This invention relates to a motor, particularly a D.C. electrical motor, for example a small direct current electric motor, although the applicability of the invention may extend beyond this specific application.

The invention will be described with respect to small electric motors in which the armature has a plurality of teeth established by reason of a laminated ferromagnetic core and insulated wire wound around, as a coil, each of the teeth of the common core. Each of the coils is connected electrically to appropriate segments of a commutator and there are brushes engaging the commutator to supply current to the respective coils as they rotate relative to a yoke.

Conventionally, the yoke is comprised of a permanent magnet or magnets with which the magnetic field created within each of the coils of the armature interact in a consecutive way.

This describes a conventional small permanent magnet direct current (D.C.) motor. Such motors are generally used in battery powered hand-held tools such as screwdrivers and drills; or propulsion vehicles that are powered by Solar Cells, batteries or a combination or both. These motors may have permanent magnets situated around the yoke of the motor (providing a stationary magnetic field) and a split commutator which distributes electric power to armature coils. Such motors are known as permanent magnet direct current motors.

Commercially available permanent magnet direct current motors can be inefficient owing to inappropriate design choices in the armature windings.

The general practice in designing commercial D.C. permanent magnet motors is to design and manufacture a motor with a fast output shaft speed and then reduce the output speed by suitable gearing. To achieve this the motors are wound with typically less than 32 turns of wire per pole-piece. In applications where the motor is frequently operated under heavy loads such motors have a severe drain effect on a battery, and hence a relatively short life before the battery or batteries has/have to be replaced or recharged. This is expensive in both time and money.

It is accordingly an object of the invention to seek to mitigate these disadvantages.

According to a first aspect of the invention there is provided an electrical motor comprising two parts adapted to be caused to have relatively reacting forces between the respective parts upon the supplying of electrical current into at least a first one of the parts to effect a magnetic field to react magnetically with the other of the parts, the first one of the parts including a plurality of coils of electrically conducting insulated wire aroimd a motor, and a second one of the parts has a magnetic field adapted to interact with the magneto motive force from the coils, characterised by the number of turns of windings around the former for each of the coils on the first one of the parts being such that a relatively high output torque per unit armature current input is obtained.

There may be a relatively high number of turns of the coil. This provides for a desired high output torque per armature current at stall.

The motor may comprise a direct current flow motor, suitably a permanent magnet motor.

The plurality of coils may be wound upon the rotor to provide an magneto motive forces for providing a torque between the permanent magnetic field and the electromagnetic field, and the coils providing the electromagnetic field may be wound such that when the motor's rated voltage is applied thereto, and the motor is loaded sufficiently for the speed to fall to a small fraction of the no-load speed, the percentage drop in battery voltage from the open circuit voltage is relatively small and the armature reaction in the motor is insufficient to cause appreciable reduction in the motor's torque per armature current.

The coils and core may be in the form bf an armature. This is a relatively simple construction.

According to a second aspect of the invention there is provided a tool, characterised by an electric motor as hereinbefore defined, particularly a hand-held tool.

The tool may be characterised by a gear box with selectable gear ratios whereby the difference between ratios is N, suitably 2.

Using the invention it is possible, by increasing the number of turns of wire aroimd each pole as compared to that which has previously been used, to produce unexpected advantages.

Thus by having a somewhat larger number of turns for each coil than has hitherto been used, motors which are able to provide higher output torque for a given input as compared to traditional motors of similar apparent characteristics, and improved overall efficiencies under heavy load conditions, are achievable.

In experiments, an increase in the number of turns by a factor of approximately 2 to 3 times or greater with a reduction in gauge of the wire and then experimentation through a range of number of turns have found to provide preferred combinations which will vary depending upon whether an overall efficiency improvement is being sought, a minimal current for a given output is being sought or a desirable speed and output at a selected torque.

Using the invention, it is possible to provide a motor having an improvement in some of the performance characteristics of the motor by reason of the increase in number of turns as compared to that which ha been the case given conventional design criteria in existing motors.

It is thus possible to improve certain performance aspects of a direct current motor when powered by a battery or solar cell means or at least provide the public with a useful alternative to currently available direct current motors.

In preference, the armature has a core which may comprise a ferromagnetic material.

In preference, the winding of each coil is formed from wire having a gauge which enables the number of turns and hence total length of wire to be high, thereby offering substantial resistance to current flowing therethrough at the motors rated voltage.

In preference, each of the coils of the armature may have a resistance such that the resistance across the brushes is no less than about 1.2 ohms.

In preference, the motor is connected to an electric power supply with a voltage which is the motors rated voltage.

The motor may be a permanent magnet direct current motor adapted to be electrically connected to the direct current supply means, the permanent magnet direct current motor being wound such that each individual coil between two armature commutator segments has more than 60 turns.

The direct current supply means may be a battery or a solar cell.

The battery may suitably comprise at least one Nickel Cadmium battery cell, suitably one which may have an internal resistance of less than 250 milli Ohms per cell.

The tool may comprise portable power tools such as a drill or screwdriver, or alternatively in a propulsion vehicle.

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 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.

Each individual coil between two armature commutator segments may have between 75 to 150 turns.

Each individual coil between two armature commutator segments may have between 100 to 150 turns.

Each individual coil between two armature commutator segments may have between 105 to 150 turns.

Each individual coil between two armature commutator segments may have between 110 to 150 turns.

Each individual coil between two armature commutator segments may have between 115 to 150 turns.

Each individual coil between two armature commutator segments may have between 120 to 150 turns.

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

Each individual coil between two armature commutator segments may have between 100 to 120 turns.

Each individual coil between two armature commutator segments may have between 105 to 120 turns.

Each individual coil between two armature commutator segments may have

between 100 to 105 turns.

Each individual coil between two armature commutator segments may have between 105 to 115 turns.

Each individual coil between two armature commutator segments may have between 110 to 115 turns.

A motor embodying the invention is hereinafter described, by way of example, with reference to the accompanying drawings, together with the performance criteria compared with motors previously wound according to conventional winding formulas.

Fig.l (a,b,c,d) to Fig.15 (a,b,c,d) illustrate the charaαeristics discovered by increasing the number of windings of an armature;

Fig.16 is a comparison of the number of turns per coil and the current required to provide a torque of 1 Newton metre at stall.

Fig. 17 illustrates a standard magnetization curve;

Fig.18 illustrates the circuitry used to obtain the test results;

Figs. 19A to 19C show graph results for a test motor in relation to the number of turns of its coil; and

Figs. 21 & 22 respectively show a 3 pole-piece and a 7 pole-piece direct current permanent magnetic motor at stall.

Referring to the results as illustrated in Figs. 1 to 16 the motor used to obtain these results was a Johnson 50274 D.C. permanent magnet motor having a 3- tooth armature. The results of Figs. 1 to lb were obtained using the same motor and rewinding the armature with the different number of turns and wire gauges. For instance, Fig. 1 (a,b,c,d) relates to a 32 turn per coil wound Johnson 50274 motor, each of the wound poles having a 0.75mm diameter wire of 0.2 ohms across the brushes, whereas Fig. 5 (a,b,c,d) has relates to the same motor wound with 110 turns per coil of 0.34mm diameter having a resistance of 1.4 ohms across the brushes.

A conventionally wound motor, the results of which are illustrated in Fig.l (a,b,c,d), usually has approximately 32 turns of relatively thick wire gauge (eg. 0.75). Hence, at or near stall armature currents that are above the current rating of the supply (battery, solar cell or other similar supplies) may result (typically substantially greater than 110% of the battery's current rating) due to the lower armature resistance. Accordingly, this can rapidly discharge the battery, or reduce the effectiveness of the supply. Furthermore, it may also damage the supply.

Referring to Fig. l(a,b,c,d) at stall the current flowing in the armature is approximately 20 amps. At this current, armature reaction effects are severe, giving rise to significant reductions in torque per armature turn: I R heating is increased leading to higher rates of temperature rise and possibly early damage to the motor. Furthermore, the supply voltage (line V) has dropped from 10 volts (no load) to 4 volts at stall this being a 60% reduction in the voltage supplied to the motor, caused by the internal resistance to the supply. In contrast the results of Fig. 5 (a,b,c,d) show that the voltage reduction is from 24 volts to 19 volts, this being a reduction of only 21%.

Analysis of the results illustrates that the large voltage supply voltage reductions that occur in a battery-fed system with a standard motor when the motor is heavily loaded results in battery damage or unduly rapid discharge of the battery. Accordingly, motor designers and manufacturers have primarily relied upon improving battery technology (or solar cell technology) and have not addressed the issue of increasing the number of turns in the motor to decrease the voltage reduction under load whilst providing a motor with suitable torque characteristics. This increase in the number of turns reduces the current drain from the supply and therefore it is desirable but not essential to operate below the supply's current rating even when the motor is at or near stall (a preferable range is between 200% to 65% of the supply's nominal current rating).

In addition to improving the performance of the motor under heavy load conditions, the increase in the number of turns preferably reduces the possibility of damage occurring to the supply. Accordingly a preferable feature is that the motor's armature resistance is at least 80% of that of the internal resistance of the supply. For example, if the supply is a Nickel Cadmium battery of 250 milli Ohms per cell then for a 24 volt battery the internal resistance is 6 Ohms, and the motor's armature resistance should be at least 4.8 Ohms.

As can be seen from the results, the characteristics (even when taking into account gearing) where high turns per armature coil with smaller wire diameter are used provides an improved performance, taking into account the current required to achieve a specific torque, over the standard type of motor of Fig. 1 (a,b,c,d). Further the results for the Johnson 50274 show (Fig. 16) that at approximately 100 to 110 turns per coil of 0.34mm wire diameter an optimum occurs for a required torque output (note a different optimum

number of turns may result for a different permanent magnet motor). Hence, upon deciding upon a required torque output and a wire gauge then the optimum number of turns can be determined by non inventive experimentation. However, it should be noted that the number of turns is limited by the physical dimensions of the armature and slots.

When a high number of turns per coil is chosen at the design stage, the armature magneto motive force at or near stall conditions is reduced so that the armature reaction is lessened and reductions in torque per armature turn minimised.

The above results were obtained by using the circuit as illustrated in Fig. 18. with a dynamometer arrangement to facilitate torque measurements. Furthermore, it should be noted that the Torque measurements were taken with the same Torque meter and therefore any errors in the accuracy of this meter are common to all the results. To emulate the supply (battery, solar cell or otherwise) two variable resistors VR1 and VR2 were used to model the internal resistance of the supply. The ammeter Am measured the armature current la, the Torque was measured with the torque meter Rm and a tachometer Rm measured the speed of the motor's shaft under different loads from free running to stall.

Referring now to Fig. 21, there is illustrated a 3 tooth direct current permanent magnet motor having the approximate dimension:

armature diameter a = 23mm; commutator diameter b = 7.5mm; pole core thickness c = 43mm; core end depth d = 3.2mm; shaft diameter e = 3.1mm; core gap f = 4.3mm; shaft length g = 71mm; commutator length h = 10mm; core and commutator gap i = 4.5mm; core length j = 22.5mm;

permanent magnetic yoke inner diameter k = 23.5mm; permanent magnetic yolk outer diameter 1 = 35mm; length of the permanent magnetic yolk m = 29mm; motor casing length n = 48mm; bearing housing diameter p = 12.5mm; and bearing housing depth o = 4mm.

Referring to Fig. 22, there is illustrated a 7 tooth D.C. permanent magnet motor having the approximate dimensions: armature diameter z = 33mm; commutator diameter t = 710mm; pole core thickness y = 3mm; core end depth dd = 2mm; shaft diameter u = 5mm; core gap aa = 2mm; shaft length s = 97mm; commutator length w = 14.75mm; core and commutator gap ϋ = 12mm; core length v = 29mm; permanent magnetic yoke inner diameter r = 34.25mm; permanent magnetic yoke outer diameter q = 50mm; length of the permanent magnetic yoke cc = 40mm; motor casing length ee = 68.5mm; bearing housing diameter bb = 18mm; and bearing housing depth dd = 6mm.

In applications where a reduction in motor speed and motor maximum output power are not critical, selection of a higher turns per coil than usual is hence beneficial because the battery drain is reduced, extending battery life. Reduced speeds and power outputs result and are feasible for screw-driver applications, for example.

Adopting a number of turns per coil that is significantly higher than usual is beneficial for screw-driver motors. Trying to maintain speeds by reducing the combined gear ratio is generally not feasible due to the resulting impact on torque. Adopting a higher battery voltage will reduce somewhat the impact on speeds of the increased turns per coil. A battery with a higher voltage and lower current capacity could be similar in size, weight and cost to the original. Since the modified motor operates with lower output and input

powers, battery life is improved particularly when operation near motor stall occurs regularly, as is the case with screw-driver applications. This is because in operation the gross battery over-loading and severe armature reaction effects that occur with an unmodified motor is eliminated.

Results of tests show that increasing the motor's turns per coil:

reduces no-load speeds; reduces maximum output power; reduces stall current; can sometimes leave stall torque barely affected when terminal voltage and total armature copper weight wire diameter are maintained constant.

The motor has lower I ² R losses under heavy load conditions which is beneficial on a battery's life when heavy loads are frequent.