GB2257844A - Balanced field direct current motor - Google Patents

Abstract

A direct current electric motor primarily designed to drive electric vehicles. Torque is developed by repulsion and attraction see Figs 2a, c from all motor cars during part of the commutated cycle, sets of winding being shorted, Fig 2b, at alignment. The magnetic field of both stator and armature pole faces at 2c are balanced to reduce magnetic drag. This is achieved through all values of current by having a windings ratio of 1:2 respectively. The commutator brushes may be mounted in an advance and return mechanism. The short length allows greater numbers of conductor turns for relatively low resistance, increasing amp-turns potential, giving greater economy of power and permits a modular arrangement. <IMAGE>

Description

Ballanced Field Direct Current Motor There are many types of electric vehicle in use today and all share a common advantage over other vehicles (which use combustible fuels), they are pollution free. There is however one major dissadvantage with some electric vehicles1 namely the type which derive its power from batteries, and that is the distance that can be traversed from one full charge. Economical use of power in these vehicles is therefore of paramount importance if greater distance is to be achieved. The motors (which are normally direct current), that drive these vehicles would therefore be the obvious place to make savings in power. Further economy could be made if the drive units where made more flexible in their use. The following text describes an invention which hopes to achieve both economy and flexibilty.

This invention relates to a direct current electric motor designed specifically for electrically propelled vehicles, although it may be used for other purposes. It provides a powerful, economical and flexible source of drive. Its torque is produced from three principles of magnetomotive force which are alignment, repulsion and interaction.

Construction: The motor will now be described by way of an example. Diagram shows the main components of the motor and is viewed looking at the centre of the shat from the commutator end. The outer casing, cooling fan and shaft support bearing are not shown for clarity. The parts are indicated by letters and are; a) The motor shaft, which is made of mild steel and is supported centralyby two bearings (not shown).

b) Mounted on the shaft is a four segmented commutator made of copper alloy which is insulated from the shaft by tough insulation. Each segment is identical in shape and size.

c) Spring loaded carbon brushes bear down onto the commutator for the purpose of electrical connection. The brushes are mounted on an automatic advance and return mechanism (not shown or specified).

d) Securely fixed to the shaft are laminated silicon steel sheets which are shaped to form a four pole rotor, and may be riveted or welded together.

e? Formers, made of insulation are firmly mounted around the pole pieces.

-h Enamel coated copper wire is wound on each former to form solenoids and are connected in series via the commutator segments. Some indings are cut away to show the shape of the steel core.

This now describes the armature of the motor and may be a typical decription ot an armature already in use in some electric motors. The stator however differs from the norm in its shape and function and will now be described using the same diagram.

g) The stator or stator ring is also made up of silicon steel sheets, which may be riveted or welded together. It consists of two pole pieces which are diametrically opposite and shaped to take windings. The rest of the ring is formed by two yokes which contain slots for the purpose of indlngs. The stator ring is coated with lacquer for the purpose of insulatIon and further insulation of paper lines the slots of the yokes and the inner walls of the poles.

There are several ways in which the stator may be wired, in this example enamel coated copper wire is wound onto the poles in solenoid form and also around the yokes. The windings of one yoke is then connected in series with the windings of one pole. The remaining windings are connected in a likewise manner but in vice versa order. The two sets of series windings are now connected in parallel forming a series/parallel circuit. Some of the windings are cut away to reveal the shape bf the stator ring.

The length of the motor is not shown, but will be approximately 20 to 25 percent of the armatures overall diameter. There are two reasons for this relatively short length. The fist is to keep the overall length of the conductors to a minimum. Thus reducing the resistance of the windings and allowing more turns to increase the amp-turns potential. The second reason is to allow two, three or more motors to be constructed in tandem, with each motor's armature sharing a common shaft.

From this point on in the text each individual motor will be refered to as a MOTOR UNIT and the combination of two or more motor units as a MOTOR SET.

Operation: Diagram 2 illustrates the three manetomotive forces which come into play when the motor unit is energlsed.

Stage one a: By following the direction of current in this diagram, it can be seen that the magnetic polarity of the stator poles and that of the armature poles advancing towards them, are opposite. This is the attraction or alignment force, and creates torque on the armature's shaft.

Stage two Db; At this stage full alignment has taken place and by noting the position of the commutator, it can be seen that commutation has occurred and the winnings on the aligning poles are shorted out. The windings on the adjacent poles now share the full current in the circuit. By noting the magnetic polarity of these poles and the direction of the current in the yoke windings, it can be seen that torque wIll be produced from interaction between the pole faces and the current, thus turning the armature in the direction indcated. The interaction force will of cause effect all armature pole faces within the region of the yoke windings during all positions of commutation.

Stage three 2c: On further rotation a new position of commutation takes place and all windings on the armature now conduct current. The two poles that have moved from alignment change magnetic polarity, and repulsion now takes place between these pole faces and those of the stator.

It is important to note at this stage that the strength of the repulsing magnetic fields of both armature and stator poles are equal or balaced. This will be best explained by way of analcgy.

Diagram 5 shows two bar magnets of equal size, shape and most importantly, magnetic field strength. Held by hand they are brought together with like poles facing each other. As the distance between the faces decreases repulsion will be felt and will increase in strength untill a maximum is reached when the pole faces touch. If however the same process is repeated with identical magnets but of different field strengths the outcome is somewhat different. As the magnets are brought together repulsion will be felt. This will increase in strength as the distance between the pole faces decreases. However at some distance apart, (depending on the difference in field stengths) the stronger of the two magnets will begin to atract the other. The strength of the atraction thereafter will increase as distance decreases.

Because of the relatively short distance between the pole faces at stage three it is imperative that the field strengths are balanced throughout all current values to prevent magnetic drag. To achieve this, (in this example) the ratio of conductor turns of the armature and stator are made 2 to 1 respectively. Diagram 4 shows a schematic circuit of a motor unit, where the conductor turns on each armature pole are equal in number. The stator yokes each carry the same number of conductor turns as thier counterpart as do the stator poles. However these values may not be equal between the pole and the yoke windings. Infact it may be necessary to place the bulk of the stator windings on the poles, to prevent any flux leakage formed by the yoke wlndings.

C.mmutatIn: As stated before the commutator brushes are mounted on an automatic advance and return mechanism. This device, advances the brushes as the shalt's speed increases and greatly improves performance.

In humming up all the operations of torque production and the fact that greater numbers of conductor turns can be used in this design, it can be seen that the most economical use of available current is made possible. It follows then, that power output must be greater for any given input, compared with conventional d. c. motors. Added to this, the diameter of the armature may be made much larger, thus creating greater torque on the shaft.

Flexibility: Any drive unit in a vehicle will need to be flexible enough to handle the numerous conditions and loads that are demanded of it. At the one end of the scale, power and torque are needed to handle starting off, heavy loads, steep hills ect. At the other end, less emphasis is needed on torque and more on revs per minute to attain the desired speed. Although d. c. motors are very flexible in speed control, a single motor would need to be geared through it's transmission to meet these conditions. To overcome this problem several motor units are constructed together to form a motor set.

Motor set: This will be best explained by taking as an example, three motor units forming a motor set. Each motor unit is of the same size in all respects i.e. size, rating, resistance ect. and fixed together in straight line form, with the armature of each motor unit sharing the same shaft, and each having ;it's own separate corimutator on the same end of said shaft.

Three pairs of carbon brushes, xone pair for each commutator) are held in a common advance and return mechanism ensuring all brushes mcve together relative to the speed of the shaft. eed conductors are brought from each motor unit and connected to a three position switch.

C ntrol: The following descriptIon of control is not part of the invention, but serves to illustrate the flexibiblbty of the invention in motor set form. The three position switch will work in conjunction with a voltage regulator and an accelerator pedal. The operation of all three devices will be best explained as first, second and top gear.

Fist gear: In this position the switch connects the supply to all three motor units in parallel via the voltage regulator and accelerator pedal. The voltage regulator limits the amount of voltage to a percentage of the total voltage available and the accelerator pedal controls this percentage from zero to maximum. This first position is allocated the lowest voltage but because the total risistance is the lowest, the current dram will be the highest. This not only gives first gear the highest power rating but also allows the load to be shared by the three motor units. The current is also shared equally and-as the amount of current determines the speed of the motor unit, this limits the r.p.m. to well below maximum.

Second gear: After the vehicle is set in motion and has attained some speed or reached its maximum, the driver may require more speed from the vehicle.

By moving the switch to the second position the percentage of available voltage is increased-and one of the motor units is switched out of the circuit. The total resistance now increases and allows less current to flow, thus reducing the overall power input. Hovever the current is now shared by only two motor units and each motor unit may now carry a higher current than the previous setting. The maximum r.p.m. may now be increased with the use of less power.

Top gear: In the third and final switch position, two motor units and the voltage regulator are switched out of the circuit. All-the available voltage may now be applied via the accelerator pedal, to the single motor unit left in the circuit. The resistance has again increased and allows even less current to flow. However all the available current may now be passed through this one motor unit, giving it the highest current rating and therefore the highest r.p.m.

By way of example the table below shows the maximum power available at each of the switch settings. A maximum voltage of 120 volts from a d. c. supply is used with a resistance of 5 ohms for each motor unit. It- is assumed that the voltage regulator and the accelerator pedal have zero resistance.

total ~ current total switch available total total per motor power in setting voltage resistance current unit watts first 80 volts 1 ohms '8 amps 16 amps 384C second 90 volts 212 ohms 36 amps 18 amps 3240 third 120 volts 5 ohms 24 amps 24 amps s88o From the table it can be seen that power can be allotted to where it is needed most and speed is attained by a reduction in power. This is important if the vehicle is to get greater travelling distance from its batteries.

This arrangement will also allow the skill of the driver to make further savings in power.

Claims (4)

Claims

1 A direct current electric motor comprising an armature with means of accepting an electrical supply whilst in motion, a stator so shaped and wound with insulated conductors as to make use of three magnetomotive forces namely alignment and repulsion and interaction, the afore mentioned armature and stator having such numbers of conductor turns as to maintain equal magnetic field strengths of both armature and stator poles during the stage of repulsion throughout all values of current, the afore mentioned armature and stator being relatively short in length allowing large numbers of condutor turns for minimal electrical resistance and also giving facility for two or more said motors to be constructed in tandem, the armature of each motor thereof sharing a common shaft.

2 A direct current electric motor or motors as claimed in claim 1 wherein a means of cooling the said motor or motors whilst in operation is provided.

3 A direct current electric motor or motors as claimed in claim 1 or claim 2 wherein the carbon brushes of the said motor or motors are mounted in an automatic advance and return mechanism which moves relative to the shaft.

4 A direct current electric motor substantially as described herein with referee to diagrams 1 - 4 of the accompanying drawings.