IE56555B1 - Power supply systems - Google Patents

Description

This invention relates to power supply systems for feeding inductive elements and in particular to power supply systems for feeding switched inductive windings such as the phase windings of a switched variable reluctance motor.

A motor of this kind, to which the present invention may be applied, is disclosed in our co-pending Patent Application No. 8525560 entitled Variable Speed Variable Reluctance Electrical Machines*.

In a variable reluctance motor provided with a unipolar drive, current may be switched into the phase windings by electronic devices under PWM control. In considering the electrical behaviour of the power supply circuit for the phase 10 windings, the windings may be regarded as inductors under certain operating conditions, in that their response to current flowing through them under such circumstances ls largely determined by their inductance rather than by their resistance. When the switch for a particular winding is closed, current flows through the inductor in question, which may then be connected between a supply rail and ground. Energy is thus stored in the ί magnetic field of the winding, the amount of this energy being M2L, where 1 Is the current and L Is the Inductance of the winding. When the switch opens again, this energy stored In the magnetic field has either to be dissipated or, preferably, returned regeneratlvely to the supply. In a particular supply arrangement for reluctance motors, the switched end of the winding 1s also connected to a second supply rail through a normally reverse biassed diode. Thus 1n this arrangement, winding current transfers to the diode after the switch has opened and deceys If the second supply rail has the appropriate polarity* However It 1s frequently the case that such supply rails are unable to regenerate energy, with the possible result that the voltage of the second supply rail may rise to a destructive level unless an equal or greater current 1s drawn from It than that being supplied by the phase winding. Dealing with or disposing of this Inductively stored energy Is thus a considerable problem In the application of variable reluctance motors, especially In situations where they are required to operate at low rotational speeds.

A twin-rail power supply with an equal number of motor phases connected to the positive and negative rails mqy be adequate for reluctance motors operating In a continuously-rotating motoring mode only, with non-usable Inductively stored winding energy being returned to the complementary rail for use 1n the phases connected thereto, but this Is not necessarily the case In a motor required to provide torque at zero speed In order to hold a load, where the current of the driving phases, less losses, may be transferred continuously between the rails and may pump up a supply capacitor located, for example, between the second rail and earth. At certain rotor positions the effects of two phases will cancel, and between these points, peaks of upward and downward current transfer will be reached. Thus, the effect at zero speed is to unbalance the rails. On the other hand, In a reluctance motor rotating at speed and -3acting to decelerate an Inertial load or otherwise regenerate energy» the effect will be to pump up both supplies.

This second-mentioned effect Is the same as that which exists in any conventional servodrive» and since the total energy involved In a typical duty cycle 1s not great» it may be dealt » with by burning It off in a dump resistor disposed between the rails. As a rule of thumb, the dump resistor Is usually sized to Intermittently draw a current equal to the continuous rating of one axis In a DC drive (e.g. 20A or 40A). The first effect, I.e. that at zero speed, is not seen in DC servodrives with a single rail supply. One solution to the problem Is to switch the reluctance motor phases at both top and bottom but this doubles the number of main devices.

A second technique to regenerate phase energy Into the main supply Is to use special blfllar windings In the motor. While this may seem attractive from many points of view there are also, serious problems with this approach, as noted below, since the number of connections to the motor is doubled. In particular, to allow for worst case duties, the secondary winding would need to have virtually the same cross-section as the primary, thus greatly reducing the utilization of winding area and motor rating. In addition, In a blfllar winding, two closely coupled colls are connected to opposite supply rails and may have very high potential differences between them, leading to unreliable operation and breakdown. While appropriate for low voltage battery operation, this could cause serious problems with supplies over 100 V. Also as a main transistor switches off and a secondary winding takes over current conduction, very fast current rises and falls would take place In the leads to the winding. This, along with poor coupling between primary and secondary windings, could give rise to severe electromagnetic noise radiation. Blfllar windings may thus be seen to be appropriate only when the drive electronics are nounted close to the motor.

Since In virtually every application a servomotor requires to hold the friction torque of the mechanism It Is driving when at stall, and stall current can be up to half the motor continuous rating, and since also 1n many applications, the motor will be holding an uncounterbalanced load, the provision of some effective and economical means of transferring energy away from a supply undergoing pump-up Is regarded as a necessary feature of at least servomotor drives.

It may be argued that 1n large multi axis systems, conditions at large should cancel out, so as to make the problem a relatively minor one. On the other hand, a solution to the unbalance problem must be available for Implementation 1n systems where 1t is required. A very crude solution would be to have Individual dump resistors on the rails to bum off the unbalance. However, since this might Involve burning off the rated motor current continuously, It would hardly be acceptable.

According to the Invention, there 1s provided a power supply system for a plurality of Inductive elements, said system having first, second and third rails, said first and second rails being energisable at differing potentials and each said element being swltchlngly connectible between the rails, and the system Including means for current flow between each said element and said third rail when the connection of said element between the first and second rails Is broken, a further Inductive element of substantially the same value of inductance as each of said plurality of Inductive elements being swltchlngly connectible between said third rail and one of said first and second rails, and the system also comprising means for connecting said further element between said third rail and said one of the first and second rails when the voltage on the third rail Is equal to or greater than a predetermined value -5and means for current flow between the further Inductive element and the other of said first and second rails when the connection of said further element between the third rail and said one of the first and second rails 1$ broken.

An Inductive element 1$ to be regarded as any component of the system having Inductance as the primary parameter determining ; β Its electrical behaviour. Thus when the current flows through an Inductive element as herein defined, 1t Is inductance rather than resistance or any other measure of Its electrical characteristics that determines Its behaviour. The or each inductive element connectible between the first and second supply rails of the system may be a magnetising or exciting winding of a variable reluctance motor.

In the power supply system according to the Invention, a resistive element may also be switchlngly connectible between said third rail and said one of the first and second rails, and the system may comprise means for connecting said resistive element between said third rail and said one of the first and second rails when the voltage between said third rail and said one of the first and second rails is equal to or greater than a predetermined value.

Current flowing from the or each Inductive element connectible between the first and second rails of the system Is preferably directed to the third rail by a diode, while the current flowing from the further Inductive element may be similarly directed to said other of the first and second rails by a further diode. The connection of each of the elements connectible between supply rails of the system most suitably takes place under the control of proportional-Integraldifferential type control means or systems in which an excess voltage represents an error signal, in dependence on the value of which, switches connecting the Inductive element across the v rails may be opened or closed as appropriate under PMM control.

I - 6 Embodiments of the invention will now be described having regard to the accompanying drawings in which: r Figure 1 is a schematic diagram of a unipolar drive for a switched variable reluctance motor, ' 5 Figure 2 is a schematic diagram of a power supply system according to the present invention, and Figure 3 is a schematic diagram of a further configuration of power supply system according to the present invention.

As shown in Figure 1, a phase winding L of a variable reluctance motor is connected between a first rail at a voltage Vp and a second rail at earth 0 through a switch S. When the switch S is closed, a current indicated by i builds up through the inductor L. When the switch is opened again, the current will transfer to the diode 0 as ig. If the rail to which this current flows through the diode 0 is unable to regenerate the returned energy, the voltage on the capacitor Cg between the first and third rails will rise, with possibly destructive results, if no precautionary action is taken.

As shown in Figure 2, the four phase windings 1, 2, 3 and 4 of a four-phase reluctance motor are fed from a single-ended power supply consisting of a negative rail 5 and a ground rail 6, through respective switches 7, 8, 9 and - In order to dispose of the inductive energy stored in the phase windings, which behave as inductive elements as herein defined to return energy to the power supply system on phase switch-off, diodes 11, 12, 13 and 14 direct the returned energy to an upper rail 15, which is not tied to any voltage and serves merely as a means for recirculating and disposing of returned energy. A dummy phase winding 16 in the form of an inductive coil of substantially the same value of inductance as each of the phase windings 1 to 4 bridges between the upper floating rail 15 and the ground rail 6. The floating rail is connected through the 0 t - 1 duray winding 16 to the ground rail by a switch 17, similar to the switches 7 to 10 of the main motor phase windings. In order to take away returned inductive energy from the element when the switch 17 is opened, a diode 16 is connected s between the inductive element 16 and the negative rail for the same direction of current flow as the diodes associated with [ the phase windings proper. A dump resistor 19 between the floating rail and the ground rail is switched In or out by switch 20 and serves for disposal of returned energy during regeneration, when all motor phases are active.

The system incorporating the features of the invention as described above recirculates any unbalanced transfer of energy from the negative rail during unbalanced operating conditions, such as may prevail when a servomotor is at standstill, so that pump-up or excessive build-up of voltage on that rail may be avoided. A logic circuit associated with the power system detects the build-up of voltage and functions to operate switch at appropriate intervals so that it recirculates such energy in a manner complementary to the way 1n which the active phase winding or windings remove it from the negative rail, and normal voltage conditions may thereby be maintained in the power supply system. This detection circuit will normally be a proportional/integral/differential type control system with excess auxiliary power supply voltage as Its error signal. The dump resistor 19 will be activated by switch 20 when operation of the anti-pump-up circuit as described above causes the main power supply to build up excess voltage. In one Implementation, excess main rail voltage inhibits the operation of the . dummy winding 16 and the dump circuit is activated when the auxiliary rail voltage passes a preset threshold.

In the three-rail system of Figure 3, a substantially similar arrangement is provided but the upper rail 15 is in this instance a positive rail at +V. Two dummy windings 16a and 16b 1 i - 8 are then necessary to link between the positive 15 and negative 5 rails respectively and the ground rail 6, since in this instance the negative rail 5 is also subject to being pumped-up by the positive rail 15. Similarly, each dummy winding has an associated switch 17a, 17b and a diode 18a and 18b respectively. Two dummy resistors 19a and 19b are also provided for disposal of energy during regeneration, and are switched by respective switches 20a, 20b.

Operation of this system is essentially similar to that of the first arrangement, the appropriate dummy winding being switched depending on which . 10 of the rails is being Upumped-up under standstill or unbalanced conditions of phase winding energisation. The scheme of Figure 3 substantially equates to a back-to-back positioning of two circuits as illustrated in Figure 2.

Claims (3)

1. A power supply system for a plurality of inductive elements, said system having first, second and third rails, said first and second rails being energisable at differing potentials and each said element being switchingly ί connectible between the rails, and the system including means for current flow between each said element and said third rail when the connection of said element between the first and second rails is broken, a further inductive element of substantially the same value of inductance as each of said plurality of inductive elements being switchingly connectible between said third rail and one of said first and second rails, and the system also comprising means for connecting said further element between said third rail and said one of the first and second rails when the voltage on the third rail is equal to or greater than a predetermined value and means for current flow between the further inductive element and the other of said first and second rails when the connection of said further element between the third rail and said one of the first and second rails is broken.

2. A power supply system according to Claim 1, wherein a resistive element is also switchingly connectible between said third rail and said one of the first and second rails, and the system comprises means for connecting said resistive element between said third rail and said one of the first and second rails when the voltage between said third rail and said one of the first and second rails is equal to or greater than a predetermined value.

3. A power supply system substantially as described herein with reference to and as shown in Figure 2 or Figure 3 of the accompanying drawings.