GB2193384A - Cyclic flux density variation in induction motors - Google Patents

Abstract

An induction motor in which each of the stator windings is connected to the associated phase of the electrical supply via a capacitor 12 whose value is selected so that the magnetic flux density of the stator core is cyclically varied from a non-saturated to a saturated and back to a non-saturated condition thereby maintaining the stator core at a higher average flux density and limiting the energy transfer to the rotor. The stator windings are provided with tappings 14 which can be energised to produce partial saturation of the stator core. Star winding arrangement is also disclosed (Fig. 2). <IMAGE>

Description

SPECIFICATION An improved asynchronous electric machine This invention relates to an improved asynchronous electric machine and more particularly, but not exclusively, to an improved three-phase induction machine.

Conventional asynchronous electric machines, particularly induction motors of, for example, the squirrel-cage type suffer from a number of disadvantages. When such induction motors are subject to heavy loading, the motor draws excessive current as the rotor slows down which can result in the motor burning out unless auxiliary over-load protection equipment is provided.

It is necessary for such conventional induction motors to have a high break-away torque to running torque ratio to prevent damage to the motor when the motor is overloaded and as a result, the magnetic flux density in the stator must be maintained at considerably less than saturation level.

This utilisation of a relatively low magnetic flux density during normal operation is also made necessary in such conventional induction motors to allow for any variation in input voltage which could also result in over-loading of the motor and damage thereto.

It will be appreciated that the use of a relatively low magnetic flux density in the stator results in the motor size for a given output power being substantially larger than would be necessary in an ideal motor where the magnetic flux density in the stator is maintained at a maximum level.

In addition, conventional induction motors also suffer from the disadvantage that the output power obtained is dependent to a significant extent on the input voltage and to a certain extent on the input voltage frequency.

Furthermore, conventional induction motors draw high starting currents until the rotor reaches operating speed and induction motors of any significant output power have to be provided with external current limiting devices or special and expensive designs of rotor have to be utilised.

The object of this invention is to provide an asynchronous machine in which one or more of the above disadvantages of the known machines are alleviated.

'According to this invention, an asynchronous electric machine comprises a stator core carrying stator windings, a rotor mounted for rotation in the stator under the action of the magnetic field produced by electrical currents flowing through the stator windings, wherein means are provided for cyclically varying the magnetic flux density of the stator core during operation of the machine from a non-saturated condition to a saturated condition and back to a non-saturated condition whereupon the cycle recommences.

Preferably, the means comprise connecting the stator windings to the electrical supply via associated capacitors.

Preferably, also, each capacitor is connected in series between the associated stator wind ing and the associated electrical supply.

The stator windings are, preferably, of delta configuration and a capacitor is connected in series between each apex of the delta confi guration and the associated phase of a three phase electrical supply.

Alternatively, the stator windings are of star configuration and a capacitor is connected in series between each end of the star configuration and the associated phase of a three phase electrical supply.

The capacitors are, preferably, of a value which enables the voltage stored therein in combination with the input voltage to cause during operation of the machine the volt-second capacity of the stator core to be ex ceeded thereby cyclically changing the mag netic flux density of the stator core from the non-saturated condition to the saturated condition and back to the non-saturated condition.

The rotor is, preferably, a squirrel-cage rotor but it will be appreciated that the rotor may be a wound rotor.

Preferably, also, means is provided for varying the effective magnetic capacity of the stator core, said means comprising providing the stator with tapped windings which when energised produce partial magnetic saturation of the stator core.

The current energising the tapped winding is, preferably, varied in accordance with the load on the machine and/or the supply voltage and/or the speed of rotation of the rotor or any combination thereof.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a schematic circuit diagram of an asynchronous machine according to the invention; and Figure 2 is a schematic circuit diagram of an alternative asynchronous machine in accordance with the invention.

Referring now to the drawings, an asynchro nous electric machine such as an induction motor comprises a stator core (not shown) carrying stator windings 10 in which a rotor .(not shown) is mounted for rotation. In Figure 1 of the drawings, the stator windings 10 are shown connected in delta configuration to the three-phases A, B, and C of a three-phase electrical supply. In an alternative machine shown in Figure 2 of the drawings, the stator windings 10 are shown connected in star configuration to the three-phases A, B, and C of the supply.

In asynchronous machines such as, for example, squirrel-cage induction motors, the force generated in a conductor of the rotor is defined by the equation: F = BxLxl where F = force, B = flux density, L = length of the conductor, I = current in the conductor.

It will be appreciated that if maximum values of flux density and current are achieved for a given length of conductor, the maximum force and consequently torque and power output can be obtained from a specific size of motor.

In order to achieve maximum flux density in the stator core of the machines, such as squirrel-cage induction motors, which are the subject of this invention, in the machine shown in Figure 1 of the drawings, capacitors 12 are connected in series between the apex of the delta configuration of stator windings 10 and the associated phases of the supply in the machine and in the machine shown in Figure 2 of the drawings, the capacitors 12 are connected in series between the ends of the star configuration of stator windings of the machine and the associated phases of the supply. The values of the capacitors 12 are selected so that the voltage stored therein is sufficient, in combination with the voltage of the supply, to periodically cause the volt-second capacity of the stator core to be exceeded during operation.This results in the stator core periodically changing non-linearly from a non-saturated to a saturated condition and then back to the non-saturation condition and the cycle is then repeated. The average flux density in the stator core is thus maintained at a high value without having to utilise high input supply voltages which would result in high input currents. The capacitors limit the amount of energy that can be transferred to the rotor even where the rotor has a very low impedance thus enabling maximum rotor currents to be utilised The inductance of the rotor can also be made lower than in a conventional squirrel-cage induction motor and the current induced at zero rotor speed can be made greater than is possible in a conventional motor even though the current will still have a normal value at normal motor operating speeds and normal loads.This thus produces the advantage that the motor of the present invention is more suited for use in a large number of applications or for any given application than a conventional induction motor. The connection of the capacitors in series .with the stator windings and the operation of the motor magnetic path in soft saturation due to the limiting effect of the total energy transfer of the capacitors results in a motor which can be operated at maximum flux density under most conditions of supply voltage without resulting in extremely high input currents for high input supply voltages. In the machines which are the subject of this invention, the input current and the flux density do not have the extreme non-linear relationship to the voltage of the supply which is the case with conventional induction motors.

In operation, in the machines which are the subject of this invention, the inductances of the stator windings can only absorb so much energy before the magnetic material of the stator core saturates and discharges through the stator winding and the electrical supply and charges up the capacitors 12 with the opposite polarity. The current flowing through the stator windings 10 then reverses and the capacitors 12 are then the source of energy and maintain the current flowing through the stator windings 10. This continues until the voltage of the supply changes in polarity and until the total volt-seconds applied to the stator winding exceeds the volt-second capacity of the stator winding and the magnetic material of the stator whereupon the magnetic material of the stator again saturates.The capacitors 12 then discharge through the stator windings 10 since they are saturated and the supply voltage charges up the capacitors 12 with the opposite polarity. The current then reverses once more through the stator windings 10 and the capacitors 12 again provide the source of current through the stator windings 10. This continues until the supply voltage again changes polarity. As the supply voltage amplitude continues to increase, the volt-second of the supply voltage together with that of the capacitors 12 are again in phase and are added together until the voltsecond capacity of the stator windings 10 and the associated magnetic material of the stator core are exceeded. The stator windings 10 and magnetic material of the stator core again saturate and the inductance of the stator windings 10 decreases causing the capacitors 12 to discharge through the stator windings 10.This process is repeated each half-cycle and results in the motor running at maximum flux density and thus maximum force, torque and output power.

In the machines which are the subject of the present invention although maximum flux density of stator core is utilised, as the voltage across the capacitors 12 is usually much higher, although not necessarily so, than the voltage of the supply, the flux density in the stator core is relatively independent of the supply voltage over a fairly wide range of voltage amplitude. Furthermore, the capacitors 12 prevent excessive currents from passing through the stator windings when the magnetic material of the stator core saturates as only the energy in the capacitors 12 which is represented by the equation E = 2CV2, where E = energy, C = capacity and V = voltage, can be transferred through the winding. This limited amount of energy transfer prevents excessive currents flowing from the supply line through the stator windings 10.

It will be appreciated that this results in an asynchronous machine that will operate over a wide range of input voltages and will also op erate at high levels of efficiency with good operating characteristics. As each of the capacitors 12 limits the amount of energy transferred through the associated stator winding 10 during each half-cycle, the danger of overloading the motor and burning it out is alleviated. In the case where the motor is overloaded, all that will normally occur is that the motor will stall and the input power to the motor would be greatly reduced. This is due to the fact that the capacitors 12 connected in series with the stator windings 10 will have a much lower voltage across them than in a conventional motor as the motor is not operated in the controlled phase, and the energy level represented by the equation where E = 2CV2 is greatly reduced.

It has also been found that even greater operating efficiencies can be obtained by varying the amount of magnetic material available in the stator in accordance with the supply voltage, the load, or other selected conditions such as the rotor speed. This variation of the amount of magnetic material in the stator which is effectively available enables stator magnetic losses and stator copper losses, which make up a significant uncontrollable portion of the total losses of a conventional induction motor, to be varied in accordance with the demands on the motor.

Control of the amount of magnetic material effectively available in the stator of the machines which are the subject of the present invention is accomplished by providing the stator windings 10 with tapped windings 14 which, when energised cause the stator core to be partially saturated, thus effectively re ducing the magnetic cross-sectional area of the stator core. This reduction in magnetic cross-sectional area reduces the volt-second capacity of the stator core with the result that the voltage across the motor is also reduced.

This results in the voltage across the capacitors being also reduced as the currents must at all times be such that all voltages round the circuit add up to zero. The reduction in capacitor voltage results in a corresponding reduc tion in the energy stored in the capacitors represented by the equation: E = BCV2 referred to above.

The energy circulating in the motor during each half cycle is thus greatly reduced and the motor losses are also correspondingly reduced.

In operation, where the motors according to the present invention are operating at no load, the amount of magnetic material effectively available is thus small so that the stator magnetic losses and stator copper losses are low.

The magnetic material effectively available, however, is sufficient to produce enough power to drive the motor at no load. When the motor is loaded, the amount of magnetic material available is increased to a point sufficient to still provide sufficient power output from the motor. Of course, the internal losses of the motor increase during this loading period but will still be less than would be the case if the motor was under full load. Because the losses are made to vary with the load on the motor, and because motors are rarely operated under full load for their entire operating period, the average internal losses are significantly decreased and the overall efficiency of the motor is thus increased.The result is a motor which is capable of satisfactory operation under any conditions of load and which consumes less electrical energy than conventional motors which have essentially the same losses whether they are operating at no load or full load.

Once again, it is emphasised that by making the current in the tapped windings 14 dependent on the conditions under which the motor is to operate, for example, load, the motor which is the subject of this invention can be made to operate efficiently for all conditions of load. Thus if the motor is not loaded, the control current can be made to be high with the result that the effective cross-sectional area of the stator core is small so that the circulating energy, and the corresponding losses, are also small. As the load increases, the control current can be made to decrease with the result that the power handling capacity of the motor increases to whatever level is necessary to drive the motor under the increased load. The control current of course can be made responsive to other motor conditions such as line voltage or speed or to any desired combinations of motor conditions or other desired external conditions.

It will also be appreciated that the present invention can be applied to various types of polyphase alternating current machines, not only induction motors, without departing from the scope- of this invention.

This invention is not restricted to the details of the foregoing embodiments but extends to any novel one, or any novel combination, of the features disclosed in the specification and/ or drawings, or to any novel one, or any novel combination, of the steps of any method or process disclosed herein.

Claims (12)

1. An asynchronous electric machine comprising a stator core carrying stator windings, a rotor mounted for rotation in the stator under the action of the magnetic field produced by electrical currents flowing through the stator windings, wherein means are provided for cyclically varying the magnetic flux density of the stator core during operation of the machine from a non-saturated condition to a saturated condition and back to a non-saturated condition wherein the cycle recommences.

2. An asynchronous electric machine according to Claim 1, wherein the means com prises connecting the stator windings to the electrical supply via associated capacitors.

3. An asynchronous electric machine according to Claim 2, wherein each capacitor is connected in series between the associated stator winding and the associated electrical supply.

4. An asynchronous electric machine according to Claim 3, wherein the stator windings are of delta configuration and a capacitor is connected in series between each apex of the delta configuration and the associated phase of a three-phase electrical supply.

5. An asynchronous electric machine according to Claim 3, wherein the stator windings are of star configuration and a capacitor is connected in series between each end of the star configuration and the associated phase of a three-phase electrical supply.

6. An asynchronous electric machine according to any one of Claims 2 to 5, wherein the capacitors are of a value which enables the voltage stored therein in combination with the input voltage to cause during operation of the machine the volt-second capacity of the stator core to be exceeded thereby cyclically changing the magnetic flux density of the stator core from the non-saturated condition to the saturated condition and back to the nonsaturated condition.

7. An asynchronous electric machine according to any one of the preceding claims, wherein the rotor is a squirrel-cage rotor.

8. An asynchronous electric machine according to any one of Claims 1 to 6, wherein the rotor is a wound rotor.

9. An asynchronous electric machine according to any one of the preceding claims, wherein means is provided for varying the effective magnetic capacity of the stator core.

10. An asynchronous electric machine according to Claim 9, wherein the means for varying the effective magnetic capacity of the stator core comprises providing the stator with tapped windings which when energised produce partial magnetic saturation of the stator core.

11. An asynchronous electric machine according to Claim 1Q, wherein the current energising the tapped windings is varied in accordance with the load on the machine and/or the supply voltage and/or the speed of rotation of the rotor or any combination thereof.

12. An asynchronous electric machine constructed, arranged and adapted to operate substantialíy as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.