CA1063167A - Control circuit for variable reluctance motor - Google Patents

Abstract

CONTROL CIRCUIT FOR VARIABLE RELUCTANCE MOTOR

ABSTRACT OF THE DISCLOSURE
A control circuit for a variable reluctance motor comprises a solid state switching device having its output circuit connected in series with the electrical winding of the motor. A circuit is coupled to the motor winding and to the control electrode of the solid state switching device and senses the EMF induced in the motor winding resulting from residual magnetism while the motor is rotating and the switching device output is nonconductive. The switching device output circuit is rendered conductive when the EMF
attains a predetermined level. The motor acts as its own position sensor.

Description

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This invention relates to a control circuit for a variable reluctance motor. ~lore particularly, it relates to a control circuit which utiliæes an EMF induced in the winding of a variable reluctance motor to provide information concerning the position of the rotor to permit ampere-turns to be supplied to the motor winding during intervals o~
increasing magnetic permeance in its magnetic circuit, thereby to produce motoring torque and continuous motor rotation.
This application is a division of our copending ;~
Canadian application Serial No. 236,054 ~iled September 22, 1975. -Variable reluctance motors are well-known in the prior art and various exemplary designs of such motoxs may be seen in U.S. Patents 3,700,943 to Heintz; 3,700,944 to Heintz et al; 3,714,533 to Unnewehr; and 3,401,288 to French.
These brushless motors employ an exciting winding and a magnetic circuit to produce mechanical torque which is sub-stantially proportional to the sguare of the winding ampere-turns and to the time rate of change of permeance (reciprocal of reluctance), which is a function of the displacement of the rotor in the motor. Typically, these motors employ a stator containing a motox winding and a rotor containing ~erro-magnetic elements spaced from one another~ Displace-ment of the rotor relative to the stator produces a variation in reluctance, and, hence, permeance of the magnetic circuit of the motor winding. Of course, displacement of the rotor -~relati~e to the stator also produces a variation in the self--inductance of the motor winding, this self-inductance being directly related to the permeance of the magnetic circuit.
The torque or force produced by a variable reluctance ~;~
motor is proportional to the product of the square of the

- 2 -:,, ~ ' ' ' ~1)63167 winding ampere-turns and the rate of change of permeance as a function of rotor displacement. From t:he preceding, it is apparent that motor torque or force that is positive with respect to some arbitrary reference can only be developed when winding ampere-turns are sustained cluring an interval in which the permeance increases with rotor displacement.
Conversely, negative motor torque or force is developed when ;~
winding ampere-turns are sustained during an interval in which the permeance decreases with rotor displacement. Thus, in order to secure continuous rotation of the variable reluctance motor, it is necessary to apply ampere-turns to the motor :, .
winding ~uring intervals of increasing permeance and to decrease or eliminate such ampere-turns during intervals of decreasing permeance.
From the above discussion, it is apparent that the winding of a variable reluctance motor must be excited `'': ' from a time varying source. Furthermore, the time varia~
tions of the source must be synchroniz~d with the mechanical rotation of the machine rotor so that winding current is supplied to the motor during intervals in which the permeance increases with displacement and so that such current is inter-rupted during the intervals in which the permeance is decreasing with displacement. When a time-invariant source of electrical energy, such as a direct current source, is used, a controller is required to produce synchronized pulsations of winding ampere-turns.
Control circuits for variable reluctance motors in the past have utilized an external position sensor to determine the onset of each of the intervals of increasing magne$ic permeance. In U.S. Patent 3,673,476 to D.X.Hamburg a signal produsing apparatus for use with a three-phase variable reluctance motor is described in detail. French

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~ 0~31~7 Patent 3,401,288 mentioned above, also discloses a position sensing apparatus for a variable reluctance motor. V.S.
Patents 3,321,685 to Johannes and 3,466,519 to Platnick also .
disclose position sensing apparatus for use with motors, ~.
although the motors are not of the variable reluctance type.
In accordance with one aspect of the present inven~
tion, there is provided a control circuit for a variable ¦
reluctance motor having an electrical winding and a magnetic ~1 circuit which retains residual magnetism in the absence of current flow through the motor winding, the control circuit comprising in combination: a solid state switching device having an output circuit and a control electrode, the output circuit being coupled to the motor winding to control the flow of current therethrough; and circuit means, coupled to the motor winding and to the control electrode of the switching device, for sensing the EMF induced in the motor winding while the rotor is rotating and the switching devLce output circuit is non-conductive, the EMF resulting from the ~:
: presence of the residual magnetism, and for rendering the switching device output circuit in a state of conductivity . causing current to flow through the motor winding when the EMF reaches a predetermined potential.
In accordance with a second aspect of the present invention, there is provided a control circuit for a variable : reluctance motor having an electrical winding, a rotor and a magnetic circuit the inductance of which varies cyclically as the rotor is rotated, the control circuit comprising in combinat1on: controllable switching means for coupling the motor winding directly across a DC source of elec~rical ~0 energy; means for sensing the EMF induced in the motor winding in the absence of current flow therethrough via the switching means; and means for actuating the switching means to permit current to be supplied to the motor winding through ~he

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~ 631~7 switching means when the EMF reaches a predetermined potential.
The invention is described further, by way of illustration, with reference to the accompanying drawings, in which:
Figure 1 is a schematic electrical diagram of a control circuit for a variable reluctance motor; and Figure 2 consists of four time-varying waveforms pertinent to the circuit of Figure l; the waveforms are identified by the letters "a" through "d" and hereinafter ::
are referred to as waveform 2(a), 2~b), etc.
With reference now to the drawinys and in particular ~ .
to Figure 1, there is shown a variable reluctance motor winding 10 having an internal resistance 12. The terminals 14 and 16 of the motor winding are coupled to a control circuit for the variable reluctance motor, this control circuit being generally designated by the numeral 18. The control circuit 18 controls the application of electrical energy ~rom a DC source 20, which may be a conventional twelve-volt storage battery or the equivalent, to the motor winding 10. The positive lead 22 of the DC source is con-nected through a switch 24 to a positive voltage supply lead 26 of the control circuit.l8. The negative lead 28 from the DC source 20 is connected to ground at 30 and forms the negative voltage supply lead for the control circuit 18 and the motor winding 10. -The control circuit 18 is particularly intended, and has its component values and circuit element types selected, for use in conjunction with a variable reluctance motor pump of the kind described in U.S. Patent No. 3,932,069 in the names of D. Giardini and L. E. Ennewehr entit].ed 'Variable ~eluctance Motor Pump" and assigned to Ford Motor ~063167 Company. Of course, the control circuit of the invention may be used with other variable reluctance motors of single ~;
or multi-phase configuration and various modifications of the circuit 18 may be made to accommodate the exigencies of the applications.
In the specific circuit shown in Figure 1, a pair of identical transistors 32 and 34 have their emitter-collector output circuits connected in parallel with one another and coupled to the motor winding 10 to contro:L the flow of current from the positive supply lead 18, through the motor winding 10, to the ground lead 28. Thus, the parallel-connected emitter-collector output circuits of the transiRtors 32 and 34 are connected in series with the motor winding 10.
A resistor 36 is connected in parallel with the output cir-cuits of the transistors 32 and 34 and also is connected in series with the motor winding 10. A bias current flows con~
tinuously through the resistor 36 and through the motor winding - 10 to the ground lead 28 as long as the switch 24 is closed.
The internal resistance 12 of the motor winding 10 may, for example, be about one-half ohm and, therefore, a current of approximately one-half ampere flows through the motor winding 10 with the resistance 36 being of the value indicated and with a twelve-volt DC source 20. Of course, when the emitter-collector output circuits of the transistors 32 and 34 are conductive, the resistor 36 is shunted and the current through the motor winding 10 i5 considerably larger and of a magnitude ' ' . ' determined by the impedance of the motor winding 10.
With the output circuits of the transistors 32 and 34 nonconductive and with the rotor of the variable reluc-tance motor rotating, the terminal 14 of the winding 10 has a time ~arying ~oltage with a DC component. If the bias current ' 1C~63~67 ~
~owing through the winding 10 is one-half ampere, terminal 14 has a DC voltage component of about 250 millivolts with an AC waveform in the millivolt range superimposed upon it as a result of an EMF induced in the motor winding 10 during rotation of the motor rotor with the bias current flowing through the winding 10. The use of the bias current to induce ~he EMF is used in the invention of parent applica-tion ~erial No. 236,054 referred to above. Figure 2(b) depicts this induced alternating E~ which is superimposed on the reference level of about 250 millivolts above ground potential.
It should he noted that the transistors 32 and 34 supply unidirectional current to the motor winding 10 and, therefore, the magnetic circuit of the variable reluctance motor associated with this winding possesses resiaual magnetism when the transistors are nonconductive. This -occurs even in the absence of the bias current feed resistor 36. Thus, even if the bias current were not present, an EMF
would be induced in the motor winding 10 while the rotor of the motor is rotating with the transistors 32 and 34 non- :: ?
conductive and the sensing of the EMF induced thereby is used in the invention of this application. ~owever, the :-presence of the bias current feed resistor 36 adds to the EMF induced in the motor winding 10 when the transistor :
output circuits are nonconductive. .
The control circuit of the invention senses the induced millivoltage at the motor terminal 14, when the output circuits of the transistors 32 and 34 are nonconduc~
tive, to provide position sensing information with respect to the rotor of the variable reluctance motor so that the output circuits of the transistors 32 and 34 may be ~63~L~;7 rendered conductive to directly couple the motor winding 10 to the positive voltage supply lead 26 during intervals of increasing permeance in the magnetic circuit o~ the variable reluctance motor. Waveform 2(a) illustrates the time variation of the inductance of a variable reluctance motor of the type illustrated in U.S. P,atent No. 3,932,069 previously mentioned. The magnetic circuit inductance L
and permeance varies cyclically as the rotor of the variable ``
reluctance motor rotates. Between times tl and t2, the permeance of the motor magnetic circuit is increasin~. It is desirable during this interval to render the emitter-collector output circuits of the transistors 32 and 34 con-ductive to supply ampere-turns to the motor winding 10 to produce motoring torque. During the time interval between times t2 and t4l the permeance of the motox's magnetic circuit is decreasing, and it is desirable during this - '~
interval to prevent`current flow through the motor winding 10 by rendering the transistors 32 and 34 nonconductive.
In waveform 2(b) it may be noted that the reference level (DC voltage component level~ on the motor winding ter-minal 14 is crossed in a positive-going direction by the alternating voltage waveform superimposed thereon at time tl which corresponds to the onset of the interval of in-creasing magnetic permeance in the magnetic circuit of the~
variable reluctance motor. The control circuit 18 detects this crossing of the reference level and renders the output clrcuits of the transistors 32 and 34 conductive for a pre-determined length of time.
Waveform 2(c) shows the actual voltage across the terminals 14 and 16 of the motor winding. It shou}d be understood that the millivoltage waveform 2(b) is super-.;.~ ~ . . : ' ' ''`' ~0631~;~

imposed on the voltage waveEorm 2(c). The portion of wave-form 2(c) between times tl and t2 has a magnitude equal to i that of the DC source 20 less the voltage drop across the ~ ~
emitter-collector output circuits of transistors 32 and ~ ;
34. At time t2 when the transistors 32 and 34 are turned off, there is a sharp negative transient followed by a small decaying voltage oscillation across the motor winding 10. In the interval between times to and tl and the corres-ponding interval between times t3 and t4, the only voltage ; lO signal across the terminals 14 and 16 of the motor winding is the induced EMF millivoltage signal of waveform 2(b~
This permits detection of the positive-going zero-crossing points, at times tl and t~, at which the alternating induced EMF signal crosses the reference millivoltage level.
The base or control electrodes 38 and 40, respec- ;?
tively of the transistors 32 and 34 control the conduction of their emitter-collector output circuits. The base of '! the transistor 32 is connected through a resistor 42 to a junction 44. Similarly, the base 40 of the transistor 34 is connected through a resistor 46 to the junction 44. The resistors 42 and 46 tend to equalize the emitter-base currents of the transistors 32 and 34. The base-collector . :
junct~ons of the transistors 32 and 34 are protected against negative transients on the motor terminal 14 by a network . .
including resistors 48 and 50 connected, respectively, to the bases 38 and 40 of the transistors 32 and 34. The junction of the resistors 48 and 50 is connected to the cathode of a zener diode 52 whose anode is connected to -the junction formed between the collectors of transistors 32 and 34 and motor winding terminal 14.
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1063~7 Junction 44 is connected to the emitter of a transistor 54 whose collector is connected through a current limiting resistor 56 to the ground lead 28. Transistor 54 provides current amplification and when its emitter-collector output circuit is conductive, the emitter-collector output circuits of the transistors 32 and 34 also are conductive.
Conduction of the transistor 54 is controlled by the portion of the control circuit 18 not yet described. This circuitry includes a potentiometer 58 whose resistance is connected between the voltage supply leads 26 and 28. The movable arm 60 of the potentiometer is connected through a resistor 62 to the negative input 64 of a comparator 66.
A resistor 6a is connected between the negative input 64 and the ground lead 28. Potentiometer 58 and resistors 62 and 68 form voltage dividers that produce a millivoltage on the negative input 64 which is substantially equal to the millivoltage reference level of waveform 2(b). In other words, the vol~age dividers produce a voltage on the negative input 64 which balances out the DC component of the bias current ~0 Er~F waveform 2(b) on terminal 14 of the motor winding 10.
The comparator 66, as well as the other comparators in the circuit have an internal output transistor which is rendered nonconductive when the positive input to the com~
parator is more positive than its negative input and which is rendered conductive when the positive input is more nega-tive than its negative input. The output transistor of this and the other comparators is connected through a pull-up resistor to the plus voltage supply lead 26. Thus, the output lead 70 from the comparator 66 is connected to a junction 72 o~ a voltage divider that includes a resistor 74, connected to the supply lead 26 and to the junction 72, and a potentio-~063167 ~ ~
meter 76, connected between the junction 72 and the ground voltage supply lead 28. This voltage divider acts as a pull-up circuit for the output of the comparator 66 and provides a reference potential at the junction 72 when the internal output transistor in the comparator 66 is nonconductive. A
feedback resistor 78 is connected between the output 70 of the comparator 66 and its positive input 80. The potentio-meter 76 has a movable arm 77 on which a voltage, less than the voltage at junction 72, appears.
The positive input 82 o~ a comparator 84 is connec-ted to the junction 72. The negative input 86 of the com-parator 84 is connected to the positive input of a comparator 88. The negative input to the comparator 84 and the positive input to the comparator 88 are connected by a lead 90 to the anode o~ a blocking diode 92 whose cathode is connected to the junction 72. The anode of the diode 92 also is connected to -the junction 94 of an RC timing circuit includiny a variable ;~
resistor 96 connected in series with a fixed resistor g8 and a timing capacitor 100. The series combination of circuit elements 96, 98 and 100 is positioned between voltage supply ~;
leads 26 and 28. The output 102 of the comparator 88 i5 `
connected through a currant limiting resistor 104 to the base or control electrode 106 of the transistor 54. Thus, the signal on the output lead 102 of the comparator 88 controls the conductivity of the transistor 54 which, in turn, controls ~
the transistors 32 and 34 as previously described. A pull-up ~ -resisto~ 108 for the comparator 88 is connected between the voltage supply lead 26 and the base electrode o~ transistor 54.
The output lead 110 of the comparator 84 is connec-ted through a pull-up resistor 112 to the voltage supply lead ~6316~7 26 and to the base or control electrode of a current ampli-fying transistor 114. The collector of the transistor 114 is connected to the voltage supply lead 26 and its emitter is connected through a current limiting resistor 116 to the ground lead 28. The emitter of the transistor 114 also is connected through the series combination of a resistor 118 and a blocking diode 120 to the positive input lead 80 of the comparator 66. The blocking diode 120 ls polarized in the circuit so that when the transistor 114 is conductive in its collector-emitter output circuit, its emitter potential, which then is substantially equal to the potential o~ posi-tive voltage supply lead 26, is applied to the positive input of the comparator 66. A capacitor 122, which provides noise filtering, has one of its terminals connected to the junction ;~
formed between the resistor 118 and the anode of the blocking diode 120 and has its other terminal connected to the ground voltage supply lead 28.
The arm 77 of the potentiometer 76 is connected to the negative input of the comparator 88 to provide a reference potential on this comparator input. This potential is less than the potential at junction 72. Also, terminal 14 of the motor winding 10 is connected through a current limiting ~ ;
resistor 124 to the positive input 80 of the comparator 66.
Thus, the bias current EMF induced in the motor winding 10 during motor rotation is supplied to the positive input of comparator 66.
In order to understand the operation o~ the con-trol circuit 18, let it be assumed that the variable reluc-tance motor is operating at its maximum speed. The wave-forms in Figure 2 depict conditions as they exist at maximum motor speed. At time to~ a bias current is flowing through lOG3167 the resistor 36 and the motor w;.nding 10. The bias- :
current EMF induced in the motor winding is sensed at :
terminal 14 and is supplied throush the resistor 124 to the pos~tive input 80 of the comparator 66. This induced EMF
is at its negative peak value with respect to the reference level indicated in Figure 2~b). At this time, the positive input to the comparato.r 66 is more negative than the poten-tial applied to the negative input of the comparator 66, which potential balances the DC component or reference level ~ .
of the EMF induced in the motor winding. Therefore, the out- :
put lead 70 of the comparator 66 is at a very low voltage ~:~
level corresponding to the saturation voltage of the internal ~;
output transistor in the comparator 66. As a result of this aondition in the output of the comparator 66, the capacitor 100 will have been discharged to ground potential through the circuit path including the diode 92, the output lead 70 of the comparator 66, and the internal transistor in the . .-comparator 66. Waveform 2(d) depicts the output voltage on :
the lead 70 of the comparator 66. This output voltage is very near ground potential between the times to and tl. -As the motor continues to rotate, the induced E~
signal on motor winding terminal 14 increases from its nega- .. .
tive peak and crosses the reference voltage level indicated . .
in waveform 2~b) at time tl. At this reference level crossing :
point, the positive input 80 of the~comparator 66 becomes . : , more positive than its negative input and the internal output transistor in the comparator 66 becomes nonconductive. As a result, the output lead 70 of the comparator 66 attains the voltage level established at junction 72 by the voltage divider comprising series-connected resistances 74 and 76.

. . , ~631~7 This voltage level is maintained at the OlltpUt of the com- :
parator 66 for the interval between times tl and t3. Feed-back resistor 78 enhances the switching action of the com-parator 66.
At time tl when the comparator 66 switches to cause the voltage at junction 72 to rise, the capacitor 100 having been discharged, acts as a short circuit to ground potential so that the junction 94 is at ground potential.
Thus, the blocking diode 92 is reverse-biased and the ground potential at junction 94 is applied via the lead 90 to the negative input 86 of the comparator 84. Since the positive input 82 of the comparator 84 is at the increased potential at ~unction 72, the internal output transistor of the com-parator 84 is nonconductive so that the base electrode 110 of the transistor 114 is pulled up to the potential on voltage supply lead 26 through pull-up resistor 112. This provides the base-emitter drive current for the transistor . 114 and it is rendered conductive In turn, the emitter of the transistor 114 rises to the potential on voltage supply lead 26 and this is supplied through the resistor 118 and the diode 120 to the positive input 80 of the comparator 66 latching the comparator 66 in its high output voltage con-dition.
Between times tl and t2, the capacitor 100 is charged from the voltage supply lead 26 through the series-connected timing resistances 96 and 98. As the capacitor 100 charges, between times tl and t2, the output of the comparator 88 is a low (nearly ground) potential and the transistor 54 is conductive maintaining parallel transistors ~ :
32 and 34 conductive and supplying motoring current to the winding 10 of the variable reluctance motor. At this time, :. .

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the voltage across the motor winding is at the positive maximum voltage level indicated in waveform 2(c~ between ~;
times tl and t2-As the capacitor 100 continues to charge, the voltage at junction 94 reaches the reference level established by the potentiometer arm 77 connected to the negative input of the comparator 88. When the voltage at junction 94 exceeds this potentiometer reference voltage by a few milli-volts, the positive input to the comparator 88, which i5 ~'~
io supplied with the junction 94 voltage via lead 90, becomes more positive than the potential on its negative input and ~ ;
the internal output transistor of comparator 88 becomes non-conductive. This occurs at time t2. The voltage on the base 106 of the transistor 54 is pulled up to the potential o voltage supply lead 26 and transistor 54 becomes nonconductive ... .
rendering the output circuits of transistors 32 and 34 non~
conductive. Thus, except for the bias current 10wing through the motor winding 10 via resistor 36, current through ~
the motor winding is cut off producing the negative voltage ~, spike indicated in waveform 2(c).
The capacitor 100 continues to charge until the ;
voltage at junction 94 exceeds the voltage at junction 72, which occurs at time t3. At this time, the voltage at ~
junction 94 causes the negative input 86 of the comparator ~ ~;
84 to be more positive than the junction 72 voltage applied to its positive input 82, and, therefore, the output of the co~parator 84 becomes a low potential rendering the transistor 114 nonconductive. With the transistor 114 nonconductive, ~
the only voltage signal applied to the positive input 80 of ;
the comparator 66 is that obtained via the motor winding terminal 14 and resistor 124, that is, the induced motor 1~63167 EMF signal. At time t3, this induced EMF siynal is at its negative peak resulting in the output of comparator 66 going to its low saturation potential. This again provides a dis-charge path for the capacitor 100 through the internal out-put transistor of compara-tor 66.
In connection with the preceding discussion, it should be no~ed that the output voltage of the comparator 84 substantially follows waveform 2(d), but has a different maximum voltage level. Also, the output of the comparator 88 is the electrical complement of the motor voltage wave-form 2(c), except that the transients shown therein are not present on the output of comparator 88. Moreover, it should be noted that the waveforms in Figure 2 have a requency which is directly proportional to the speed of the controlled motor. The interval between times tl and t2 for waveform 2(c) is constant and not a function of motor speed because this time interval is determined b~ the timing circuit in-cluding resistances 96 and 98 and capacitor 100. At low motor speeds, the interval of increasing permeance in the magnetic circuit of the variable reluctance motor is much longer than the timing established by the timing circuit.
Therefore, at low speeds, maximum motor current flows only for a portion of the interval of increasing permeance. As motor speed increases, the maximum current flows for an increasingly greater portion of the interval of increasing magnetic permeance until the entire interval is occupied establishing the maximum speed limit of the motor and its control circuit. Current supplied to the motor winding 10 during an~ portion of an interval of decreasing magnetic permeance tends to brake the motor establishing its upper speed limit. Of course, the maximum motor speed may be varied ~al63167 . by changing the time constant of the RC t.iminy circuit in-cluding resis~ances 96 and 98 and capacitor 100. However, a practical limit on motor speed is established by the reactive impedance of the motor winding 10 and its associated magnetic circuit.
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Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A control circuit for a variable reluctance motor having an electrical winding and a magnetic circuit which retains residual magnetism in the absence of current flow through said motor winding, said control circuit comprising in combination:
a solid state switching device having an output circuit and a control electrode, said output circuit being coupled to said motor winding to control the flow of current therethrough; and circuit means, coupled to said motor winding and to said control electrode of said switching device, for sensing the EMF induced in said motor winding while said rotor is rotating and said switching device output circuit is non-conductive, said EMF resulting from the presence of said residual magnetism, and for rendering said switching device output circuit in a state of conductivity causing current to flow through said motor winding when said EMF reaches a predetermined potential.

2. The control circuit of claim 1 wherein said EMF
alternates above and below a reference potential and wherein said circuit means renders said switching device output circuit in said state of conductivity to cause current to flow through said motor winding when said EMF crosses said reference potential.

3. A control circuit for a variable reluctance motor having an electrical winding, a rotor and a magnetic circuit the inductance of which varies cyclically as said rotor is rotated, said control circuit comprising in combination:

controllable switching means for coupling said motor winding directly across a DC source of electrical energy;
means for sensing the EMF induced in said motor winding in the absence of current flow therethrough via said switching means; and means for actuating said switching means to permit current to be supplied to said motor winding through said switching means when said EMF reaches a predetermined potential.

4. The control circuit of claim 3 which further in-cludes means for supplying a bias current to said motor winding when said switching means is nonconductive, said EMF induced in said motor winding prior to actuation of said switching means resulting at least in part from said bias current flowing through said motor winding.

5. The control circuit of claim 4 wherein said pre-determined EMF potential corresponds to the onset of in-creasing inductance in the magnetic circuit of said variable reluctance motor.