Abstract: A speed reduction switch for a multiple speed induction motor includes a speed selector switch and a delay element. As the motor is switched from a high speed to a lower speed, the delay element temporarily prevents electrical connection of a motor auxiliary winding for low speed operation. The motor is therefore de-energized and slows down. After a preselected delay, the delay element connects the motor auxiliary winding when the motor is at a lower speed so that little magnetic braking of the motor occurs, and the motor smoothly transitions from a higher speed to a lower speed.

Abstract: Control device for a multiphase electric motor (M) comprising firstly two two-directional static switches (IS1, IS2) each associated with a terminal of a shunt contactor (S) installed in parallel, these switches (IS1, IS2, S) being controlled by a control circuit (C) controlling progressive operation of the motor, characterized in that the shunt contactor (S) is of the four-pole type and is fitted with solid connection parts (6, 7) each connecting two terminals of the said contactor, these connection parts (6, 7) being connected in pairs (L1, L3) to the terminals of the static switches (IS1, IS2) such that each (IS1 or IS2) is connected to two terminals.

Abstract: An electrical control system for an induction motor for achieving optimum efficiency at both full and partial loads at markedly reduced electrical components costs while maximizing reliability over the range of load, and for achieving a two step motor power output in a greatly simplified manner, the circuit having an electrical induction motor adapted for connection to line electrical power at maximum motor operating frequency, and to a second source of electrical power at approximately one half of the maximum operating frequency, wherein an electrical switch is provided for selectively connecting each power source to the motor, wherein the second source is a generator for producing a reduced frequency sine wave form.

Abstract: Method for controlling the speed of a single-phase asynchronous induction motor, the stator of which is equipped with a main winding (BP) and an auxiliary winding which consist of half-windings (B1, B2) which are magnetically coupled and wound in opposite directions, in which method the main winding is powered directly using an AC source and the auxiliary winding is powered using a current delivered by a full-wave rectifier and using two switching devices (T1, T2), so as to generate in the auxiliary winding a current which is phase-shifted by 90.degree.. The method consists in varying the motor torque by altering the time that at least one of the switches spends closed/open.

Abstract: The present application discloses a system and associated method for lowering heat dissipation created by running a motor. The system comprises a voltage regulator, the motor, and a controller. The voltage regulator is connected to a supply voltage and has an output voltage with an associated output current dependent upon impedance of the system. The motor, which may be AC or DC, is driven by the output voltage and an output current at a current running speed. The controller controls the supply of the output voltage and associated output current to the motor toward varying the current running speed of the motor while minimizing the power dissipated throughout the system.

Abstract: A control circuit for an electric motor having start, low speed and high speed windings reactivates the start winding at different reduced motor speeds depending upon whether the motor is running on the low speed winding or the high speed winding. Reactivation of the start winding while the motor is running on the low speed winding automatically deactivates the low speed winding and activates the high speed winding.

Abstract: A multi-speed switching device particularly suitable for providing multi-speed starts for a motor, especially a two-speed motor, is provided with a high-reliability solid state switch for controlling a phased start winding of the motor, and an inexpensive switch for accommodating a lesser number of starts at an auxiliary speed. The high-reliability switch is a triac switched off during every motor start as the motor comes up to an operating speed. The inexpensive switch is an electromechanical relay that switches between a main run winding and an auxiliary speed run winding, and which is energized by voltage appearing across the solid state switch when it is switched off.

Abstract: A motor controller for a single phase induction motor (SPIM), wherein the SPIM is driven by a square wave above or below rated frequencies. The square wave may be shaped by introducing one or more notches to eliminate or suppress undesirable harmonics, to reduce the amplitude of the fundamental, to provide desirable voltage control, or to provide desirable voltage to frequency control. The control topology may include switches, main winding taps and switchable capacitors to accommodate selection between line and square wave driving, temporary capacitance increases, or other advantages.

Abstract: A winding circuit for use with different input signals includes a first main winding, a second main winding, and an auxiliary branch having an auxiliary winding permanently and serially connected to a capacitor. The windings and auxiliary branch are alternatively configurable in one of a first configuration or a second configuration for use with respective first and second input voltages. When in the first configuration, the first main winding, the second main winding, and the auxiliary branch are connected in parallel. The input voltage signal then may be applied across the first main winding. When in the second configuration, the first and second main windings are serially connected, and the auxiliary branch is connected in parallel with one of the first and second main windings. The input voltage signal then may be applied across the first main winding and the second main winding.

Abstract: A method and circuit design for establishing the operating status of a direct drive, multiple speed motor. More specifically, the invention furnishes a method for providing feedback to an air distribution system indicating first whether the system motor is operating and second which speed winding has been selected. The circuit design senses the voltage differences between the speed windings of the motor and also between the first speed motor winding and the power supply. The voltage difference sensed by the circuit will determine whether a light emitting diode (LED) illuminates. The illumination of the LED indicates to the system controller whether the motor is operating and the speed winding that has been selected, thus providing the necessary feedback to the air distribution system controller.

Abstract: The invention concerns an improved system for windshield wipers in vehicles. An induction motor is used, in which speed is accurately controlled by controlling frequency of power supplied to the motor. The induction motor may be of the consequent-pole type, which ordinarily does not produce sufficient torque. Addition of (a) auxiliary tooth slotting and (b) skew has raised the torque produced to acceptable levels.The invention includes a control system, which senses excess load on the motor, which occurs as the windshield dries, and reduces motor speed, subject to a minimum, in response.

Abstract: A controller is provided for use with electric motors which will achieve a two mode motor operation. In this operation, a first mode is utilized having a torque configuration thus more efficiently providing high torque outputs from the motor. However, a second configuration is also used which more efficiently accommodates high speed operations albeit at the expense of some output torque. The system accomplishes this two mode operation in a low cost manner so as to minimize the overall cost of products or devices utilizing this power control system.

Abstract: Control circuitry for a multi-speed, rotational, direct current motor can include a means to select one of a plurality of discrete circuits to select a desired speed of the motor, each circuit being activated by a separate analog switch which can complete the circuit to select a potentiometer voltage which then becomes a speed reference voltage output; a motor encoder which generates a pulse train proportional to motor speed and is converted to a proportional voltage by means of a frequency-to-voltage converter; an error amplifier, connected to the outputs of the speed reference voltage and the motor speed voltage such that when the speed reference voltage differs from the motor speed voltage, an error signal is produced; a pulse width modifier circuit which is driven by the error signal and which adjusts motor logic outputs so as to adjust the speed of the motor to produce an essentially zero error; a power supply circuit to feed voltage to the controlling circuitry and motor.

Abstract: A fractional-power electric motor is provided to generate varying amounts of output torque supplied to a driven member over a wide range of RPM speeds so as to reduce power consumption on a battery-powered source. The electric motor includes a rotor and a stator assembly. The stator assembly is disposed around the rotor and is inductively coupled to the rotor. The stator assembly further includes a plurality of poles, each having a main winding and at least one fractional-power winding. The main winding and associated at least one fractional-power winding on each of the stator poles are wound so that the magnetic fluxes generated therefrom are added. Power switching circuits are provided for independently energizing the main and fractional-power windings on each of the stator poles. A controller is used to control selectively the frequency and duty cycle energization of the power switching circuits to vary the rotational speed of the rotor and the amounts of torque supplied to the driven member.

Abstract: A method and device are disclosed for speed control of an AC induction motor by means of a control circuit which generate a motor drive voltage consisting of a combination of AC and time-spaced DC pulses. During the occurrence of the time-spaced DC pulses in the motor's AC drive voltage a back EMF suppression network is activated to absorb the motor's generated back EMF energy. A further feature of the invention is a slow down mode where the motor is decelerated from continuous full cycle AC to a requested slow speed setting by means of a higher contend of DC pulses in the AC drive voltage.

Abstract: A method and apparatus for controlling an induction motor having at least high speed windings and reduced speed windings divides the reduced speed windings into first and second portions. The first and second portions of the reduced speed windings are electrically connected in series and a voltage is applied to the first and second portions of the reduced speed windings when it is desired to operate the induction motor at a reduced speed. The first and second portions of the reduced speed windings are electrically disconnected and a voltage is applied to the high speed windings when it is desired to operate the induction motor at a high speed. The induction motor has three sets of windings extending from a middle region, each set of windings having an inner winding toward the middle region and an outer winding away from the middle region.

Abstract: An apparatus for controlling the speed of an induction motor includes a first winding and a second winding. The first winding and second winding are adapted to be coupled to an AC source for supplying an AC input signal. A first switching device is coupled to the first winding and a second switching device is coupled to the second winding. Each of the switching devices is operative in a low impedance state enabling current to flow through the associated winding of each switching device and a high impedance state preventing significant current flow through the associated winding of each switching device. A controller switches each of the switching devices from the high impedance state to the low impedance state in a sequence for controlling the current in the windings, which induces a phase shift between the AC voltages across the windings.

Abstract: To control the output from an induction motor with a main winding and an auxiliary winding with multiple taps, varying output stages of the motor are adjusted by the activation of the auxiliary winding or one of its part-windings. According to the invention, the coarse adjustment of the power to be output by the motor is effected by the selection of a certain output stage and a repeating digital m-bit keying pattern generated simultaneously for the purpose of fine adjustment, so that the motor is driven alternately at the set output stage and the next higher or next lower output stage as a function of the sequential logic values of the keying pattern. By means of the process complying with the invention, precise power control is possible which can be used to advantage for the adjustment of output.

Abstract: A control circuit for a two speed electric motor includes high and low speed electronic switches. The high speed electronic switch is biased off when the low speed electronic switch is biased on. The low speed run winding of the motor is activatable only through the low speed electronic switch. The high speed run winding of the motor is activatable either directly through a high speed setting of a speed selector switch or through the high speed electronic switch in a low speed setting of the speed selector switch. Upon activation of the motor start winding, the low speed electronic switch is off and the high speed electronic switch is on.

Abstract: A method and apparatus used with a motor controller for determining transient inductance and rotor resistance upon commissioning of the motor. The estimator drives a single phase of the motor with an excitation current at an operating frequency, determines the d-q axis stator synchronous feedback voltages, divides the excitation current by 2 to produce a positive current signal, multiplies the operating frequency by the positive current signal to produce a current frequency signal and then divides the q-axis voltage feedback by the current frequency signal to produce the transient inductance estimate, divides the d-axis voltage feedback signal by the positive current signal and subtracts stator resistance estimate therefrom to obtain a rotor resistance estimate.

Abstract: Dynamic operation of a brushless N-phase DC motor results in a high torque constant (Kt) line-to-line phase configuration during spindle motor power-up, and in a lower effective Kt configuration once normal run speed operation is attained. When the spindle motor is rotating below a predetermined run speed, drive transistors coupled to the motor phases cause at least two phases to be series-coupled between the source of operating voltage and ground. This ensures a large initial torque because the Kt contribution of each series-coupled phase is combined vectorially. Once run speed is achieved, effective Kt is lowered by causing the drive transistors to energize but a single phase, reconfiguring the spindle motor for line-to-neutral operation. So doing reduces the effective back electromagnetic force, permitting sufficient run current from a given magnitude operating voltage source.

Abstract: The improved motor drive unit includes a zero-crossing voltage detector circuit composed of components (7) to (12) for detecting whether the voltage from an AC power supply (1) is zero-crossing and a switching circuit for performing on-off control by means of a thyristor 27 so as to switch between energization and de-energization of the AC power supply to a motor (2) and controls the speed of the motor by changing the ratio between the times of energization and de-energization of the AC power supply to the motor. The start of energization coincides with the zero-crossing of the AC power supply voltage whereas the timing of end of energization coincides with the zero-crossing of an energization current by means of the thyristor and wherein the energization time is fixed at a value either equal to or twice the power supply period whereas the de-energization time is varied in units that are integral multiples of one half the power supply period.

Abstract: A method and apparatus to be used with a motor controller for determining the speed of the motor rotor using current zero crossing times of the stator winding currents. An error signal set, consisting of phase angle errors between consecutive zero crossings over a sampling period, is generated and analyzed in a region of interest in either the time or frequency domain, producing a signal frequency, the signal frequency being the frequency of the motor rotor.

Abstract: A motor control system for a multi-speed electric motor electrically excited by a power source having at least two terminals. The motor controller system includes a switching device electrically connected to the terminals of the power source for alternating current flow from the power source, a first speed circuit, and a second speed circuit. The first speed circuit includes a first speed primary winding electrically connected to the switching device, a first speed secondary winding electrically connected to the first speed primary winding, a start capacitor electrically connected to the first speed secondary winding, and start relay electrically connected to the start capacitor. The second speed circuit includes a second speed primary winding which is connected to the switching device.

Abstract: The present invention provides a control apparatus that stably controls a mechanical system incorporating a variable speed motor by suppressing mechanical vibrations without requiring the use of a motor speed detector. In accordance with the present invention, a power converter driving the motor is controlled based on a result fed from a weighting adder that calculates a weighted sum of the mechanical system's state variables, which are estimated by a first state observer to which detected current and voltage values are fed, and a target current value or a target voltage value. Alternatively, the power converter is controlled based on a result fed from a weighting adder that calculates a weighted sum of the state variables, which are estimated by a second state observer to which detected current value and a target voltage value are fed, and a target current value or a target voltage value.

Abstract: An apparatus for controlling the supply of power to a variable-speed induction motor converts an AC power source voltage to a motor supply voltage at a designated frequency, with the amplitude of the motor supply voltage being controlled in accordance with the motor load such that the degree of slip conforms to a predetermined optimum slip charactistic, whereby high efficiency and stability are maintained over a wide range of load values. The motor supply voltage amplitude is detected to provide an indication of the motor load, an optimum value of an operating parameter of the motor which varies in accordance with degree of slip is derived based on the designated frequency and the detected voltage amplitude, using a predetermined function corresponding to that frequency, and the motor supply voltage amplitude is controlled such as to reduce an amount of difference between the optimum value of the operating parameter and a detected value of that parameter.

Abstract: An AC motor is provided with a motor housing and a photoelectric sensor associated with the housing. The sensor is part of a motor control circuit which is thereby responsive to the ambient light condition to control the motor by selectively switching At on and off, or alternatively by selectively switching between higher and lower speeds. The control circuit derives its power from the motor's AC input terminals, thereby yielding a self-contained motor control unit, which requires no timer unit and no wiring or mounting by the user. In accordance with another aspect of the present invention, the photoelectric sensor extends through an aperture in a motor housing end cap with the associated control circuitry fitable within the end cap, thereby providing a retrofit for existing motors.

Abstract: A control method and apparatus for actuating an induction motor which is capable of generating a sufficiently large output in a wide speed region. In such an induction motor, each of Y-connection phase windings of the induction motor is divided into two split windings 5t and 3t at a predetermined split ratio (for example, 5:3). When a rotational speed of the induction motor is in a low speed region, only the changeover switch MC2 is turned on to supply exciting current to both of the split windings 5t and 3t. When the rotational speed of the induction motor is in a middle speed region, only the changeover switch MC3 is turned on to exclusively supply exciting current to the split windings 5t. Furthermore, when the rotational speed of the induction motor is in a high speed region, only the changeover switch MC1 is turned on to exclusively supply exciting current to the split windings 3t.

Abstract: A blender is provided with a microprocessor for controlling energization of the blender drive motor, and a plurality of manually actuated switches for controlling the stepping of the microprocessor through a program which determines the mode and pattern of energization of the drive motor. There are N speed range selection switches, M speed selection switches and a pulse mode switch. Depending on the sequence in which these switches are actuated, the microprocessor controls the drive motor in a manual continuous mode at any one of N.times.M speeds, a manual pulse mode at any one of N.times.M speeds or an autopulse mode with any one of three patterns of energization. In the first pattern of energization the motor is energized to run at one of M constant speeds during intervals T1 which are separated by intervals T2 during which the motor is not energized.

Abstract: A method for control of the speed of an asynchronous electric motor by powering the motor with a single-phase sine wave voltage for its running at rated speed, and supplying it with at least one first partialized voltage formed from the sine wave voltage for a half-period or half cycle thereof, for running the motor at speed reduced to a fraction of its rated speed. A device for applying the method including a first power supply applied to the motor with a sine wave voltage for running at rated speed and a second means power supply applied to the motor with at least one first partialized voltage formed from the sine wave voltage for a half-period of the sine wave every uneven number ns of half-periods thereof for running at a motor speed reduced to a fraction of rated speed. Control apparatus sends activation signals alternately to the first and second power supplies upon reception of control signals requiring running at rated speed or reduced speed.

Abstract: A spindle motor for a machine tool is required to operate at a low rotational speed and to deliver a high output in a machining mode, and is required to operate at a high rotational speed and to deliver a high output in another machining mode. An AC motor employing a winding arrangement of the present invention meets those operating conditions for different machining modes, in that the coils (X1, X2; X1', X2') of each winding for a phase are divided into a plurality of sets of coils, and the plurality of sets of coils are held in a slot (SL). The connection of terminals (U1, U2; V1, V2; W1, W2) for phase dividing the coils into the sets is changed selectively by switching unit (SW, SW') such as an electromagnetic relay, to thereby supply power selectively to the coils for the output regulation.

Abstract: An ac spindle motor (10) for driving a spindle motor of a machine tool provided with a rotor (14) having an output shaft (12) connected to the spindle, and a stator (16) surrounding the rotor and having a laminated core (20) serving as a yoke, and stator winding assemblies of three phases (U-phase, V-phase, W-phase). The stator winding assembly of each phase (18u, 18v, 18w) has: windings (C.sub.2, C.sub.4) of a first group which are energized for a rotation of the spindle in both a low-speed range not higher than a given speed, and a high-speed range above the given speed; windings (C.sub.1, C.sub.3) of a second group energized only for a rotation of the spindle in the low-speed range; and electrical contact means (S1, S2, S3, S4) controlled so that the windings (C.sub.1, C.sub.2, C.sub.3, C.sub.4) of the first and second groups are energized to generate a first multipolar revolving magnetic field for the rotation of the spindle in the low-speed range, and so that only the windings (C.sub.2, C.sub.

Abstract: The invention relates to a motor output changeover control apparatus used for the spindle motor of machine tools. Conventionally, since a constant output over a wide range is realized by gears, a spindle motor is constituted by a single winding. The amount of heat produced by the winding is great, and a speed-change mechanism employing the gears is large in size.With the motor output changeover control apparatus of the invention, output characteristics over a wide range from high to low speeds is stabilized by changing over the connection of the power lines of an AC motor.

Abstract: A controller for a dual speed induction motor selectively applies operating power to the low speed motor terminal or the high speed motor terminal according to the open circuit/closed circuit condition of a low temperature thermostat and a high temperature thermostat. For low speed operation, operating power is conditionally applied to the low speed motor terminal through the low temperature thermostat and through the normally closed circuit of a single pole, double throw switch. For high speed operation, operating power is conditionally applied to the high speed motor terminal through the high temperature thermostat and the normally open circuit of the single pole, double throw switch. The operating position of the single pole, double throw switch is determined by a solenoid which is connected in series circuit relation with the high temperature thermostat.

Abstract: The speed of an induction motor is reduced by initially using dynamic electrical braking to slow the motor to a speed at which it will not remain synchronized to the A.C. supply frequency. At this point, A.C. cycle skipping is employed to apply current to the motor at an effective frequency which is a fundamental frequency component of the A.C. supply line frequency. The motor becomes synchronized at this fundamental frequency component. If desired, the motor can be brought to a complete stop by discontinuing the cycle skipping and using the dynamic braking again for a given period of time. This technique provides a very controlled stopping of the motor enabling equipment driven by the motor to be accurately positioned.

Abstract: A capacitor motor and method is disclosed which utilizes one or more tap points within the auxiliary winding of the motor which can be selectively connectd to a source of AC power via a selector switch for multi-speed operation of the motor.

Abstract: Lead wires are extended from phase coils of a head control motor, so that their effective reactances may be varied, and reactance selection circuits are additionally provided to switch values of the reactances of the phase coils to a low value. A vector position is sequentially delivered to phase current generators by a processor for designating the vector position, so that a head is displaced to a desired position. Thereafter, the reactances of the phase coils are switched to a high value by the reactance selection circuits, and then the vector position corresponding to the normal one is applied to the phase current generators, so that the head is so controlled as to be positioned at its normal position on a track on a disc and is maintained at this position. During the time of displacement of the head across the tracks, it is ensured that the head control motor is driven at a high response speed.

Abstract: The present invention discloses a speed control apparatus for an AC motor which vector-controls, for example, a drive current of the AC motor such as an induction motor, synchronous motor and AC commutator motor with a feedback current control loop providing an inverter utilizing a semiconductor element for power amplification such as power transistor and compensates for non-linear current characteristic of the drive current generated by dead time resulting from ignition delay of a semiconductor element with a current compensation control element provided within the current control loop, thereby controlling the rotation speed and command response speed of an AC motor.

Abstract: A capacitor motor and method is disclosed which utilizes one or more tap points within the auxiliary winding of the motor which can be selectively connected to a source of AC power via a selector switch for multi-speed operation of the motor.

Abstract: In a method and apparatus for regulating the rotational speed of a motor having a rotor with magnetic poles and a stator with at least two coils, wherein during a first operational mode the sinusoidal voltage included in a first of the coils is digitized to produce first and second streams of rectangular pulses, the first pulse stream being compared with a reference frequency pulse signal as regards width and phase to generate accelerating or braking driving pulses supplied to the second coil, the second pulse stream being monitored to determine the rotational speed of the rotor and a second operational mode being instituted whenever the rotor speed falls below a predetermined value, in said second operational mode the rotor speed being accelerated through the application of driving pulses alternately to the first and second coils, the improvement wherein, following the switchover from the self starting mode to the nominial running mode of operation, during a transitional period for a specified time interval,

Abstract: Circuit arrangements to improve the performance of motor speed control circuits using electronic switching on an AC waveform where it is desired to remotely control the electronic switch without either load current or the load waveforms being present at the controlling element. The voltage across the switching device, during the time it is off, is rectified, and used as the charging voltage for the phase firing circuit, with a resistor capacitor timing network determining the firing of the switching device when the capacitor voltage reaches a predetermined value.

Abstract: A capacitor motor and method is disclosed which utilizes one or more tap points within the auxiliary winding of the motor which can be selectively connected to a source of AC power via a selector switch for multi-speed operation of the motor.

Abstract: A variable speed 3-phase a-c motor is connected to a 3-phase source of alternating voltage via five controllable bidirectional solid state switches which are so arranged and controlled that motor speed can be reduced by skipping selected cycles of the source voltage. For example, for half speed operation three of the switches are arranged respectively to connect phases A, B and C of the source to phases A, C and B of the motor, and over two consecutive periods of the source voltage three separate, uniformly spaced "firing windows" are selected during which the conducting states of these switches are initiated in a predetermined pattern that results in the phase A-to-phase B source voltage being applied to the motor during the first window, the phase C-to-phase A source voltage being applied during the next window, and the phase B-to-phase C source voltage being applied during the last window.

Abstract: A dynamoelectric machine has only a pair of line terminals adapted for selective connection across a single phase power source and is operable to be started in a split-phase mode, to run at a predetermined low speed in a single phase mode, and to run at a predetermined high speed generally in a two phase mode.

Abstract: A two-speed single phase motor of the capacitor-start type is disclosed wherein the motor is switched from the start condition to the run condition by means of a single contact centrifugally actuated switch. The motor comprises high speed and low speed main windings and high speed and low speed start windings wound on the stator core and a double throw selector switch for connecting the high speed main and start windings to the terminal or the low speed main and start windings to the terminals when the motor is operated in the high speed and low speed configurations, respectively. A single pole single throw centrifugally actuated switch connected to the rotor has a single contact pair connected in series with the start capacitor and one of the terminals, and the switch contacts open when the rotor exceeds a predetermined rotational speed to disconnect the start capacitor from the circuit. The same centrifugal switch contact pair is used in both the high speed and low speed configurations.

Abstract: The drive circuit operates a shaded pole motor having in the disclosed embodiment three windings of turns N, N/2 and N/2. The control is basically two-speed control with the lower speed operation being adjustable by means of the drive circuit and with the high speed operation being directly from the AC line. The drive circuit includes a clock, frequency adjustment means, power inverters and unilateral means for limiting power consumption.

Abstract: A speed control apparatus includes a plurality of switches disposed between a three phase AC power source and a motor, a low speed control device for feeding power having a frequency lower than the frequency of the power source to the motor under the turn-on control of the switches, and a phase shifting device for shifting the phase order for changing from high speed operation to low speed operation of the motor.

Abstract: A polyphase motor is disclosed in which each coil of the stator winding of, for example, a squirrel cage induction motor is connected in series with a capacitor so that the capacitor, together with the input voltage, causes the stator core to periodically switch from a nonsaturated to a saturated condition and vice-versa so that the flux density is maintained at a uniformly high level. A control winding is provided to generate magnetic flux in the stator to vary the volt-second capacity of the magnetic material of the stator in accordance with varying line, load or other selected conditions. The current in the control winding can be externally controlled or may be provided by a feedback winding wound on the stator.