WO1994011162A1

WO1994011162A1 - A power tool

Info

Publication number: WO1994011162A1
Application number: PCT/GB1993/002293
Authority: WO
Inventors: Colin Edward Cartwright
Original assignee: Emerson International Ltd
Current assignee: Emerson International Ltd
Priority date: 1992-11-06
Filing date: 1993-11-08
Publication date: 1994-05-26
Anticipated expiration: 1995-05-06

Abstract

The invention relates to a power tool (1) which in the embodiment is portable, self-contained and which has a push button switch (3), and power means in the form of ten 1.2 volt rechargeable batteries (12), rechargeable v^_i^_a^_ a recharging socket (4). A motor (5) drives a gearbox (6) which turns a chuck (7). The chuck is attached to a spindle and the chuck can be removed from the spindle such that a screwdriver means can be inserted directly into the spindle as opposed to the screwdriver means being gripped by the chuck (7). The push button switch (3) is adapted to move within a slot (34). When the push button switch (3) is in the centre of the slot (34), as shown, no power can be applied to the motor (5) by depressing the push button switch (3). However, when the push button switch (3) is in its top position within the slot (34) the motor (5) turns in a clockwise direction. Alternatively, when the push button switch (3) is in the down position within the slot (34) the motor (5) will turn in an anticlockwise direction. Referring to Fig. 2 the self contained power tool (1) is shown inserted in means such as attachment means (2) for receiving the tool (1) as by sliding from right to left as viewed in Fig. 2.

Description

A POWER TOOL

This invention relates to a portable multi purpose power tool by direct current power supply.

Direct current multi purpose power tools have been developed so that for example they can be used both as a drill or a screwdriver. Such tools have gearing to reduce the output speed from the direct current motor which is an integral pan of the tool. The reason for the gearing is due to the design and type of motor which generally has an output shaft speed requiring speed reduction. Furthermore, such motors usually impose considerable demands upon the direct current power supply (battery) and therefore the motor can rapidly drain the stored battery charge.

When used as a drill a relatively high speed is often required (but motor gearing speed reduction is still used) , whereas with a screwdriver the speed generally required is much lower. Because the screwdriver must turn in both a clockwise and anti-clockwise direction the motor adapted to turn the screwdriver must be bi-directional so that screws can be screwed in or alternatively unscrewed. To achieve the slower drill speeds interchangeable gearing can be used or variable pressure on the power tool switch can be used to provide changes to the voltage supplied across the motor.

The above approaches to varying speed offer problems. For instance, gear changing can be expensive due to the high gearing ratios and interchanging mechanisms that may be required. The switch sensitive approach requires skill by the operator and it is possible to apply too much pressure causing the screwdriver to jump out of the screw head. In addition to the above problems, power tools are generally in the shape of a pistol. Hence, when working in tight corners or in confined spaces the pistol shape may not be suitable and therefore it may be impractical to use such power tools in these circumstances.

It is the intended object of this invention to overcome the above mentioned problems or at least provide the public with a useful alternative.

According to a first aspect of the invention there is provided a tool which can be convertible between different modes, characterised by a self-contained power tool part for one mode and by means for receiving the self-contained power tool part for operation in a second mode.

The self-contained power tool part may comprise a screwdriver in the first mode.

The self-contained tool part may comprise a drill in the second mode. These options provide for a flexible utilisation of a tool embodying the invention in providing a multi-purpose application.

The tool part and receiving means may each comprise a power means, for example a plurality of rechargeable or replaceable batteries.

The power means may each comprise respectively a battery means and or electrical circuit means.

There may be means of the receiving means to modify the circuit of the self- contained power tool part. The circuit modifying means may comprise a movable plug part which may suitably be guided by a guide part. This provides for ease of operation.

There may be an interlock part operable to allow insertion of the self- contained power tool part in the receiving means. Suitably the interlock part may be automatically operable.

The interlock part may comprise an interlock adapted for releasable interlocking engagement with the self-contained part, by a shaft, and by a resiliently mounted pivotable arm device which may be operably connected with the interlock.

The self-contained tool part may have a seating for receiving the interlock.

The seating may comprise a slot in the tool part. This provides for a positive interlock.

A switch of the circuit of the tool part may suitably be in the slot. This provides for a neat appearance and smooth operation.

The circuit modifying means may comprise a handle and the interlock may comprise two cam parts whereby when the handle is turned in opposite sense respectively the tool is operable to rotate in a clockwise or anti-clockwise sense.

The circuit modifying means may comprise a piston of the tool part and a conductor of the receiving part adapted to break the circuit of the self- contained part. The tool is suitably a portable power tool.

According to a second aspect of the invention there is provided a portable power tool, characterised by a first direct current storage means adapted to be electrically connected to a direct current motor by a switch means and by a circuit modification means adapted to allow a second direct current storage means to be inserted in series with the first direct current storage means. The portable power tool may have a means for reversing the turning direction of the motor.

The second direct current storage means may be contained within an attachment means adapted to engage the portable power tool.

The attachment means may have a circuit modification means activator, wherein when the portable power tool is engaged by the attachment means the circuit modification means activator, in conjunction with the circuit modification means, inserts the second direct current storage means in series with the first direct current storage means.

The attachment means may have a switch means adapted electrically to connect the first direct current storage means and second direct current storage means to the direct current motor.

According to a third aspect of the invention there is provided an attachment means adapted to engage a power tool having a first direct current storage means, the attachment means having a second direct current storage means adapted to be inserted in series with the first direct current storage means.

The attachment means may have a switch means adapted to provide a series connection of the first direct current storage means and second direct current storage means to a direct current electrical motor adapted to drive the power tool.

According to a fourth aspect of the invention there is provided a power tool having a first direct current storage means adapted to be electrically connected to a direct current motor by a first switch means, and a second direct current storage means contained within an attachment means, the attachment means engaging the power tool such that the second direct current storage means is inserted in series with the first direct current storage means, wherein the attachment means is adapted to disengage the power tool such that when disengaged the second direct current storage means is removed from being in series with the first direct current storage means.

The second direct current storage means may be adapted to be removed from being in series with the first direct current storage means such that the first direct current storage means remains adapted to be electrically connected to the direct current motor by the switch means.

The portable power tool may have a means of reversing the turning direction of the motor.

The attachment means may have a second switch means adapted to connect the first direct current storage means and the second direct current storage means electrically to the direct current motor.

The attachment means may have an interlock means adapted to activate the first switch means when the attachment means engages the power tool such that the second switch means provides the electrical connection between both the first direct current storage means and second direct current storage means and the direct current motor.

The motor may be a permanent magnet direct current motor, and the first and second battery means for providing electrical power to the permanent magnet direct current motor whereby it is possible to damage either of the battery means by drawing excessive currents, the arrangement being adapted such that the current drawn by the motor will not cause damage to occur to either of the battery means when the motor is in a stalled condition for a substantial period of time.

The direct current permanent magnet motor may be wound such that when it is connected to a 12 Volt battery means, the battery's terminal voltage does not reduce by more than about 20% of its no-load value when the motor is at or near a stalled condition.

The direct current permanent magnet motor may have an efficiency of greater than 40% when the motor drives a variable load over at least 15% of the total output torque range at the output of the motor, wherein the efficiency is calculated by: efficiency = output power/input power x 100 where output power is the power at the output of the motor's shaft and input power is the input power to the motor.

The direct current permanent magnet motor may have an efficiency of greater than 45% when the motor drives a variable load over at least 15% of the total output torque range at the output of the motor, wherein the efficiency is calculated by: efficiency = output power/input power x 100 where output power is the power at the output of the motor's shaft and input power is the input power to the motor.

The direct current permanent magnet motor may have an efficiency of greater than 50% when the motor drives a variable load over at least 15% of the total output torque range at the output of the motor, wherein the efficiency is calculated by: efficiency = output power/input power x 100 where output power is the power at the output of the motor's shaft and input power is the input power to the motor.

The direct current permanent magnet motor may have an efficiency of greater than 55% when the motor drives a variable load over at least 15% of the total output torque range at the output of the motor, wherein the efficiency is calculated by: efficiency = output power/input power x 100 where output power is the power at the output of the motor's shaft and input power is the input power to the motor.

Each individual coil between two armature commutator segments may have between 35 to 400 turns.

Each individual coil between two armature commutator segments may have between 50 to 400 turns.

Each individual coil between two armature commutator segments may have between 75 to 400 turns.

Each individual coil between two armature commutator segments may have between 100 to 400 turns.

Each individual coil between two armature commutator segments may have between 125 to 400 turns.

Each individual coil between two armature commutator segments may have between 35 to 375 turns.

Each individual coil between two armature commutator segments may have between 50 to 375 turns.

Each individual coil between two armature commutator segments may have between 75 to 375 turns.

Each individual coil between two armature commutator segments may have between 100 to 375 turns.

Each individual coil between two armature commutator segments may have between 125 to 375 turns.

Each individual coil between two armature commutator segments may have between 35 to 350 turns.

Each individual coil between two armature commutator segments may have between 75 to 350 turns.

Each individual coil between two armature commutator segments may have between 100 to 350 turns. Each individual coil between two armature commutator segments may have between 125 to 350 turns.

Each individual coil between two armature commutator segments may have between 35 to 325 turns.

Each individual coil between two armature commutator segments may have between 50 to 325 turns.

Each individual coil between two armature commutator segments may have between 75 to 325 turns.

Each individual coil between two armature commutator segments may have between 100 to 325 turns.

Each individual coil between two armature commutator segments may have between 125 to 325 turns.

Each individual coil between two armature commutator segments may have between 35 to 300 turns.

Each individual coil between two armature commutator segments may have between 50 to 300 turns.

Each individual coil between two armature commutator segments may have between 75 to 300 turns.

Each individual coil between two armature commutator segments may have between 100 to 300 turns. Each individual coil between two armature commutator segments may have between 125 to 300 turns.

Each individual coil between two armature commutator segments may have between 35 to 275 turns.

Each individual coil between two armature commutator segments may have between 50 to 275 turns.

Each individual coil between two armature commutator segments may have between 75 to 275 turns.

Each individual coil between two armature commutator segments may have between 100 to 275 turns.

Each individual coil between two armature commutator segments may have between 125 to 275 turns.

Each individual coil between two armature commutator segments may have between 35 to 250 turns.

Each individual coil between two armature commutator segments may have between 50 to 250 turns.

Each individual coil between two armature commutator segments may have between 75 to 250 turns.

Each individual coil between two armature commutator segments may have between 100 to 250 turns. Each individual coil between two armature commutator segments may have between 125 to 250 turns.

Each individual coil between two armature commutator segments may have between 35 to 225 turns.

Each individual coil between two armature commutator segments may have between 50 to 225 turns.

Each individual coil between two armature commutator segments may have between 75 to 225 turns.

Each individual coil between two armature commutator segments may have between 100 to 225 turns.

Each individual coil between two armature commutator segments may have between 125 to 225 turns.

Each individual coil between two armature commutator segments may have between 35 to 200 turns.

Each individual coil between two armature commutator segments may have between 50 to 200 turns.

Each individual coil between two armature commutator segments may have between 75 to 200 turns.

Each individual coil between two armature commutator segments may have between 100 to 200 turns. Each individual coil between two armature commutator segments may have between 125 to 200 turns.

Each individual coil between two armature commutator segments may have between 35 to 175 turns.

Each individual coil between two armature commutator segments may have between 50 to 175 turns.

Each individual coil between two armature commutator segments may have between 75 to 175 turns.

Each individual coil between two armature commutator segments may have between 100 to 175 turns.

Each individual coil between two armature commutator segments may have between 125 to 175 turns.

The wire gauge of each individual coil may suitable be the thickest that can be wound upon the armature for the given number of turns, and suitably the wire gauge of each individual coil may be less than 0.4mm.

Capacitors may be connected across each pole-piece winding. This offers the additional advantage of substantially reducing radio frequency interference. This is not achievable with motors currently used for example in commercially available power tools. With such tools the excessive currents required by the armature and physical size capacitor constraints do not readily allow for such capacitors to be connected across each pole winding. Stated in another way, capacitors whose size is not too large for the available space are not particularly effective in reducing RFI.

A tool embodying the invention is hereinafter described, by way of example, with reference to the accompanying drawings.

Fig. 1 illustrates a portable power tool;

Fig. 2 illustrates an attachment to the power tool which provides additional voltage to the power tool;

Fig. 3 illustrates the switch mechanism of the power tool; and

Fig. 4 illustrates the electrical circuit of the power tool and the attachment means.

Referring to the drawings, Fig. 1 illustrates a power tool 1 which in the embodiment is portable, self-contained and which has a push button switch 3, and power means in the form of ten 1.2 volt rechargeable batteries 12, rechargeable via a recharging socket 4. A motor 5 drives a gearbox 6 which turns a chuck 7. The chuck is attached to a spindle and the chuck can be removed from the spindle such that a screwdriver means can be inserted directly into the spindle as opposed to the screwdriver means being gripped by the chuck 7. The push button switch 3 is adapted to move within a slot 34. When the push button switch 3 is in the centre of the slot 34, as shown in Fig. 1 , no power can be applied to the motor 5 by depressing the push button switch 3. However, when the push button switch 3 is in its top position within the slot 34 the motor 5 turns in a clockwise direction. Alternatively, when the push button switch 3 is in the down position within the slot 34 the motor 5 will turn in an anticlockwise direction. Referring to Fig. 2 the self contained power tool 1 is shown inserted in means such as attachment means 2 for receiving the tool 1 as by sliding from right to left as viewed in Fig. 2. To assist in correct location there are two location guides 11 and a movable plug part such as a jack plug 10 which, when inserted, modifies the circuitry of the self-contained power tool 1. When inserting the self-contained power tool 1 an interlock 16 is pulled downwards by force acting upon a spring 19 which moves a pivotable arm 14 which therefore acts upon the shaft 15 to move the interlock 16. Hence, when the self contained power tool 1 is inserted into the attachment means 2 pressure can be removed from the spring 19 which therefore allows the interlock 16 to be located in a seating such as a slot 34 in the tool part. The attachment means further has a hollow handle 18 within which are located in the embodiment power means such as ten 1.2 volt batteries 17 that are adapted to be recharged via the socket 35.

Assuming a screw driver blade is used in the self-contained part in one operating mode, it may be replaced by a drill bit for the second operating mode.

Referring to Fig. 3 and Fig.4 there is illustrated the switch mechanism and the electrical circuitry of both the self contained power tool 1 and attachment means 2. A turning handle 8 is adapted to rotate the inner body 9 such that push button switch 3 will be activated by either a cam 32 or a cam 33 of the interlock 16. This therefore connects a spring means 24 to the positive of the battery 12. Furthermore, both the spring means 23 and 24 are mechanically connected to the inner body 9, therefore when the turning handle 8 is pushed clockwise (as viewed) the spring means 23 would be connected to pin 21 and the spring means 24 would be connected to the pin 22. This provides a simple mechanism for reversing the direction of the motor 5 which is electrically connected to the pins 21 and 22.

Upon the turning handle 8 being pushed either clockwise or anticlockwise the push button switch 3 will close its contacts by either being activated by the cam 32 or the cam 33. However, the motor 5 will not turn until the switch 13, located upon the handle 18, is closed.

The switch 13 is inserted into the circuit by a circuit modification means 20 which includes the jack plug 10 having an inner conductor 31 surrounded by an insulator 30 and an outer conductor 29. Before the portable power tool 1 is inserted into the attachment means 2 an electrical circuit is completed at the circuit modification means 20 by a cylindrical conductor 26 making contact with a cylindrical conducting piston 27 which is resiliently mounted as by being spring loaded by a conducting spring 28. Thus, an electrical circuit is made via the negative of the battery 12 through the conducting spring 28, through the cylindrical conducting piston 27 and the cylindrical conductor 26 to the spring means 23.

When the jack plug 10 is inserted into the circuit modification means 20 the cylindrical conducting piston 27 is pushed away from the cylindrical conductor 26 which therefore breaks their direct electrical contact between the negative of battery 12 and the spring means 23. the outer conductor 29 of the jack plug 10 is connected to the switch 13. The other side of the switch 13 is connected to the negative of the battery 17 and the positive of the battery 17 is connected to the inner conductor 31 of jack plug 10. When the jack plug 10 is inserted the circuit modification means 20 the inner conductor 31 makes electrical contact with the cylindrical conducting piston 27 and the outer conductor 29 makes contact with the cylindrical conductor 26. The insertion of the jack plug 10 reconfigures the circuit such that the battery 12 and the battery 17 are in series and power can only be supplied to the motor when both the contacts of the push button 3 and switch 13 are closed.

A further preferred feature of a tool embodying the invention is that of the motor used. This is designed and has features that are uncommon in power tools. The generated electro-motive force (e.m.f.), in such motors when used as a generator, is proportional to the rate of rotation of the armature. This is because the induced e.m.f. in an armature coil is proportional to the rate of change of magnetic flux through the coil, which in turn is proportional to the angular velocity of the coil as it sweeps past the permanent magnet. As the armature rotates, the induced e.m.f. in each coil is intrinsically an A.C. signal containing sign reversals and maximum and minimum voltages (the minimum being zero volts) . The position of brushes in contact with the motor's commutator, which are also electrically connected to external terminals, are arranged so that the induced e.m.f. at the terminals is always of the same electrical polarity for a given armature rotation direction.

When direct current is supplied to a motor a torque tending to cause armature rotation occurs. However, as stated above, the rotation itself causes an induced e.m.f. in the coils. Thus, if the effects of mechanical friction, wind resistance and eddy currents are ignored, the r. p.m. of a free running motor is defined by the generated induced e.m.f. equalling the supplied voltage. In other words, for a given supply voltage ignoring mechanical friction, wind resistance and eddy current losses, the r.p.m. is dependent upon the permanent magnet field, the geometry of the brushes, the armature and the supply e.m.f. For a real motor with friction, wind resistance and eddy current losses, the free running r.p.m. is a little less than the theoretical value. However, it is important to establish the above basic concept as a background to aid with the understanding of this invention. It should be noted that the rotational velocity of the ideal motor described above is independent of the electrical resistance of the armature windings, brushes and commutator. Hence, as the induced e.m.f. in the winding is approximately proportional to the number of turns on each coil for a given motor and armature r.p.m., it follows that in the ideal motor the free running r.p.m. is approximately inversely proportional to the number of turns per coil for a given supply voltage. That is the fewer the turns the greater the free running r.p.m. Furthermore, when considering an ideal motor the free running r.p.m. is proportional to the supply voltage.

When a motor is under load, that is made to do work, the friction, wind resistance, eddy currents, electrical resistance of the windings, brush resistance and commutator have an effect upon the motor's performance. In the extreme case where the motor is so loaded that the rotational velocity is much less than the free running speed, the induced e.m.f. can be considered as insignificant. Furthermore, the motor dynamics are now a function of the said resistances and electrical supply's effective internal series resistance, the flux of the permanent magnets, the geometry of the pole pieces, and the number of turns per pole.

We have discovered that direct current permanent magnet motors used for instance in commercially available appliances such as hand held battery operated power tools (including drills and screw drivers) are poorly interfaced with their supply. The motors have relatively thick copper wire armature windings of a small number of turns which results in a high r.p.m. for reasons given above. Consequently, such tools have a gearbox of high reduction ratio for reducing the output speed of the tool. However for reasons which follow, this type of design results in inefficiency when the motor is heavily loaded, and it is our experience that most battery operated hand held screw drivers and drills are frequently heavily loaded when used in practice.

It seems that as this practice of a relatively small number of thick armature conductors has been universally adopted for the small permanent magnet direct current motors used in such tools, there is probably a misconceived "rule of thumb" used in the design process.

We will now show that the practice of a few thick turns per coil found in these electric motors is not optimal, and in fact a thinner wire and more turns can provide a more useful motor. Furthermore, it should be noted that there is an optimum range of wire thickness for a given set of criteria.

In order to clarify the influence of the number ofturnes per coil on the motor performance a heavily loaded motor is now considered :-

If:-

-the internal resistance of the supply plus commutator and brushes is ,

-the resistance of the windings is r₂,

-then τ_ +r₂ = R which is the total effective resistance in the current

-the number of turns per coil is n,

-the cross-sectional area of the conductor of windings is a,

-the supply voltage is V,

-the armature electro motive force is F,

-the current flowing through the conductor is i,

-and k₂ to k₄ are constants that are independent of the designer's choice of n and ∑ r₂ = k₂n/a, for heavy loads and low rev/min, i ≡ V/R = V/(rl +k₂n/a)

F = k₃in = k₃Vn/(r, +k₂n/a) =k₃Vn/(r₁ +klk₂n²/A) but since the total copper cross-section per coil A = a n tends to be sent by slot area, here assumed fixed, F can be expressed in terms of A. dF = k₃V [(r, + k₂n²/A) - n, 2n k₂/A] dn [r, + k₂n²/A]²

k,V fr. - k,n²/Al [r, + k,n /A]²

k,V (Ar. - k,n²) A(r, + R₂n²/A)²

F and hence the torque at very low speeds for a given field flux and supply voltage 13 maximum when dF = 0, that is when Ar, = kji². dn

F, and hence the torque to a given field flux, is a maximum when dF/dn = 0, that is when r_ = k^². As A and r, can be assumed constant, there is an optimum number of turns which is a function of the internal resistance of the supply plus commutator and brushes. Note that this optimum is not a function of supply voltage. This maximizes the torque at heavy loads for given internal supply resistance and given brushes and commutator. In general this is approximately true of the torque per battery output power as well at high loads, so long as no magnetic saturation occurs.

For an intermediate load, the current drain is approximately;

V( 1 -rpm/rpmf ree) /R , and the power is approximately proportional to rpm V(l-rpm-rpmfree)/R, where V is the supply voltage, m is the angular velocity and φmfree is the free running angular velocity for supply voltage V.

As load on a motor is varied, the output power maximises when the motor operates at approximately at half the free running φm. However, most users stress the tool beyond this optimum. Under these circumstances, there is a danger that significant magnetic saturation will occur. If this happens, then the output torque per current is then lower than the non-saturated case and the motor's windings tend to overheat more quickly. Hence, this can also be a criteria in the design. That is V and R should be chosen to not allow too much saturation or excessive heating to occur.

A simplified summary of the above is that motors of improved efficiencies for heavy, low speed operation can be designed and manufactured by taking properly into account the assisted output speed of the motor which is often considerably less than the free r.p.m. and less than speed for other typical applications of such small motors. The free r.p.m. is proportional to 1/n, hence this determines the number of windings (turns) per armature pole-piece. Once n is determined it is preferable to select the thickest wire gauge that can be wound upon the armature. However, there are constraints upon the maximum thickness of wire gauge, set by the available slot area. Note that in practice consideration of magnetic saturation due to armature reaction and of demands upon power supplies should be taken into account in selecting n..

RESULTS

Devices such as battery powered hand held power drills and screw drivers which are commonly sold with motors having typically between 12 and 22 turns per armature coil may be considered. Efficiencies at mid-range loads are typically less than 25% for these tools. By changing the turns to 125 and using the thickest wire that can fit in the slots, we have increased this efficiency to approximately 60%. This is attributable to the improved ratio between the armature resistance and the battery internal resistance and the improved ratio between the typical on-load operating speed and the free running speed.

In general, the increasing turns per coil will improve the motor's performance when fed from a small battery. This is basically because the torque is proportional to the number of turns for a given current. Hence the more turns, the smaller the battery current drain required to produce a given torque. Also, the smaller this current, the smaller the power inefficiency losses from the internal resistance of the battery and resistive losses of the brushes, armature and conductors feeding the motor via the switch. However, there is a limit to the increasing number of turns in order to provide improved operation. This is related to the effect on the current drawn from the battery when the armature resistance becomes significant in comparison with the battery source output and to reductions in space factor in the slots caused by increasing ratio of wire insulation cross-section to copper cross-section. There is hence a range of turns which are suitable for a specific application.

When considering a battery powered motor in which the battery has a low resistance per cell (as is the case with Nickel Cadmium batteries which have typically less than 250 milli Ohms per cell and the motor stalls or is so loaded that the r.p.m. is very low, the current drawn from the batteries may cause battery damage. In contrast, with a motor having 125 or more turns per coil, this use of relatively thin wire of approximately less than 0.4mm diameter, the resistance of the armature windings is several ohms and the near stalled current will not cause much heating or battery stress in practice owing to reasonable currents of a few amperes. Using the invention disclosed in this specification with the assistance of Figs. 1 to 4, there is illustrated a multi purpose tool which can be powered by either a 12 volt battery supply or 24 volt battery supply. It should be noted that other battery voltages can be used and variations of the embodiments described can be substituted without diverting from the invention.

The following results illustrate the performance of this invention. A typical power tool motor such as the Johnson HC683G which has 22 turns per coil and a wire diamter of 0.9mm is considered. Using available data sheets (although our own tests on this motor showed a worse performance) , this motor at maximum efficiency draws 8.17 amps at 17700 r.p.m. and produces a torque of 26.12 mNM. Comparing this with the same motor (HC683G) rewound, using the invention disclosed in this specification, with 125 turns per coil of 0.145mm diameter were used the following results were obtained:

SUPPLY

2.4v

4.8v

7.2v

1500mA 1088 203.04 2100ma STALL 293.13

12v 330mA (NO LOAD) 4727 N.A.

670mA 3432 254.08

1570mA 2472 254.08

3500mA STALL 403.1

24v 450mA (NO LOAD) 9185 N.A.

1000mA 6404 389.57

2373mA 4048 572.04

4700mA STALL 985.47

The above results were obtained using 125 turns of 0.145mm diameter wire. However, this is not the optimum diameter. As stated above by changing the turns to suit a desired r.p.m. and using the thickest wire that can fit on the armature higher efficiencies can be achieved. Thus when considering the Johnson HC683G the following was observed when the armature was wound as follows:-

The motor with 125 turns per coil of 0.28mm wire diamter illustrates that once the number of turns per coil is determined then the thickest wire gauge that can be used improves the efficiency of the motor. Furthermore, small D.C. motors having a small number of turns and thick windings tend to be inefficient at maximum output power. (This inefficiency may be due to excessive armature reaction and/or to high IR losses in the coils and brushes at the maximum output point.

The above efficiency results were calculated from measurements made by coupling the shaft of a HC683G machine with 22 turns of 0.9mm diameter wire (adapted to be used as a generator) to the output shaft of the motor being tested. Thus by varying the current in the HC683G generator by means of a variable resistor across the generator's output, the load on the motor could be varied.

The motor was dynomometer mounted and its output power was calculated by multiplying the speed in radions/sec by the torque as measured by means of a digital force meter attached to the dynomometer' s torque arm.

The efficiency was calculated as follows :-

efficiency = output power/ input power x 100

Where output power = Vs. Is, Vs being the supply voltage to the motor under test and Is the supply current to the motor's armature.

The above results and theory illustrate that small D.C. permanent magnet motors having a small number of turns and thick windings are not well adapted for heavy load operation, with small battery supplies. However in general, D.C. power tools use such motors, to emphasize this point the following examples are described:

1) The Johnson HC683G motor, as sold, has 22 turns of 0.9mm diameter wire per coil.

2) The Johnson HC615L motor has 22 turns of 0.9mm diameter wire per coil.

3) The Johnson HC783G as sold has 24 turns of 0.9mm diameter wire per coil.

4) The motor illustrated in Fig.2 is a D.C. permanent magnet motor with a U-shaped pole configuration motor as sold has 9 turns per coil of 0.9mm diameter per coil and each coil is wound around 3 teeth.

Using the invention the direct current permanent magnet motor has an efficiency of greater than 40% suitably 55%, 50% or 45% when the motor drives a variable load over at least 15% of the total output torque range at the output of the motor, wherein the efficiency is calculated by: efficiency = output power/input power x 100 where output power is the mechanical output at the motor's shaft and input power is the input electrical power to the motor.

It should be noted that capacitors can be connected across each coil (if required) . This offers the additional advantage of substantially reducing radio frequency interference. This is not achievable with motors currently used for example in commercially available power tools. With such tools the excessive currents required by the armature constraints on the capacitor's physical size do not readily allow for effective capacitors to be connected across each coil.

Claims

1. A tool which can be convertible between different modes, having motor means characterised by a self-contained power tool part for one mode and by means for receiving the self-contained power tool part for operation in a second mode.

2. A tool according to Claim 1, characterised by the self-contained power tool part comprising a screwdriver in the first mode.

3. A tool according to Claim 2, characterised by the self-contained tool part comprising a drill in the second mode.

4. A tool according to any preceding claim, characterised by the tool part and receiving means each comprising a power means.

5. A tool according to Claim 4, characterised by the power means each comprising respectively a battery means and an electrical circuit means.

6. A tool according to Claim 4 or Claim 5, characterised by means of the receiving means to modify the circuit of the self-contained power tool part.

7. A tool according to Claim 6, characterised by the circuit modifying means comprising a movable plug part.

8. A tool according to Claim 7, characterised by a guide for the circuit modifying means.

9. A tool according to Claim 8, characterised by an interlock part operable to allow insertion of the self-contained power tool part in the receiving means.

10. A tool according to Claim 9, characterised by the interlock part comprising an interlock adapted for releasable interlocking engagement with the self-contained part, by a shaft, and by a resiliently mounted pivotable arm device which is operably connected with the interlock.

11. A tool according to Claim 10, characterised by the self-contained tool part having a seating for receiving the interlock.

12. A tool according to Claim 11, characterised by the seating comprising a slot in the tool part.

13. A tool according to Claim 13, characterised by a switch of the circuit of the tool part being in the slot.

14. A tool according to Claim 13, the circuit modifying means comprising a handle and by the interlock comprising two cam parts whereof when the handle is turned in opposite sense respectively the tool is operable to rotate in a clockwise or anti-clockwise sense.

15. A tool according to Claim 14, characterised by the circuit modifying means comprising a piston of the tool part and a conductor of the receiving part adapted to break the circuit of the self-contained part.

16. A tool according to any preceding claim, characterised by being portable. 17. A portable power tool, characterised by a first direct current storage means adapted to be electrically connected to a direct current motor by a switch means and by a circuit modification means adapted to allow a second direct current storage means to be inserted in series with the first direct current storage means.

18. A tool according to Claim 17, characterised by means for changing the turning direction of the motor.

19. A tool according to Claim 17 or Claim 18, characterised by the second direct current storage means being contained within an attachment means.

20. A tool according to Claim 19, characterised by the attachment means comprising a circuit modification means activator, whereby when the portable power tool and attachment means are engaged, the circuit modification means activator, in conjunction with the circuit modification means, inserts the second direct current storage means in series with the first direct current storage means.

21. A tool according to Claim 20, characterised by the attachment means comprising a switch means adapted to connect the first direct current storage means and second direct current storage means to the direct current motor.

22. An attachment means adapted to engage a power tool having a first direct current storage means, characterised by the attachment means having a second direct current storage means adapted to be inserted in series with the first direct current storage means.

23. An attachment means according to Claim 22, characterised by a switch means adapted to provide a series connection of the first direα current storage means and by a second direct current storage means to a direct current electrical motor adapted to drive the power tool.

24. A power tool, characterised by a first direct current storage means adapted to be electrically connected to a direα current motor by a first switch means, by a second direα current storage means contained within an attachment means, and by the attachment means engaging the power tool such that the second direα current storage means is inserted in series with the first direα irrent storage means, whereby the attachment means is adapted to disengage the power tool such that when disengaged the second direct current storage means is removed from being in series with the first direct current storage means.

25. A power tool according to Claim 25, charaαerised by the second direct current storage means being adapted to be removed from being in series with the first direct current storage means such that the first direct current storage means remains adapted to be eleαrically conneαed to the direct current motor by the switch means.

26. A power tool according to Claim 25, charaαerised by being portable and by having a means of changing the turning direαion of the motor.

27. A power tool according to Claim 26, charaαerised by the attachment means having a second switch means adapted to connect the first direct current storage means and second direct current storage means to the direct current motor.

28. A power tool according to Claim 27, characterised by the attachment 1162

30 means having interlock means adapted to aαivate the first switch means when the attachment means engages the power tool such that the second switch means provides the eleαrical connection between both the first direct current storage means and a second direct current storage means and the direct current motor.

28. A power tool according to any preceding claim, characterised by the motor being a permanent magnet direct current motor, and by the first and second battery means for providing eleαrical power to the permanent magnet direα current motor whereby it is possible to damage either of the battery means by drawing excessive currents, the arrangement being such that the current drawn by the motor will not cause damage to occur to either of the battery means when the motor is in a stalled condition for a substantial period of time.

30. A power tool according to Claim 19, charaαerised by the direα current permanent magnet motor being wound such that when it is connected to a 12 Volt battery means the battery's terminal voltage does not reduce by more than about 20% of its no-load value when the motor is at or near a stalled condition.