GB1597790A

GB1597790A - Variable reluctance motor drive systems

Info

Publication number: GB1597790A
Application number: GB2289178A
Authority: GB
Original assignee: Chloride Group Ltd
Current assignee: Chloride Group Ltd
Priority date: 1978-05-26
Filing date: 1978-05-26
Publication date: 1981-09-09

Description

(54) VARIABLE RELUCTANCE MOTOR DRIVE SYSTEMS (71) We, CHLORIDE GROUP LIM ITED, a Company registered under the laws of England, of 52 Grosvenor Gardens, London SWIW OAU, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to variable reluctance electric motor drive systems and is concerned with the synchronisation with the rotation of the rotor, of the timing of solid state switching devices controlling the feeding of current pulses to the motor from a DV source. This timing is of great importance since the driving torque of each phase depends on current flowing while the inductance is increasing, and any current flowing while the inductance is falling will produce a braking torque.

Various forms of transducer are known for the purpose of giving a signal synchronized with the rotation of the rotor. For example, a rotating disc with holes may be interposed between a light source and a photosensitive device. It is also known to mount a transducer so as to be adjustable about the stator axis to vary the timing of signals producing current pulses.

An object of the present invention is to provide a more versatile or readily controlled system without unnecessary moving parts.

According to the present invention a vari able reluctance electric motor system includes means for generating a low frequency signal synchronised with the motor rotor, means for generating a high frequency signal from the low-frequency signal at a frequency con trolled by, varying with, and many times higher than, that of the low-frequency signal, and means for counting cycles of the high frequency signal starting from a predetermined point of the low-frequency cycle to provide one or more control signals starting and/or finishing at a predetermined angular position of the motor rotor and serving to control synchronisation of current pulses with angular position of the rotor.

The control signal may be used to control the conduction of a solid state switching device to switch on a current from a DV source to a phase winding of the motor at a predetermined angular position of the rotor. Alternatively or in addition it may be used to commutate or switch off a solid state switching device to initiate the end of a pulse of current. The timing of the signal (in terms of angular position) may be adjustable either manually or automatically by varying the count of the counter at which it is produced. The high frequency is preferably several hundred times the rotational frequency of the rotor so that, by varying the count, adjustment of the timing is possible to a fraction of a degree. For example, the low frequency could be one pulse per 150 and the high frequency one pulse per quarter of a degree.Thus, if the motor is for propelling a vehicle the speed can be controlled from a pedal operated by the driver and linked mechanically or electrically to means for varying the timing of the signal or signals.

In one form of the invention the high frequency signal is produced indirectly by rotation of the rotor by multiplication of the frequency of the low frequency signal. Multiplication may be achieved by a phase locked loop, that is to say a variable frequency oscillator with a phase comparator arranged to adjust the frequency of the oscillator to keep it in phase with a multiple of the low frequency signal.

A preferred system includes speedresponsive logic switching means having a high-speed condition in which the supply of current to the phase windings is under the control of the high-frequency signal and counter, and a low-speed position in which it can be switched on independently thereof.

The low frequency signal may comprise at least one pulse per revolution for each of a number of phases, the duration of the pulses being such that every point of the revelation falls within a pulse of at least one phase when the inductance of the winding is rising at that point. With the logic switching means in the low-speed condition the low frequency signal may then directly control conduction of a solid-state switching device permitting it to switch on a current to a phase winding of the motor only during a lowfrequency pulse.

In such an arrangement it will be appreci ated that there are two different operating modes, which stem from the impossibility of counting pulses when starting, because no pulses are generated when the rotor is stationary. In this case the periodic lowfrequency signal has a number of periods per revolution corresponding to the number of times the phase is excited in each revolution, and duration sufficient to ensure that at any point of the revolution at least one phase can be excited.

The high-frequency pulse train is required to have sufficient pulses per revolution to give the necessary fineness of control of the switching angles, in synchronism with the low-frequency pulse train.

Further features and details of the invention will be apparent from the following description of one specific embodiment that will be given by way of example with reference to the accompanying drawing, in which the single figure is a schematic block diagram of one form of controller for a variable reluctance motor system using a threephase motor with six stator poles and a twopole rotor.

The rotor carries a timing disc 10 formed with holes 11 co-operating with stationary sensors 12A, 12B and 12C each comprising a light source and a light-sensitive cell to constitute a position detector or transducer.

Each phase has a cycle of rising and falling inductance twice in each revolution and so must be switched on twice in each revolution (requiring two "holes" in the disc) and for starting must produce positive torque throughout a 600 angle of rotor movement. Thus each "hole" spans 600 and three detectors or transducers must be 60 apart in order that each phase shall conduct for 600 in sequence.

When the rotor is stationary in any position, it is necessary for starting to know which phase winding to energise. This information is obtained directly from the detectors which provide the source of "low frequency" signals.

Thus, for example, in the form which uses as many detectors as phases and as many "holes" ("holes" may be of course be slots, or reflecting areas, or iron teeth or other defined arcs of rotation depending on the type of transudcer employed) as there are periods of conduction of each phase in a revolution, the detector associated with one phase will be excited during the appropriate position of the rising inductance region of rotation of that phase, thus guaranteeing the required starting torque. As the rotor begins to move, there comes a position where it is required to energise the next phase in sequence and to turn off the initially excited phase.

This condition is indicated by the trailing edge of the "hole", which was allowing excitation of this phase, reaching the detector and causing loss of excitation, while the leading edge of the hole reaches the detector of the next phase and allows excitation of the following phase. (If the detectors are spaced differently, the excitation of the following phase may be initiated by the leading edge of another "hole").

This is the mode of control for zero and low speeds. (The mangnitude of the current may be limited by "chopping" and this is a way in which the torque may be controlled in this mode). At some higher speed a transisition is made from this mode (in which the signals from the position detectors directly control the switching of the phases) to one in which pulse counting of high frequency pulses takes over control.

Turning to the drawing it will be seen that a speed comparator 15 controls a logic switch 16 to connect the power switching circuit 20C of a particular phase (say phase C) either direct to the transducer 12C of the same phase C for the low speed mode, or to the output of a delay angle counter 21 triggered by the output of the transducer 12A of an earlier phase, (say phase A).

Thus at low speeds the current is switched on throughout the low frequency pulse of the transducer of phase C, lasting throughout an angle of 600 i.e. the whole or a substantial part of the angular range of increasing inductance. This may comprise a current controlled chopping mode in which to prevent the current from becoming excessive, the switching device is commutated when the current rises above a given value, and fired again when it falls to a given lower value.

For the high-speed mode of operation the delay angle counter 21 has a clock input supplied through a three-input NAND gate 22 with high frequency pulses from a phaselocked loop 23 synchronised with a lowfrequency signal, actually the start of a transducer pulse from the angle transducer 12A of any phase, shown as phase A. The counter 21 starts counting downwards from an initial count fed into it from a delay count store 24, under the control of a controller 40.

When it reaches zero count the output is fed to the logic switch 16 and thence to the power switching circuit 20C of phase C. It is also fed back to the second input of the NAND gate 22 to inhibit further clock pulses to the delay angle counter, and to a two input NOR gate 25 to enable the supply of clock pulses to the clock input of an ON angle counter 26. This also counts downwards from an initial count fed in from an ON count store 27 also under the control of the controller 40. When the ON angle counter reaches zero its output is fed to a reload input of the delay angle counter 21 causing it to reload to the same initial count as before. It is also fed to a "reset" monostable 28 forming the reset input of a latch 30 of which the other, "set", input is formed by a "set" monostable 29 fed from the angle transducer of phase A.The latch com- prises cross connected NAND gates 31 and 32 and its output is connected to the third input of the three input NAND gate 22 of the delay angle counter 21 as well as to the reload input of the "ON" angle counter 26.

Hence when the ON angle counter counts back to zero it reloads the delay angle counter 21 (thus cutting off the signal to the power switch 20C) and resets the latch, which reloads the ON angle counter 26 and inhibits the supply of clock pulses to the delay angle counter 21. The supply of clock pulses to the delay angle counter is only resumed when the position transducer 12A of phase A again sends a low-frequency pulse to the set monostable 29 to set the latch again.

Thus clock pulses can only be fed to the delay angle counter 21 when the latch 30 is set, and the latch is initially set at the start of a low-frequency pulse from the transducer 12A of a previous phase (specifi cally phase A) and after the end of such a pulse the latch remains set until the ON angle counter 26 returns to zero and triggers the rest monostable 28.

The operation of phases A and B is analogous to that of phase C. Thus in the start ing mode, the detector 12C controls the power switches 20C directly and similarly for phases "B" and "A" (the sequence being ABC). But above the speed at which a change is made to the "controlled" mode, the delay counter 21C is triggered from the detector 12A, that for "B" from the detector 12C and that for "A" from the detector 12B.

It will be appreciated that various forms of transducer may be employed, for example optical, direct or reflective as described, magnetic such as Hall effect or magneto resistive or by proximity switches. Moreover the transducer may either be operated by a special disc or from a suitably adapted fan or other rotating part such as the rotor body or an air baffle.

The signals synchronised with the rotor may take various forms and may either be of short duration identifying a specific angular point in the revolution (generally arising in a high-speed mode) or of longer duration distinguishing a range of angular positions, (as in a low speed or starting mode).

Thus a prolonged signal may comprise a DC square wave pulse or a train of a periodic or AC wave, while a short duration signal may comprise the start or finish of such a prolonged signal or may comprise a short pulse.

Moreover by suitable logic a short signal may be converted into a prolonged signal as by a bistable while the start and finish of a prolonged signal may provide short pulses (possibly performing different functions) as by differentiation.

Moreover the invention is applicable to other numbers of phases for example four.

Also the number of angle transducers may be reduced by employing overlapping angular ranges with a known logic system distinguishing between combinations such as both, neither, one or the other.

WHAT WE CLAIM IS: 1. A variable reluctance electric motor system including means for generating a low frequency signal synchronised with the motor rotor, means for generating a high frequency signal from the low-frequency signal at a frequency controlled by, varying with, and many times higher than, that of the lowfrequency signal, and means for counting cycles of the high frequency signal starting from a predetermined point of the lowfrequency cycle to provide one or more control signals starting and/or finishing at a predetermined angular position of the motor rotor and serving to control synchronisation of current pulses with angular position of the rotor.

2. A system as claimed in Claim 1 in which the control signals control conduction of a solid state switching device to switch on a current to a phase winding of the motor at a predetermined angular position of the rcrtor.

3. A system as claimed in Claim 1 or Claim 2 in which the control signals serve to com- mutate a solid state switching device to initiate the end of a pulse of current.

4. A system as claimed in any one of the preceding claims in which the timing of a control signal is adjustable manually or automatically by varying the count of the counter at which it is produced.

5. A system as claimed in any preceding claim in which the high frequency signal is produced by multiplication of the frequency of the low-frequency signal.

6. A system as claimed in any preceding claim, in which the high-frequency signal is produced by a phase locked loop i.e. a variable frequency oscillator with a phase comparator to adjust the oscillator frequency to keep it in phase with a multiple of the low frequency.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. two input NOR gate 25 to enable the supply of clock pulses to the clock input of an ON angle counter 26. This also counts downwards from an initial count fed in from an ON count store 27 also under the control of the controller 40. When the ON angle counter reaches zero its output is fed to a reload input of the delay angle counter 21 causing it to reload to the same initial count as before. It is also fed to a "reset" monostable 28 forming the reset input of a latch 30 of which the other, "set", input is formed by a "set" monostable 29 fed from the angle transducer of phase A.The latch com- prises cross connected NAND gates 31 and 32 and its output is connected to the third input of the three input NAND gate 22 of the delay angle counter 21 as well as to the reload input of the "ON" angle counter 26. Hence when the ON angle counter counts back to zero it reloads the delay angle counter 21 (thus cutting off the signal to the power switch 20C) and resets the latch, which reloads the ON angle counter 26 and inhibits the supply of clock pulses to the delay angle counter 21. The supply of clock pulses to the delay angle counter is only resumed when the position transducer 12A of phase A again sends a low-frequency pulse to the set monostable 29 to set the latch again. Thus clock pulses can only be fed to the delay angle counter 21 when the latch 30 is set, and the latch is initially set at the start of a low-frequency pulse from the transducer 12A of a previous phase (specifi cally phase A) and after the end of such a pulse the latch remains set until the ON angle counter 26 returns to zero and triggers the rest monostable 28. The operation of phases A and B is analogous to that of phase C. Thus in the start ing mode, the detector 12C controls the power switches 20C directly and similarly for phases "B" and "A" (the sequence being ABC). But above the speed at which a change is made to the "controlled" mode, the delay counter 21C is triggered from the detector 12A, that for "B" from the detector 12C and that for "A" from the detector 12B. It will be appreciated that various forms of transducer may be employed, for example optical, direct or reflective as described, magnetic such as Hall effect or magneto resistive or by proximity switches. Moreover the transducer may either be operated by a special disc or from a suitably adapted fan or other rotating part such as the rotor body or an air baffle. The signals synchronised with the rotor may take various forms and may either be of short duration identifying a specific angular point in the revolution (generally arising in a high-speed mode) or of longer duration distinguishing a range of angular positions, (as in a low speed or starting mode). Thus a prolonged signal may comprise a DC square wave pulse or a train of a periodic or AC wave, while a short duration signal may comprise the start or finish of such a prolonged signal or may comprise a short pulse. Moreover by suitable logic a short signal may be converted into a prolonged signal as by a bistable while the start and finish of a prolonged signal may provide short pulses (possibly performing different functions) as by differentiation. Moreover the invention is applicable to other numbers of phases for example four. Also the number of angle transducers may be reduced by employing overlapping angular ranges with a known logic system distinguishing between combinations such as both, neither, one or the other. WHAT WE CLAIM IS:

1. A variable reluctance electric motor system including means for generating a low frequency signal synchronised with the motor rotor, means for generating a high frequency signal from the low-frequency signal at a frequency controlled by, varying with, and many times higher than, that of the lowfrequency signal, and means for counting cycles of the high frequency signal starting from a predetermined point of the lowfrequency cycle to provide one or more control signals starting and/or finishing at a predetermined angular position of the motor rotor and serving to control synchronisation of current pulses with angular position of the rotor.

7. A system as claimed in any one of the

preceding claims including speed-responsive logic switching means having a high-speed condition in which the supply of current to the phase windings is under the control of the high-frequency signal and counter, and a low-speed position in which it can be switched on independently thereof.

8. A system as claimed in any one of the preceding claims in which the low frequency signal comprises at least one pulse per revolution for each of a number of phases, the duration of the pulses being such that every point of the revolution falls within a pulse of at least one phase when the inductance of the winding is rising at that point.

9. A system as claimed in Claims 7 and 8 in which3 with the logic switching means in the low-speed condition the low frequency signal directly controls conduction of a solidstate switching device permitting it to switch on a current to a phase winding of the motor only during a low-frequency pulse.

10. A variable reluctance electric motor system as specifically described herein with reference to the accompanying drawing.