GB1576435A

GB1576435A - Fork lift truck with balance weight using batteries as power source

Info

Publication number: GB1576435A
Application number: GB2497977A
Authority: GB
Original assignee: Shinko Electric Co Ltd
Current assignee: Shinko Electric Co Ltd
Priority date: 1977-06-15
Filing date: 1977-06-15
Publication date: 1980-10-08

Description

(54) FORK LIFT TRUCK WITH BALANCE WEIGHT USING BATTERIES AS POWER SOURCE (71) We, SHINKO ELECTRIC CO., LTD., of 3-12-2, Nihonbashi, Chuo-ku, Tokyo, Japan, a Japanese company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a fork lift truck with a balance weight using batteries as power source.

In a general battery-powered vehicle ap plication, unlike an ordinary electricitypowered vehicle which is continuously supplied with electricity from an external source of power, it is inevitable that there is practically a substantial restriction in a vehicle's capability such as a distance or a payload that the vehicle can cover with a single charge of its battery. In consideration of this fact, it is essential in the design of such a vehicle to obtain an as high as possible operating efficiency so as to have a maximum extent of its capability in terms of a running distance or a payload.In recent years, there has been developed a direct current chopper system, and the operating efficiency of such a batterypowered vehicle has been improved substantially with the introduction of such direct current chopper system to the battery-powered vehicle, instead of the conventional speed control system incorporating the series-connected resistance and switches. However, with the application of the direct current chopper system, because of an increase in the iron (core) loss of the motor produced from the chopping of the voltage applied to the motor, and the ohmic loss due to a large chopping current, in comparison with a speed control system applying the shunt field control, a battery-powered vehicle having such direct current chopper system still suffers from relatively low efficiencies in the starting and power running conditions.In this consideration, in the case of a batterypowered fork lift truck in which frequent starting and stopping operations are involved, or in the case of a battery-powered locomotive in which uphill and downhill operations are frequently encountered, it is actually an indispensable technique to put the vehicles in the regenerative braking condition so as to convert the kinetic energy of the vehicle into electric energy for improving the overall operating efficiency of the vehicles.However, in the practical design and construction of power regenerating arrangements in such direct current chopper systems, which have been practiced in the ordinary external-powersourced electric trains, the circuitry and devices incorporated therein inevitably become complicated, thus resulting in greater initial cost and maintenance expenses, especially for fork lift trucks in which frequent starting and stopping operations are essential in their routines.

In addition, in the case of the chopper system, when the motor generated voltage of an electric motor greatly exceeds the power source voltage, excessive current in the armature is caused to flow in the windings thereof, thus leading to an uncontrollable state, and consequently it is practicably impossible to obtain a sufficient efficiency in the regenerative braking operation for the reason that in a high range of speeds of the armature where a maximum energy can be regenerated, the current passing through the armature must necessarily be limited to a relatively low level.

According to the invention, there is provided a fork lift truck comprising: a separately excited compound motor including an armature, a series field winding, and a separately excited field winding; a ferromagnetic balance weight mounted on the fork lift truck; a first battery mounted on said fork lift truck for supplying the armature circuit of the motor; a second battery mounted on the fork lift truck for exciting the separately excited field winding; a current transient limiting reactor having a reactor core element and a reactor winding, the reactor core element comprising at least a part of the ferromagnetic balance weight and the reactor winding being connected in series with the armature between the armature and the first battery; a diode connected in parallel with at least part of the series. field winding so as to short-circuit the at least part of the series field winding only during a power regenerating operation of the motor; an operator actuable accelerator device on the fork lift truck; and speed control means including means for controlling the separately excited field current of the motor in response to actuation of the accelerator device by operator.

Preferably there is provided a further diode connected so as to short-circuit at least part of the reactor only during a power regenerating operation of the truck.

Preferably, the first battery comprises two battery cells which are connectable in series or in parallel with each other, and the speed control means further includes a first diode electrically connected to the positive poles of the cells so as to prevent charging current flow into one of thel cells, a second diode electrically connected to the negative-poles of the cells so as to pre vent charging current flow into the other of the cells, and a switch connected between the positive pole of one cell and the negative pole of the other cell so as to connect the two cells in series when the switch is closed and so as to prevent current flowing in series through the cells when the switch is open.

Preferably, the motor armature has a first section and a second section which are connectable in series or in parallel, and the speed control means further include a third diode electrically connecting together the positive terminals of the armature sections, a fourth diode electrically connecting together the negative terminals of the armature sections, the third and fourth diodes each being connected so as to prevent driving current flow into one of the armature sections, a first contactor connected between the positive terminal of the first armature section and a positive voltage supply terminal, a second contactor connected between the negative terminal of the second armature section and a negative voltage supply terminal, a circuit comprising a fifth diode and a third contactor connected in parallel with each other and between the positive terminal of the first armature section and the negative terminal of the second armature section, the fifth diode being connected so as to direct driving current through the armature sections in series when the third contactor opens, the three contactors being arranged so that, when the third contactor is open the other two contactors are closed and vice versa so that switching the three contactors selectively connects both armature sections in either series or parallel.

The invention will be described by way of examples with reference to the accompanying drawings, in which like parts are designated with like reference numerals, and in which: Figs. 1A and 1B are schematic wiring diagrams showing the conventional art chopper system, in which Figure 1 A shows a state of power running, and Figure 1B shows a state of power regeneration; Fig. 2 is a schematic diagram showing a basic construction of a speed control device circuit of a fork lift truck constituting a preferred embodiment of this invention; Fig. 3 is a graphic representation showing a characteristic curve in terms of the armature current versus the speed of the motor shown in Fig. 2; Fig. 4 is a schematic diagram showing an alternative field control circuit to that shown in Fig. 2;; Fig. 5 is a wiring diagram showing a circuit for the power source comprising batteries arranged for either series or parallel connection; Fig. 6 is a schematic diagram showing the motor armature windings divided into two sections of windings so that they may be operated in either series or parallel operation; Fig. 7 is a front elevational view showing the balance weight of a preferred fork lift truck used as a reactor core; Fig. 8 is a cross-sectional view taken along the plane designated by the line A-A in Fig. 7; Fig. 9 is a front elevational view showing another embodiment wherein part of the body of a preferred fork lift truck comprising thick sheet steel functions as a part of the balance weight and is used as the core of the reactor; Fig. 10 is a cross-sectional view taken along the plane designated by the line B-B in Fig. 9; and Fig. 11 is a cross-sectional view taken along the plane designated by the line C-C in Fig. 9.

Figs. 1A and 1B are schematic wiring diagrams which are designed to simply show the states of power running and power regeneration, respectively, in a conventional art chopper system. In Fig. 1A, there is shown a diode D which is connected in a circuit from the positive electrode of a power source battery E, through a chopper Ch connected in series with the motor M, and through a series field coil L and an armature M of the electric motor to the negative electrode of the battery E.

In Fig. 1B, there is shown a case wherein the diode D and the chopper Ch are interchanged in comparison with the arrangement shown in Fig. 1A. As shown in Figures 1A and 1B, when the system including a single chopper therein is used and the vehicle is shifted from its power running condition to a power regeneration state, the chopper section Ch should be shifted from a series connection to a parallel one with the electric motor, while the flywheel diode D should be shifted from a parallel connection with the motor to a series connection with the motor.Particularly, in the case of a fork lift truck, since it is generally required to brake the vehicle at such a high frequency as several times for one minute during operation, it is important that a quick changing between series and parallel connections can be readily made and with a high stability in the power regenerative braking operation. In this consideration, there have been proposed certain means for reducing the number of the switches in the system circuit which are required for changing the connections of the circuit during operation, but such circuits are likely to result in such drawbacks as complexity in construction and high cost.In the case of a battery-powered locomotive which is to be used in situations affording few chances of power regeneration braking and with a conventional chopper system, when arranged to provide power regeneration braking during operation, there are many complicated and difficult problems, e.g. positive building-up of a regenerated voltage, prevention of overcurrent in the circuitry, deciding upon an optimal chopping frequency, and selection of circuit components or elements including a reactor, in order to obtain a stable power regeneration braking operation within a specified range of speeds of the vehicle.

Referring now to Fig. 2, there is shown a basic speed control circuit of a fork lift truck, comprising a power source battery 7, a direct current compound wound motor including an armature 1, a series field winding 2 and a separately energised "shunt" field winding 3, a reactor 4 connected in series between the battery 7 and the series field winding 2, two pairs of forward drive or front contacts Sf connected in series with the armature 1, and two pairs of rearward drive or back contacts Sb connected in parallel with the armature 1. One terminal of the shunt field winding 3 of the motor is connected in circuit with a variable resistor 6 for regulating the shunt field current.Also, an accelerator pedal 9 is operatively connected to the movable part of the variable resistor 6 in such a manner that a resistance value of the variable resistor 6 is changeable in proportion to the extent of depression of the pedal by the operator of a vehicle. The switch 10 can be closed or turned "on" with the depression of the pedal 9 by the operator. A diode 11 is adapted to short-circuit all or part of the series field winding 2 during power regeneration braking operation, while a diode 12 is adapted, during power regeneration braking, to short-circuit all or part of the reactor winding 4, whose purpose is for smoothing or preventing current transients or irregular surges in the circuit during starting of the motor or changing to and from regenerative braking.The other terminal of the shunt field winding 3 is connected to a positive terminal, whilst a discharge resistor 14 is connected in parallel with the shunt field winding 3. A movable extension wire 13, in the form of a coil spring, and a fixed resistance 8 for the shunt field circuit are connected in series, one terminal of the above mentioned movable wire is connected to the variable resistor 6, and one terminal of the resistance 8 is connected to a negative terminal in the circuit.

Fig. 3 is an exemplary graphic representation showing the characteristic curves of the speed control device of Fig. 2, with armature current plotted on the abscissa and "revolution number" (i.e. motor speed) on the ordinate. The contactors 5f are turned "on" (i.e. closed) when the fork lift truck is to be driven in the forward direction, whilst the contactors 5b are turned "on" (i.e. closed) when the fork lift truck is to be driven in the rearward direction.

Depression of the accelerator pedal 9 to a slight extent firstly causes the shunt field current to reach a maximum level; then a further depression of the pedal will close the main circuit switch 10 (turning it "on") thus causing the electric motor to run. By limiting the starting armature current to an appropriate level by means of the reactor 4, there is effectively prevented any such improper operating state as for example a flash-over of the motor, thus producing a smooth starting of fork lift truck.

As the armature current decreases gradually along the curve B in Fig. 3, the fork lift truck's speed will gain increasingly.

When the accelerator pedal 9 is depressed further by the operator at the point P1, so that the shunt field current decreases and the motor characteristic shown by the curve C is selected, the armature current will sharply increase from the point P1 on the curve B toward the curve C. As the fork lift truck speed increases along the curve C, the armature current will gradu ally decrease accordingly. When the accelerator pedal is depressed still further at the point P2 by the operator, the motor characteristics is now changed to the curve D, and the armature current is again increased sharply. As the fork lift truck speed increases along the curve D, the armature current now decreases along the curve D.When further depression of the accelerator pedal 9 at the point P3 changes the motor characteristics over to the curve E, the fork lift truck speed increases up to the point P4 on the curve E, where the payload torque equals the motor torque, so that the fork lift truck ceases to accelerate.

On the other hand, when the operator desires to drop the fork lift truck speed, and raises the accelerator pedal 9 to the motor characteristic represented by the curve C, the operating condition of the fork lift truck is now shifted over from the Point P4 to Point P5 on the curve C, i.e.

a power regenerative braking state. In this operating condition, the armature current now flows reversely through the motor to the battery 7 so that the battery is now charged, and at the same time the fork lift truck is subjected to a braking force, the fork lift truck speed thus reducing along the curve C and likewise the regenerated current. When the accelerator pedal 9 is raised to a point where the motor characteristic lies on the curve B, the regenerated current is further increased toward the level or point of P6 on the curve B, thus producing a braking force effected on the electric motor so as to cause the fork lift truck to be reduced in its speed along the curve B, and as the fork lift truck speed slows down the regenerated current decreases to the point P7.This is all the steps of control for a batterypowered fork lift truck equipped with the speed control device of Fig. 2.

As fully described hereinbefore, in the speed control device of Fig. 2, when in power regeneration mode of operation, the electric current passing through the series field winding 2 flows in the reverse direction and the polarity of the series field winding 2 is now opposite to that of the shunt field winding 3, thus causing the resultant field to be reduced. This would result in a low efficiency of power regeneration, but for the diode 11. The effect of the diode 11 is that all or part of the series field winding 2 is short-circuited when in a power regenerating mode of operation.

Also, in order to effect a high efficiency of power regeneration, all or part of the reactor 4 is short-circuited when in a power regeneration mode of operation. Although it is desirable to have all of the series field winding 2 and the reactor 4 short-circuited from the point of view of a high efficiency power regeneration, a part of either or each of these windings may, as a compromise, be left to be not-short-circuited from the viewpoint of the stability in operation.

By virtue of this design concept, no matter how sharply the accelerator pedal 9 may be depressed by the operator regardless of the motor's state of operation, either in a power running mode or in a power regeneration mode, the reactor 4 connected in series with the motor will effectively and automatically prevent or smooth transients in the electric current in the armature circuit.In this consideration, in comparison with the conventional chopper system of Figs. 1A and 1B, the speed control device of Fig. 2 is more advantageous in view of the efficiency in the power running mode of operation, as well as the power regeneration efficiency during retardation, and particularly at a high revolutional number (i.e. speed) of the armature, there is assured stable power regeneration irrespective of any voltage generated in the motor armature, therefore resulting in a substantial improvement in the power regeneration efficiency of the device.

In Fig 4, there is shown an alternative field control circuit to that shown in Fig.

2. In this circuitry, there is a constantvoltage device RPS connected in parallel with an operating power source B, the constant-voltage device RPS being connected further to a power source + Vcc.

Series-connected resistor R1, variable resistor or potentiometer VR and resistor R2 are connected between the power source + Vcc and a terminal Ov of the power source B, series-connected resistors R3 and R4 being likewise connected between the power source + Vcc and the terminal Ov. A parallel-connected circuit comprising a resistor R7 and a "shunt" field winding F, and a Darlington connection comprising power transistors Q1 and Q2, and a resistor R6 are connected in series between the terminals 48V and OV. The output of an operational amplifier OP is connected through a resistor R5 to the base of the transistor Q1, the non-inverting input terminal of the above mentioned operational amplifier OP being connected to the junction between the resistors R3 and R4, the inverting input terminal of that amplifier being connected through a resistor R8 to the emitter of the transistor Q2 and further connected through a resistor R9 to the tapping of the variable resistor or potentiometer VR. The accelerator pedal P is designed to change the tapping of the variable resistor VR in proportion to an extent of depression of the pedal by the operator in such a manner that a voltage Vi across the resistor R2 and a part of the variable resistor VR may be increased when the pedal P is depressed.For instance, if the non-inverting terminal of the operational amplifier OP is fixed at 3 volts by selecting the resistors R3 and R4, and the resistors R1 and R2 are selected so that the voltage Vi may become 6 volts at a maximum speed, whilst it may be 3 volts at a minimum speed when the pedal P is depressed by the operator, and if the resistors R5 and R8 are selected to be equal in their resistance value, the inverting input terminal of the operational amplifier OP may be controlled at 3 volts, and as the voltage drop (Vi-3) due to the resistor R5 and the one (3-Vo) due to the resistor R8 become equal, thus establishing a control, Vi + Vo = 6 volts.Consequently, if the voltage Vi is 3 volts, for instance, the voltage Vo is 3 volts, thus allowing a maximum field current of approximately 3/R6 amperes in the "shunt" field winding F, and if the voltage Vi is 6 volts, the voltage Vo turns out to be zero volt, allowing a minimum field current of zero ampere, thus attaining a vehicle speed control by function of the accelerator pedal P.

Fig. 5 is a circuit or wiring diagram showing a preferred embodiment of a battery circuit wherein there are provided two battery cells El and E2 divided from the battery 7 shown in Fig. 2, and having the same voltage and capacity, and these battery cells may be connected switchably in either series or parallel connection so that the armature voltage can be changed selectively. The battery cells El and E2 are connected in parallel with each other via diodes Dl and D2. There is provided a series-parallel shifting switch S between the cathode of the above mentioned diode D2 and the anode of the diode D1, which switch is adapted to function in such a manner that when it is closed, there is a series connection, and when it is open, there is a parallel connection between the two battery cells El and E2.

The foregoing embodiment shows an example of two-way switching connection between two battery cells. If and when it is desired to provide for a multiple stage switching of more than two stages, there may be provided such an arrangement that considering the whole battery 7 as shown in Fig. 2 as a single unit of battery such as El or E2 as in the embodiment of Fig. 5.

if there are provided two sets of such unit cells to be connected in the same manner as in the case of Fig. 5, it is possible to provide for a three-stage switching arrangement. Likewise, if. there are added welch sets of unit cells of a- desired number in the same manner, it is practicable to provide for a multistage swit'chjng arrangement accordingly. With the practice of sucil switching arrangenient, it is possible to obtain a constantly stable and well-balanced discharge and charge among each unit battery cells.

Fig. 6 shows an example of a circuit wherein the motor armature is divided into two sections so that they may be switched in either series or parallel connection. In this arrangement, the two armatures 1 a 1, la 2 are connected in parallel with each other, and there are provided two contac- tors Sa arranged one on the plus side of the armature la 1 and one on the minus side of the armature la 2, diodes d 1 and d 2 being connected in parallel with the above mentioned contactrs Sa, respectively, and further there are a diode d 3 and a contactor Sb connected in parallel between the plus side of the armature la 1 and the minus side of the armature 1a 2.

These contactors Sb, Sa function in an opposite manner such that when the contactors Sa are open, the contactor Sb is closed, whilst the contactors Sa are closed when the contactor Sb is open. When the contactors Sa are closed and the contactor Sb is open, the armatures la 1 and 1a -2 are put in a parallel connection, whilst when the contactors Sa are open and the contactor Sb is closed the armatures la 1 and la 2 are now put in a series connection.

In the exemplary arrangement shown in Fig. 6, there is shown the case where the motor armature is divided into two sections. However, it is practicably possible to provide for more4ban-two division of the armature for a further multistage switching purpose.

It is to be understood that there can advantageously be provided a greater range of speed control by incorporating in combination the series-parallel switching arrangements as shown in conjunction with Figs.

5 and 6 in the fork lift truck speed control circuitry shown in Fig. 2. When the fork lift truck speed is controlled by applying a series-parallel-switching in the battery cells, and the divided armatures with such ar- rangement as mentidned hereinbefore, there is the possibility that there might occur an over-current in the circuitry in either a power running condition or a power regeneration condition. In fact,- however,- the reactor 4 is connected in series with the circuitry- and has a value sufficient to effectively prevent or reduce current transients during changing to and from regenerative braking or during starting to effect smooth starting and - smooth speed change of the fork lift truck.

Figs. 7 to 11 inclusive show ferromagnetic balance weights of the fork lift truck forming the core of the reactor 4 of Fig. 2. Fig. 7 is a front elevational view showing the balance weight of the fork lift truck to be used as the core of the reactor, and Fig, 8 is a cross-sectional view taken alorig'the ijli designated by the line A-A in- Fig. -7 There are provided coils 2' in grooves 6' - for accommodating the coils in part!.of the balance weight 1', and a coil holder plate 3''its fixedly mounted on the remainder of the balance weight by using boltS'4', which coil holder plate is adapted to should the. coils in position in the grooves and form a .magnetic path or circuit together with the part of the balance weight 1'. A plurality of lead wires 5' extend externally out of the coils. The grooves for accommodation. of the coils may be formed in the process of production of the part of the balance weight 1', when the balance weight is made of cast iron. Fig.

9 is a front elevational view showing another arrangement wherein a part of the body of a fork lift truck made of sheet steel is \1sed as the core of the reactor and also! functions as a balance weight, Fig. 10 is a cross-sectional view taken along the plane designated by the line B-B, and Fig.

11 is a cross-sectional view taken along the plane designated by the line C-C. In this arrangement, there is provided a yoke core 12' in an opposite relationship with a thick steel sheet 11' constituting part of the fork lift truck bödy) both of the steel sheet 11' and yoke core 12' being adapted to form a magnetic circuit together.There is - in- stalled tas coil 14' in a groove 17' formed between the mentioned two components, and fixedly mounted in position by using bolts 15'. - Lead wires 16' extend outside the edit 14'-. As fully described in the foregoing examples of reactor structure, by structural combination of the reactor core with..a part- of the fork lift truck structure fotining the or part of the balance weight, there result substantial savings from the point'of view of space, material, production cost as well as total weight of the fork lift truck, which will substantially contribute to the materialization of a practical and economical battery-powered fork lift truck equipped with a large capacity reactor.

As is anparent from the foregoing de; scription, in contrast with the conventional chopper type system (of Figs. 1 A and 1 B) in - which a constant-torque oriented characteristic is made available due to its voltage control so as to provide for a wide range of speed control, the fork lift truck speed control device described hereinbefore is of a constant power oriented characteristic due to its field control, which would inevitably bring a relatively narrow range of speed control. However, in consideration of the general load characteristic of a work lift truck, there is required a maximum driving torque at starting, while the requirement for such driving torque decreases - at high motor speed.Consequently, in general, it is practicably possible to provide a speed control range of 1 :6 by applying a field control system to the báttery-powered fork lift truck, and the maximum speed can be relatively low, and therefore, it is mostly the case that a speed control range of 1:5 is practically safely applicable. However, when- it is desired to apply a wider range of speed control, it is practically possible to provide a wider range of speed control by using the series-parallel switching arrangement of a plurality of battery cells, as well as the series-parallel switching of the motor armatures which are divided in plurality as described hereinbefore.

As fully described hereinbefore, the fork lift truck speed control devices have the advantage that a high operating efficiency of the battery-powered fork lift truck may be made available with a relatively simple and inexpensive construction, a high runnable distance or heavy payload thereof should be possible substantially from a single charge of power, and also a wider range of speed control is made available when necessary.

WHAT WE CLAIM IS: - 1. A fork lift truck comprising: a separately excited compound motor including an armature, a series field winding and a separately excited field winding; a ferromagnetic balance weight mounted on the fork lift truck; a first battery mounted on said fork lift truck for supplying the armature circuit of the motor; a second battery mounted on the fork lift truck for exciting the separately excited field winding; a current transient limiting reactor having a reactor core element and a reactor winding, the reactor core element comprising at least part of the ferromagnetic balance weight and the reactor winding being connected in series with the armature between the armature and the first battery; a diode connected in parallel with at least part of the series field winding so as to short-circuit the at least part of the series field winding only during a power regenerating operation of the motor; an operator actuable accelerator device on the fork lift truck; and speed control means including means for controlling the separately excited field current of the motor in response to actuation of the accelerator device by operator.

2. A truck as claimed in claim 1, wherein there is provided a further diode connected so as to short-circuit at least part of the reactor only during a power regenerating operation of the truck.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

showing the balance weight of the fork lift truck to be used as the core of the reactor, and Fig, 8 is a cross-sectional view taken alorig'the ijli designated by the line A-A in- Fig. -7 There are provided coils 2' in grooves 6' - for accommodating the coils in part!.of the balance weight 1', and a coil holder plate 3''its fixedly mounted on the remainder of the balance weight by using boltS'4', which coil holder plate is adapted to should the. coils in position in the grooves and form a .magnetic path or circuit together with the part of the balance weight 1'.A plurality of lead wires 5' extend externally out of the coils. The grooves for accommodation. of the coils may be formed in the process of production of the part of the balance weight 1', when the balance weight is made of cast iron. Fig.

9 is a front elevational view showing another arrangement wherein a part of the body of a fork lift truck made of sheet steel is \1sed as the core of the reactor and also! functions as a balance weight, Fig. 10 is a cross-sectional view taken along the plane designated by the line B-B, and Fig.

11 is a cross-sectional view taken along the plane designated by the line C-C. In this arrangement, there is provided a yoke core 12' in an opposite relationship with a thick steel sheet 11' constituting part of the fork lift truck bödy) both of the steel sheet 11' and yoke core 12' being adapted to form a magnetic circuit together.There is - in- stalled tas coil 14' in a groove 17' formed between the mentioned two components, and fixedly mounted in position by using bolts 15'. - Lead wires 16' extend outside the edit 14'-. As fully described in the foregoing examples of reactor structure, by structural combination of the reactor core with..a part- of the fork lift truck structure fotining the or part of the balance weight, there result substantial savings from the point'of view of space, material, production cost as well as total weight of the fork lift truck, which will substantially contribute to the materialization of a practical and economical battery-powered fork lift truck equipped with a large capacity reactor.

As is anparent from the foregoing de; scription, in contrast with the conventional chopper type system (of Figs. 1 A and 1 B) in - which a constant-torque oriented characteristic is made available due to its voltage control so as to provide for a wide range of speed control, the fork lift truck speed control device described hereinbefore is of a constant power oriented characteristic due to its field control, which would inevitably bring a relatively narrow range of speed control. However, in consideration of the general load characteristic of a work lift truck, there is required a maximum driving torque at starting, while the requirement for such driving torque decreases - at high motor speed.Consequently, in general, it is practicably possible to provide a speed control range of 1 :6 by applying a field control system to the báttery-powered fork lift truck, and the maximum speed can be relatively low, and therefore, it is mostly the case that a speed control range of 1:5 is practically safely applicable. However, when- it is desired to apply a wider range of speed control, it is practically possible to provide a wider range of speed control by using the series-parallel switching arrangement of a plurality of battery cells, as well as the series-parallel switching of the motor armatures which are divided in plurality as described hereinbefore.

As fully described hereinbefore, the fork lift truck speed control devices have the advantage that a high operating efficiency of the battery-powered fork lift truck may be made available with a relatively simple and inexpensive construction, a high runnable distance or heavy payload thereof should be possible substantially from a single charge of power, and also a wider range of speed control is made available when necessary.

WHAT WE CLAIM IS: - 1. A fork lift truck comprising: a separately excited compound motor including an armature, a series field winding and a separately excited field winding; a ferromagnetic balance weight mounted on the fork lift truck; a first battery mounted on said fork lift truck for supplying the armature circuit of the motor; a second battery mounted on the fork lift truck for exciting the separately excited field winding; a current transient limiting reactor having a reactor core element and a reactor winding, the reactor core element comprising at least part of the ferromagnetic balance weight and the reactor winding being connected in series with the armature between the armature and the first battery; a diode connected in parallel with at least part of the series field winding so as to short-circuit the at least part of the series field winding only during a power regenerating operation of the motor; an operator actuable accelerator device on the fork lift truck; and speed control means including means for controlling the separately excited field current of the motor in response to actuation of the accelerator device by operator.
2. A truck as claimed in claim 1, wherein there is provided a further diode connected so as to short-circuit at least part of the reactor only during a power regenerating operation of the truck.
3. A truck as claimed in claim 1 or 2

wherein the first battery comprises two battery cells which are connectable in series or in parallel with each other, and wherein the speed control means further includes a first diode electrically connected to the positive poles of the cells so as to prevent charging current flow into one of thel cells, a second diode electrically connected to the negative poles of the cells so as to prevent charging current flow into the other of the cells, and a switch connected between the positive pole of one cell and the negative pole of the other cell so as to connect the two cells in series when the switch is closed and so as to prevent current flowing in series through the cells when the switch is open.
4. A truck as claimed in any one of the preceding claims wherein the motor armature has a first section and a second section which are connectable in series or in parallel, and wherein the speed control means further include a third diode electrically connecting together the positive terminals of the armature sections, a fourth diode electrically connecting together the negative terminals of the armature sections, the third and fourth diodes each being connected so as to prevent driving current flow into one of the armature sections, a first contactor connected between the positive terminal of the first armature section and a positive voltage supply terminal, a second contactor connected between the negative terminal of the second armature section and a negative voltage supply terminal, a circuit comprising a fifth diode and a third contactor connected in parallel with each other and between the positive terminal of the first armature section and the negative terminal of the second armature section, the fifth diode being connected so as to direct driving current through the armature sections in series when the third contactor opens, the three contactors being arranged so that when the third contactor is open the other two contactors are closed and vice versa so that switching the three contactors selectively connects both armature sections in either series or parallel.