coils LM.sub.2 and LM.sub.3 respectively. When the running body comes only above the driving coil LM.sub.3 after having left the driving coil LM.sub.2 the power converter CC.sub.2 is disconnected and only the power converter CC.sub.1 continues to feed the driving coil LM.sub.3 until the moment immediately before the running body reaches the driving coil LM.sub.4.

FIG. 2 shows waveforms in this operation. In FIG. 2, a and b represent the power supplied by the power converters CC.sub.1 and CC.sub.2, respectively. The labels "LM.sub.1 " and "LM.sub.3 " in a respectively indicate that the driving coils LM.sub.1 and LM.sub.3 are fed by the power converter CC.sub.1 and the labels "LM.sub.2 " and "LM.sub.4 " in b respectively indicate that the driving coil LM.sub.2 and LM.sub.4 are fed by the power converter CC.sub.2. During the period in which the running body is passing over two adjacent driving coils, both the power converters CC.sub.1 and CC.sub.2 supply power simultaneously. According to this feeding method, the running body may travel through its entire track with a constant driving force. However, the resultant power of the power converters CC.sub.1 and CC.sub.2, i.e. the apparent power on the primary side of the transformer Tr, is greatly varied as shown in the waveform c of FIG. 2, thereby causing an adverse effect om the a.c. input source SS unless proper measures are taken. The variations in the apparent powr are attributed mostly to the variations in the reactive power caused by the simultaneous exitation of two driving coils. As shown in the waveform d of FIG. 2, the reactive power is considered to include two portions; one portion RP.sub.B being a basic portion determined by the power factor or the like of the linear motor and the other portion .DELTA.RP being an increment caused by the simultaneous exitation of two driving coils. If the basic reactive power RP.sub.B as shown in the waveform e of FIG. 2 may be compensated for by the first reactive power controller RPC.sub.1 and described above and the reactive power increment .DELTA.RP caused by the simultaneous exitation of two driving coils as shown in the waveform f of FIG. 2 may be compensated for by the second reactive power controller described above, the a.c. input power source may be designed to supply effective power Pw which is substantially constant as shown in the waveform g of FIG. 2.

Each of the reactive power controllers may be of the conventional type comprising capacitors and reactors. As shown in FIG. 3, the second reactive power controller RPC.sub.2 achieves its control operation in conjunction with feeding commands GS.sub.1 and GS.sub.2 produced by a feeder-section switching control device SC for controlling the operations of the switches and the power converters. More specifically, when both the respective feeding commands GS.sub.1 and GS.sub.2 to the power converters CC.sub.1 and CC.sub.2 indicate the actuation of the power converters, a gate circuit TSG actuates a thyristor switch TS to turn it on so as to connect the second reactive power controller RPC.sub.2 to the a.c. power line. Thus, the variations in reactive power which are caused while the running body is passing over two adjacent driving coils may be compensated for.

In the case where the second reactive power controller is of a thyristor controlled type as shown in FIG. 4, the thyristor switch TS of FIG.3 may be omitted. In this reactive power controller of FIG. 3, thyristors TS.sub.11 to TS.sub.32 are turned on when the command from the gate circuit TSG is on, while turned off when the command from the gate circuit TSG is off. C.sub.1 to C.sub.3 in FIG. 4 designate parallel capacitors. In the case of the reactive power controller of the thyristor controlled type, the compensation for reactive power may be made more finely if reactive power is controlled every moment in accordance with a predetermined reference value which is provided as a command corresponding to the variations in reactive power while the running body is passing over two adjacent driving coils.

The first reactive power controller RPC.sub.1 is provided to compensate for the basic or steady portion of the reactive power as described previously. In the case where the linear motor is a linear synchronous motor, its characteristics, such as power factor (curve X), reactive power (curve Y) and efficiency (curve Z), vary with the motor speed as shown in FIG. 5. Commonly, since a linear synchronous motor is driven by a constant current in accordance with a current reference, the reactive power varies with the magnitude of the reference current flowing through driving coils. For example, in acceleration a relatively large current is required, while in constant speed running a current is required less than that corresponding to the driving force against the resistance to the movement of the running body. In the case of a linear motor, the reactive power to be compensated for thus depends on the operation conditions such as motor speed and current. FIGS. 6 and 7 show arrangements to be used to compensate for the variations in this basic portion of reactive power by means of the first reactive power controller.

In FIG. 6, a reactive power setting device RPC.sub.s1 receives signals representative of operating conditions of the linear motor, such as the motor speed and current, and sets the reactive power to be compensated for by the first reactive power controller. The reactive power compensation may most desirably be performed continuously in accordance with the operating conditions of the linear motor, but such an arrangement of the reactive power controllers will be complicated and expensive. As a second best alternative, the reactive power compensation is performed stepwise by dividing the operation conditions in accordance with which the compensation is effected into three steps in the reactive-power setting device RPC.sub.s1 of FIG. 6 and the first reactive power controller RPC.sub.1 of FIG. 7. In FIG 7, a maximum powering controller unit RPC.sub.11, a powering controller unit RPC.sub.12, and a regenerative braking controller unit RPC.sub.13 are provided and they are selectively alternatively connected through switches S.sub.11, S.sub.12 and S.sub.13 respectively to the a.c. source so as to compensate for reactive power in a stepwise manner. This enables the system to be constructed simply and inexpensively. The alternative selection of the reactive power controller units RPC.sub.11 to RPC.sub.13 may be made for example, in such a manner as shown in FIG. 6 in which the output of a function generator FA representative of reactive power characteristics with respect of the motor speed is multiplied by a reference current value by a multiplier MA and the resultant is used by comparators CP.sub.11 to CP.sub.13 to determine which unit is to be selected. The comparators CP.sub.11, CP.sub.12 and CP.sub.13 correspond to the maximum powering, normal powering, and regenerative braking running conditions. The setting value of the comparator CP.sub.12 occupies a positive region, the setting value of the comparator CP.sub.11 also occupies a positive region which is, however, larger than that of the comparator CP.sub.12 and the setting value of the comparator occupies a negative value. When the output signal of the multiplier MA falls within any one of the setting value regions of the comparators CP.sub.11, CP.sub.12 and CP.sub.13, the comparator produces an output to actuate one of the switches S.sub.11 to S.sub.13 associated to the comparator to make the contact thereof. When the output from the multiplier MA is relatively small, the reactive power to be compensated for is small and thus the switches S.sub.11 to S.sub.13 are opened to disconnect the reactive power controller from the system. In order to perform the reactive power compensation more finely, the reactive power setting device RPC.sub.s1 and the first reactive power controller RPC.sub.1 shown in FIGS. 6 and 7 may be divided into more units.

The above-described method of changing the reactive power to be compensated for in accordance with the operating conditions is also effective in the second reactive power controller. For example, when the reference current value is small, the variations in reactive power are small and thus the quantity to be compensated for should be small, otherwise an overcompensation will occur. If a reactive power controller of an advanced power factor is connected to a load of a lagging power factor in powering, the power source line is considered to be connected to a load of an excessively advanced power factor, resulting in adverse effects such as an increase in voltage. Taking into consideration the fact mentioned above, in the embodiment shown in FIG. 7, the quantity of the reactive power to be compensated for by the second reactive power controller RPC.sub.2 may also be varied in accordance with the operating conditions by the reactive power setting device RPC.sub.s2 in the same manner as that effected by the first reactive power setting device RPC.sub.s1 and the first reactive power controller RPC.sub.1.

Although in the system of the above-described embodiments two power converters and two feeders are utilized to feed adjacent driving coils by different power converters, the present invention is not limited to this system. An alternative system may be embodied in which a single power converter is used and the feeding the driving coils is controlled merely by switches. Alternatively, three or more power converters may also be used.

The present invention provides a power supply system for a linear motor capable of compensating for the reactive power occurring in the case where a plurality of driving coils are sequentially fed with electrical energy in accordance with the movement of a running body, i.e. the reactive power varying with the movement of the running body, while preventing adverse effects from occurring on the associated a.c. power source line.