An apparatus and method for starting and stopping an AC induction motor

Technical Field

The invention concerns an apparatus and method for controlling the starting and stopping of an AC induction motor.

Background of the Invention

An AC induction motor is a type of electric motor for powering driven equipment. They are robust, reliable, inexpensive and standardized throughout the world. Starting an AC induction motor requires a careful compromise between the cost of the motor starter apparatus, starting performance and the electric supply authorities' regulations.

A basic method of starting an electric motor is by closing a contactor to allow the motor to start at full voltage as a direct- on-line starter. Although it is a compact and inexpensive method, it is not the best method to use. Direct- on-line starting is marked by inrush current surges of six to eight times the motor's full load ampere value, resulting in electrical surge transients as well as mechanical strain on driven equipment. It results in a fast breakaway and acceleration up to full speed in an uncontrolled fashion.

The effect upon a hydraulic pump is mechanical stress applied on the rotating components followed by surges in the hydraulic system. This may include a high initial flow rate causing a vacuum to be drawn on the suction side, pump cavitations and pressure pulsations at the pump discharge. Similar effects are found with mechanical loads such as conveyers (driven by a motor) which when subjected to sudden jerks or severe applications of torque, may lead to load displacement, conveyor belt slippage or breakage.

Equally, when stopping the rate of deceleration is totally uncontrolled, this leads to further mechanical stress on pipelines for pumping applications, mountings and check valves from the inertia of the flowing fluids. It also produces pressure surges in the hydraulic circuit leading to pipeline rupture and leakage of product.

To maintain continuity and quality of the supply of electricity, electric supply utilities apply service rules and regulations that limit the kW size of motors that can be connected with the

direct-on-line method of starting and this may affect other electric power consumers.

For large motors, from 35kW up to 500OkW, the supply authorities may require the application of reduced inrush current starting, to limit the starting current surge to a lower value.

Although there are numerous variations of motor starting with a reduced voltage, the prior art auto-transformer Korndorfer circuit is a common methodology for starting a large motor. As it is a reduced voltage starter it that can provide maximum starting torque with minimal line current; this is due to the transformation ratio of the number of turns of the primary and secondary windings of the auto-transformer.

Other known non-electronic types of reduced voltage starters are the primary reactor and primary resistance starters. Both primary reactor and primary resistance starters have similar starting characteristics and disadvantages as the direct- on-line starting methodology, which causes transient current and torque peaks when changing from a reduced voltage to full line voltage.

The technical literature identifies the prior art auto-transformer Korndorfer circuit as a producer of fast rise-time voltage surges during a forced transition from the first starting stage. The information presented shows that the reported high-voltage stress failures are mainly for medium voltage motor starters, and occur during the motor starting period either with an overload fault such as a bearing failure or an operational error that prevents the motor from accelerating to full speed. When a forced transition occurs, the starting sequence is stopped by the protection relays in the control circuit and all switches are de-energized.

The testing program results reported in the technical literature for a forced transition during the motor starting period shows the extent of such high voltage surges. When autotransformer starters are forced to transition before they reach near full speed they generate high voltages on the 0% taps with respect to the line voltages. The 80% tap generates dangerously high voltages and the lowest voltage being relatively benign on the 50% tap.

The prior art does not identify why high- voltage surges are generated at a forced transition or how to reduce the risk of generating high-voltage surges. The only conclusion given is installing a means for reducing the amplitude of the voltage surge such as applying distribution metal oxide arrestors to prevent the buildup of dangerous voltage levels. The prior art does not identify the physical process that generates the high voltage surges during a forced transition. The arresters do not limit the multiple re-ignitions associated with vacuum circuit breakers. Properly sized C-R (capacitor-resistor) surge suppressors eliminate multiple re-ignitions and voltage escalation. Surge arresters limit the magnitude of the voltage surge but do not modify its rate of rise. Capacitors reduce the rate of rise, lower the surge impedance and may reduce the TRV (transient recovery voltage) sufficiently to prevent multiple re-ignitions.

From an inspection of the schematic circuit for the prior art Korndorfer reduced voltage starter, it can be seen that at the instant of transition, the motor current supplied from the secondary winding is transferred to the primary winding circuit. The action of the separation of the contacts of the "star" contactor causes the motor current to instantaneously change (180 electrical degrees) from a "secondary" winding supply to a "primary" winding supply, which is a very fast dv/dt event.

The amplitude of these fast rise-time (dv/dt) voltage surge at transition are of concern because each voltage peak may cause a microscopic "pinhole" failure or partial discharge in oxidizing the winding insulation material. Repeat starting of the motor adds to the partial breakdowns. Partial breakdowns gradually destroys the polymer insulation with corona discharges. This occurs when the air- filled voids in the insulation material is ionized by the rapid changes of the electric field between the winding turns. The insulation failure could be related to some polarization and/or space charge in addition to partial discharge development.

An overload condition at starting may mean the motor is not able to reach operational speed and the protection relays will trip the circuit. All switches will be de-energized and power is disconnected by a "forced transition". The high motor current surge and a step-up voltage transformer connection that generates the destructive winding flash-over or burn-out is an unplanned event. It is also dependant on the instantaneous angular position of the voltage

wave at which the star point switch contacts separate.

Random switching for the transition from the first starting stage means that with a step-up autotransformer connection of the prior art starter, a forced transition will eventually generate the destructive voltage surges. Therefore "controlled switching" is desirable and advantageous compared to random switching.

Summary of the Invention

In a first preferred aspect, there is provided an apparatus for controlling the starting and stopping of an AC induction motor, the apparatus comprising: a first magnetic permeable portion having first, second and third legs; a second magnetic permeable portion arranged relative to the first magnetic permeable portion such that a magnetic control flux of the second magnetic permeable portion has an opposite magnetic polarity to a magnetic control flux of the first magnetic permeable portion, the second magnetic permeable portion having fourth, fifth and sixth legs; a non-magnetic spacer separating the first and second magnetic permeable portions to prevent magnetic control flux cancellation between the first and second magnetic permeable portions; a first AC power phase winding wound around the first and fourth legs; a second AC power phase winding wound around the third and sixth legs; a first pair of DC excitation control windings mounted around the first magnetic permeable portion; and a second pair of DC excitation control windings mounted around the second magnetic permeable portion; wherein a magnetic power flux opposes the magnetic control flux in the first magnetic permeable portion and a magnetic power flux assists a magnetic control flux in the second magnetic permeable portion during a first half cycle, and a magnetic power flux assists a magnetic control flux in the first magnetic permeable portion and a magnetic power flux opposes a magnetic control flux in the second magnetic permeable portion during a second half cycle, such that the impedance of the AC power phase windings is changed by an equal and symmetrical variation of the density of the combined magnetic fluxes in each of the magnetic permeable portions during a complete power cycle to produce a balanced

voltage with a symmetrical waveform from each of the AC power phase windings.

The apparatus may further comprise a third AC power phase winding wound around the second the fifth legs.

The DC excitation control windings may all have the same number of turns.

Each magnetic permeable portion may comprise: an upper cross-bar yoke connecting an upper portion of each leg, and a lower cross-bar yoke connecting a lower portion of each leg.

Each AC power phase winding may have substantially equal magnetic saturation levels.

The AC power phase winding may be a single continuous winding with a plurality of voltage taps.

The AC power phase winding may be at least two separate coils, each coil having a plurality of voltage taps.

The apparatus may further comprise a central switch connected between selected voltage taps of the AC power phase winding and by opening the central switch, the lower coils are disconnected from the circuit and reduces stress on insulation of the AC power phase winding.

The apparatus may further comprise a third magnetic permeable portion, the third magnetic permeable portion omitting DC excitation control windings.

The third magnetic permeable portion may have an air gap and is isolated by non-magnetic spacers to prevent magnetic control flux cancellation between the first and second magnetic permeable portions.

The DC excitation control windings may be a plurality of control windings for inducing magnetic control fluxes in the magnetic permeable portions, and the flow of magnetic

control flux in the magnetic permeable portions have substantially equal flux densities and the magnetic flux flows in the first magnetic permeable portion in opposition to the second magnetic permeable portion.

The first pair of DC excitation control windings may be wound around the upper cross-bar yoke of the first magnetic permeable portion between the first and second legs and between the second and third legs, and the second pair of DC excitation control windings is wound around the upper cross-bar yoke of the second magnetic permeable portion between the fourth and fifth legs and between the fifth and sixth legs.

The first pair of DC excitation control windings may be connected in series and supplied with DC amperes, and the DC amperes induce a magnetic control flux to flow in an upwardly direction on the second leg, and in a downwardly direction on the first and third legs

The second pair of DC excitation control windings may be connected in series and supplied with DC amperes, and the DC amperes induce a magnetic control flux to flow in an downwardly direction on the fifth leg, and in a upwardly direction on the fourth and sixth legs

The apparatus may further comprise an input connection to the AC power phase windings for connection to a three phase AC power system; an output connection to a load from selected reduced voltage taps of the AC power phase windings; a common connection to each AC power phase winding for connection to a common circuit point; and an input connection to each pair of DC excitation control windings.

The apparatus may further comprise additional voltage taps on each AC power phase winding adjacent to a zero voltage tap; wherein a first additional voltage tap is above the zero voltage tap at 5% of the number of turns; and a second additional voltage tap and an auxiliary winding is connected below the zero

voltage tap at 5% of the number of turns.

The apparatus may further comprise additional voltage taps on each AC power phase winding adjacent to a 100% voltage tap; wherein a first additional voltage tap is below the 100% voltage tap at 95% of the number of turns; and a second additional voltage tap and an auxiliary winding is connected above the 100% voltage tap at 5% of the number of turns.

Additional reduced voltage values may be selected for a first starting stage as an autotransformer and the plurality of voltage taps are at 5% voltage increments.

The apparatus may be a single phase construction having a magnetic core assembly in a CI laminated form.

It is an advantage of at least one embodiment of the present invention to use impedance of a variable reactor to symmetrically ramp-up the available voltage to the motor terminals during the second starting stage and control deceleration of the driven load during a motor stop sequence.

The progressive increase of impedance of the present invention and also an improved magnetic core assembly symmetrically reduces the available voltage to the motor terminals. This allows a motor stop with a ramp-down from the motors full load speed in a controllable manner.

Another advantage of at least one embodiment of the present invention is that the continuously variable, control of reactor impedance values for the second step allows a wider range of motor kW sizes to be utilized for a given physical size of a variable reactor/auto-transformer apparatus allowing more versatility.

A further advantage of at least one embodiment of the present invention is having the first step as a reduced voltage auto-transformer starter that produces more torque-per-ampere of line current than any other type of reduced voltage starter and a second starting step as a

reduced voltage reactor apparatus that includes an improved means of DC control. The improved magnetic core assembly enables a smooth switch-over and acceleration for a variable reactor control connection.

One feature of the present invention is that pairs of DC control windings are connected in series opposition such that any AC induced in these DC control windings by the AC power phase windings cancels out and the resultant voltage across the excitation circuit is substantially zero.

Another feature of the invention is that the physical construction and electrical engineering requirement for manufacture does not depart from typical transformer construction techniques used by prior art auto-transformer starters. In other words, the construction may utilize a 3-coil set of windings, identified as a "wye (star point)" configuration with one set of coils per phase. This allows balanced load sharing between the three phases or utilizes 2-coil set of windings where the central phase does not include a coil set of windings. This results in a slight imbalance compared with the above mentioned 3-coil set construction. The 2-coil set of windings is identified as an "open-delta" configuration. There is no significant disadvantage to using a 2-coil autotransformer design since the starting currents will be approximately balanced in each phase.

It is an advantage of at least one embodiment of the present invention to provide a new apparatus and method that minimizes the production of such voltage surges by including a central switch circuit that disconnects redundant windings from the circuit at the transition for:

1. A 2- step autotransformer starter, a first starting step as a reduced voltage starting stage and the second step to full voltage at the motor terminals, and

2. A 3-step autotransformer starter, a first starting step as a reduced voltage starting stage and the second starting step as a variable reactor stage with the means of control of reactor impedance and a third step to full voltage at the motor terminals.

It is another advantage of at least one embodiment of the present invention that the central switch may be a 'controlled switch' as that term is understood by persons skilled in the art, where the separation of the switch contacts may be controlled for a pre-determined angular

position of the voltage waveform.

Another advantage of at least one embodiment of the present invention is to make available additional reduced voltage taps for the first starting stage to improved matching of the available motor torque to a given load, with allowance for any changes in system impedances and compensate for supply voltage sag during motor starting. With prior art starters, several sets of taps are usually available to the user to provide different values of reduced voltage (NEMA standards are 80%, 65%, and 50% of the full line voltage). The starter manufacturer can provide taps other than the NEMA preferred taps of 50%, 65%, and 80%. Common mistakes application engineers make is not allowing for changes in system impedance, changes motor starting torque requirements, lower motor starting torque due to manufacturing variation, changes in load torque requirements due to changes in temperature or leaking valves.

The 15% voltage increments between the tap setting of prior art starters is improved by the +5% and -5% increments of the present invention by selecting taps at the 100% voltage tap and a selection at the zero % volt tap. The nine reduced-voltage settings are available and permit an optimal adjustment of the motor starting voltage to a required minimum motor torque or starting current value.

Another advantage of at least one embodiment of the present invention is to provide a means of re-energizing of safety critical motors after a power interruption to a continuous process plant to minimize production losses. Random re-closure of power to motors that are still in rotation can generate high transient torques and currents that dynamically stress the motor insulation, the drive shaft, coupling and driven equipment. Such random re-application of out of phase voltages to a motor may be destructive. Adding stator resistance at the moment of re-closure reduces the peak torque by limiting the length of the electromagnetic re-closure transient. The increase of the variable reactor impedance will alter the stators time constant value and therefore reduces any shaft resonance transient.

For higher magnitudes of voltage sag, the drive speed may decay further, determined by the inertia and load characteristics, a time delay window for voltage recovery may be required for load shedding of non-essential drives and a time- staggered reacceleration schedule in

process manufacturing facilities. The viability of the re-starting sequence is aided by the motor starting characteristics of the invention, the ability of the first starting stage as a reduced voltage autotransformer that produces the highest starting torque with the least current demand is the most effective means of providing torque for the reacceleration of a drive, a second starting stage as a variable, primary reactor starter aids the voltage recovery of the power network by limiting motor current surge at the changeover to full voltage for the third step.

The application of a variable reactor for the second starting stage allows the reactor impedances to be set to suit the required second stage reduced voltage. This may reduce the need for an air gap in the magnetic core of the apparatus and a corresponding reduction in the no-load magnetizing current of the apparatus.

It is an advantage of at least one embodiment of the present invention to provide an apparatus and method of a reduced voltage starter with a means of reducing the risk of generating high -voltage surges during the switching sequences between starting stages by application of a central switching means, a smooth change-over from the first starting stage to a second reactor stage without forming any step-up transformer connection that may with random switching methods cause destructive flash-over during a "forced transition" of prior art reduced voltage motor starters.

Brief Description of the Drawings

An example of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a 2-coil variable reactor/auto-transformer apparatus for use in a polyphase induction motor starter according to a first embodiment of the invention;

Figure 2 is an electrical diagram of the apparatus of Figure 1;

Figure 3 is an exploded view of the magnetic core of Figure 1 ;

Figure 4 is an exploded view of the magnetic core of Figure 1 showing the direction of flow of the magnetic control flux;

Figure 5 is a perspective view of a 3-coil variable reactor/auto-transformer apparatus for use in a polyphase induction motor starter according to a second embodiment of the present invention;

Figure 6 is an electrical diagram of the apparatus of Figure 5;

Figure 7 is a schematic circuit diagram of the apparatus of Figure 5;

Figure 8 is a perspective view of a 3-coil variable reactor/auto-transformer apparatus for use in a polyphase induction motor starter according to a third embodiment of the present invention;

Figure 9 is an electrical diagram of the apparatus of Figure 8;

Figure 10 is a schematic circuit diagram of the apparatus of Figure 8;

Figure 11 is a perspective view of a 2-coil variable reactor/auto-transformer apparatus for use in a polyphase induction motor starter according to a fourth embodiment of the present invention;

Figure 12 is an electrical diagram of the apparatus of Figure 11;

Figure 13 is a schematic circuit diagram of the apparatus of Figure 11;

Figure 14 is an electrical diagram of a single continuous coil AC power phase winding assembly, with +5% and -5% voltage taps, according to a fifth embodiment of the present invention;

Figure 15 is a table of comparison of prior art voltage taps and the plurality of additional open circuit voltage taps of Figure 14;

Figure 16 is a perspective view of a multi- winding AC power phase assembly, with additional +5% and -5% voltage taps at 0% and 100% connections;

Figure 17 is a graph of the motor starting current, with a transition from the first starting stage to full voltage; and

Figure 18 is a schematic circuit diagram of a 2-coil auto transformer with central switching.

Detailed Description of the Drawings

Referring to Figure 1, a perspective view of a 2-coil, variable reactor/auto-transformer apparatus 10 according to a first embodiment is shown. The apparatus 10 is used in a motor starter (not shown) to supply starting current to a polyphase induction motor. The apparatus 10 may be used for starting, and stopping the motor with deceleration. The apparatus 10 generally comprises a magnetic core assembly having magnetic properties that is assembled from transformer core laminations arranged in an "EI" arrangement. In another embodiment, the apparatus 10 is a single phase construction having a magnetic core assembly in a "CI" laminated form. The apparatus 10 has at least two magnetically permeable portions or part cores 20, 40 made of a magnetic material having a magnetization curve with a pronounced

knee. The first and second part cores 20, 40 are separated by a non-magnetic spacer 30. The first part core 20 is arranged relative to the second part core 40 such that a magnetic control flux of the first part core 20 has an opposite magnetic polarity to a magnetic control flux of the second part core 40. This enables symmetrical control of the output waveform of the AC to the induction motor. The components of the magnetic core assembly are held in position by upper brackets 11 and lower brackets 12.

Each part core 20, 40 has three legs. The first part core 20 has two lateral legs 21, 23 and a central leg 22. The second part core 40 also has two lateral legs 41, 43 and a central leg 42. The lateral legs 21, 23, 41, 43 have equal cross sectional areas and have respective AC power phase windings 90, 100 of AC phase coils mounted on them. The AC power phase windings 90. 100 have voltage taps similar to prior art auto-transformers. The first AC power phase winding 90 is wound around the first pair of lateral legs 21, 41. The second AC power phase winding 100 is wound around the second pair of lateral legs 23, 43. The AC power phase windings 90, 100 are interlinked by the magnetic flux in the lateral legs 21, 23, 41, 43 they are mounted on.

A pair of DC excitation coils 70, 80 is mounted around the first part core 20. Another pair of DC excitation coils 50, 60 is mounted around the second part core 40. The pairs of DC excitation coils 50, 60, 70, 80 enable the magnetic control flux in each part core 20, 40 to be varied. The DC excitation coils 70, 80 of the first part core 20 are connected in series and have the same number of turns as each other. The DC excitation coils 50, 60 of the second part core 40 are connected in series and have the same number of turns as DC excitation coils 70, 80 of the first part core 20. The windings of the DC excitation coils 50, 60, 70, 80 are interlinked by the magnetic flux in the respective part core 20, 40 they are mounted around. Thus, the first part core 20 has a set of DC excitation coils 70, 80 and the second part core 40 has a set of DC excitation coils 50, 60. Each pair of DC excitation coils 50, 60, 70, 80 are connected with an opposite polarity.

The DC excitation coils 70, 80 of the first part core 20 and the DC excitation coils 50, 60 of the second part core 40 are connected to a DC source for inducing a magnetic control flux to flow in each of the part cores 20, 40 since they are closed magnetic circuits. The direction of flow of the magnetic control flux in the part cores 20, 40 are in opposite directions to each

other such that any AC induced in those DC excitation coils 50, 60, 70, 80 by the AC power phase windings 90, 100 cancels out and the resultant voltage across the excitation DC control circuit is substantially zero. A magnetic power flux opposes the magnetic control flux in the first part core 20 and a magnetic power flux assists a magnetic control flux in the second part core 40 during a first half cycle. A magnetic power flux assists a magnetic control flux in the first part core 20 and a magnetic power flux opposes a magnetic control flux in the second part core 40 during a second half cycle. This causes the impedance of the AC power phase windings 90, 100 to be changed by an equal and symmetrical variation of the density of the combined magnetic fluxes in each of the part cores 20, 40 during a complete power cycle to produce a balanced voltage with a symmetrical waveform from each of the AC power phase windings 90, 100.

Referring to Figure 2, an electrical diagram of the apparatus of Figure 1 is shown. An input connection 200 to each AC power phase winding 90, 100 is provided for connection to an AC power system. There is an output connection 210 to a load from selected voltage taps of the AC power phase winding 90, 100. A common point connection 220 to each AC power phase winding 90, 100 for connection to a common star point is provided. An input connection 230 to each pair of DC excitation coils 50, 60, 70, 80 mounted on the part cores 20, 40 is provided.

Figure 3 shows an exploded view of the magnetic core of Figure 1. The arrangement of the magnetic part cores 20, 40 have substantially equal magnetic saturation levels.

Figure 4 is an exploded view of the magnetic core of Figure 1 showing the direction of flow of the magnetic control flux. The direction of flow of the magnetic control flux through each of the part cores 20, 40 is indicated by the arrows. When the first part core 20 is excited by applying DC excitation, a magnetic control flux is induced that flows in an upwards direction along the central leg 22. The second part core 40 induces a magnetic control flux that flows in a downwards direction along its central leg 42 when DC excitation is applied. Since the non-magnetic spacer 30 is sandwiched between the part cores 20, 40, any magnetic coupling is reduced and cancellation of the two equal and opposite magnetic control fluxes is prevented.

The impedance of the AC power phase windings 90, 100 is dependant upon the magnetic permeability of the magnetic core assembly. When DC is applied to the DC excitation coils 50, 60, 70, 80, the magnetic control fluxes in each part core 20, 40 are varied. An increase in DC to the DC excitation coils 50, 60, 70, 80 results in a decrease in the impedance of the AC power phase windings 90, 100 because of the increase of core saturation. This reduces the permeability of the magnetic core assembly and the impedance of the AC power phase windings 90, 100.

The amplitude of the DC applied to the DC excitation coils 50, 60, 70, 80 varies in relation to a desired electrical parameter that may be provided by a microprocessor based motor protection relay controller. The controller monitors the motor phase current during each stage of starting the motor. The controller has a memory to store a first predetermined current value which is indicative of a started motor and is able to compare the current levels with a second predetermined current value after the motor has started for causing the motor starter to switch to subsequent second stage as a variable reactor after a predetermined transition time/current value. The controller compares the current levels with a third predetermined current value after the motor has reached near operating speed for causing the motor starter to switch to full supply voltage. The controller switches a digital output for the transition from the auto-transformer connection for the first starting period. The controller also controls an analog DC output signal for the manipulation of DC excitation of the part cores 20, 40 during the second starting period as a variable reactor unit. The motor controller manipulates the analog DC output signal for desired motor operating parameters for rate of change during starting and deceleration during a stop command.

The amplitude of the DC magnetic control flux saturates the part cores 20, 40. This in turn deeply controls the permeability of the magnetic core assembly and therefore the impedance of the AC power phase windings 90, 100.

The AC power phase windings 90, 100 and the DC control windings 50, 60, 70, 80 are disposed with respect to the magnetic core assembly so that the AC and DC induces an AC magnetic power flux and a DC magnetic control flux in each leg 21, 22, 23, 41, 42, 43 of the magnetic core assembly. These fluxes assist each other or oppose each other when the AC has a positive or negative value respectively.

The pairs of DC control windings 50, 60, 70, 80 are connected in series opposition such that any AC induced in those DC control windings 50, 60, 70, 80 by the AC power phase windings 90, 100 cancels out. Any resultant voltage across the excitation circuit is substantially zero.

Referring to Figure 5, a 3-coil, variable reactor/auto-transformer apparatus 110 in accordance with a second embodiment is shown. The apparatus 110 is used in a motor starter to supply starting current to a polyphase induction motor (not shown). The apparatus 110 may be used for starting the motor and stopping the motor with deceleration.

The apparatus 110 comprises a magnetic core assembly of transformer laminations of a magnetic material having a magnetization curve with a -pronounced knee. The apparatus 110 is assembled into a typical EI construction with at least two part cores 120, 140. The first 120 and second 140 part cores have three legs 121, 122, 123 and 141, 142, 143, respectively. The part cores 120, 140 are separated by a non-magnetic spacer 130. A pair of DC excitation coils 170, 180 is wound around the first part core 120. Another pair of DC excitation coils 150, 160 is wound around the second part core 140.

All three pairs of legs 121, 141, and 122, 142 and 123, 143 have a respective AC power phase winding 190, 191, 192. The three AC power phase windings 190, 191, 192 have voltage taps similar to prior art auto-transformers. A first AC power phase winding 190 is wound around the first lateral legs 121, 141. A second AC power phase winding 191 is wound around the central legs 122, 142. A third AC power phase winding 192 is wound around the third lateral legs 123, 143. The windings of the AC power phase windings 191, 192, 193 are interlinked by the magnetic flux in the legs 121, 122, 123, 141, 142, 143 they are mounted on.

Referring to Figure 6, an electrical diagram of the apparatus 110 of Figure 5 is shown. An input connection 300 to each AC power phase winding 190, 191, 192 is provided for connection to an AC power system. An output connection 310 to a load from selected voltage taps of the AC power phase winding 190, 191, 192 is provided. A common point connection 320 to each AC power phase winding 190, 191, 192 for connection to a common

star point is provided. An input connection 330 to each pair of DC excitation coils 150, 160, 170, 180 mounted on the part cores 120, 140 is provided.

The relationship of prior art % reduced voltage and corresponding % starting current values and torque values are shown in the table below and also on Figure 6:

Referring to Figure 7, a schematic circuit diagram of the apparatus 110 of Figure 5 is shown. The system 400 comprises a circuit that is similar to the prior art Korndorfer closed transition configuration. The system 400 has three (single coil) AC power phase windings 190, 191, 192 which are continuous (single). The system 400 also has a common point with a star switch 408, 409, 410. There is also a main switch 401, a bypass contactor 402 and motor overload relay 403. The motor overload relay 403 may be a thermal or an electronic relay. There are transformer contacts 404, 405, 406. The transformer contacts 404, 405, 406 may be a solid state type or electromechanical type.

Referring to Figures 8 to 10, a 3-coil, variable reactor/auto-transformer apparatus 510 in accordance with a third embodiment is shown. Turning to Figure 9, a schematic circuit diagram of the apparatus 510 of Figure 8 is shown. The system 500 has at least two separate coils for each AC power phase winding. The two coils are an upper coil 593, 594, 595 with a plurality of voltage taps, and a lower coil 596, 597, 598 of equal number of turns as the upper coil 593, 594, 595. There is a magnetic core assembly comprising at least two part cores 520, 540. A pair of DC excitation coils 570, 580 is wound around first part core 520. Another pair of DC excitation coils 550, 560 is wound around second part core 540. An input connection 300 to each upper coil 593, 594, 595 of AC power phase windings is provided for connection to an AC power system.

An output connection 310 to a load from selected voltage taps of the upper coil 593, 594, 595 of AC power phase windings and to centre switch terminals is provided. A common point connection 320 to each lower coil of AC power phase windings 596, 597, 598 for connection

to a common star point is provided. An input connection 330 to each pair of DC excitation coils 550, 560, 570, 580 mounted on the first and second part cores 520, 540 is provided. An output connection 315 to centre switch terminals is provided. The centre switch terminals enable switching the circuit from the auto-transformer connection of the first starting step to the variable reactor connection of the second starting stage. There is also a main switch 501, a bypass contactor 502 and motor overload relay 503. The motor overload relay 503 may be a thermal or an electronic relay. There are transformer contacts 505, 506, 507. The transformer contacts 505, 506, 507 may be a solid state type or electromechanical type. The system 500 also has a common point that is hard wired.

Referring to Figures 11 to 13, a perspective view of a 2-coil variable reactor/auto-transformer apparatus 610 according to a fourth embodiment is shown. The system 600 comprises a circuit that is similar to the prior art Korndorfer closed transition configuration, with two separate coils for each AC power phase winding 690A, 690B, 69 IA, 691B. The two coils are an upper coil 690A, 690B with a plurality of voltage taps, and a lower coil 69 IA, 69 IB of equal number of turns. There is centre switch terminal to enable switching the circuit from the auto-transformer connection of the first starting step to the variable reactor connection of the second starting stage. There is also a main switch 601, a bypass contactor 602 and motor overload relay 603. The motor overload relay 603 may be a thermal or an electronic relay. There are transformer contacts 605, 606, 607. The transformer contacts 605, 606, 607 may be a solid state type or electromechanical type. The system 600 also has a common point that is hard wired.

Referring to Figure 14, a single continuous winding, with a plurality of voltage taps at 80%, 65% and 50%. Additional taps are provided above and below the zero volt tap. The upper tap is made at + 5% winding connection, and an auxiliary 5% windings turns are added below the zero volts tap. A similar, pair of 5% voltage taps are provided at the 100% voltage tap and an auxiliary 5%.winding turns are added above the 100% volts tap. By selecting the appropriate taps the three (3) prior art open-circuit voltage taps can be varied for nine (9) reduced voltage settings of equal 5% voltage increments between adjacent taps.

Referring to Figures 15 and 16, a multi-winding phase assembly for connection with a central switching means incorporates a plurality of 5% voltage taps adjacent to the zero volt

tap and at the 100% volt tap. The connection links 894 are relocated at other taps to suit the connections for the central switch as shown in Figure 15.

Referring to Figure 17, the motor starting current 1 supplied by from the secondary winding of the autotransformer is graphically shown during the first starting stage and the rapid change over of the motor current to the supply from the primary winding of the autotransformer. The change from the supply from a secondary winding is through a 180 degree phase-change on to the primary supply 3. This is an extremely high dv/dt event 2 and is the source for the generation of destructive voltage surges with random switching means.

Referring to Figure 18, an electrical schematic circuit of the apparatus of Figure 13 is shown. A central switch 704 is connected at the 80% tap for utilizing a two-pole switching means. The disconnected windings comprising coils B, C and D are prevented from floating by remaining connected to the line voltage via the second phase connection at the common "star" point. The disconnected windings may also be clamped by the connection of surge arrester/suppression units 708, at the load side of the central switch 704.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the scope or spirit of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects illustrative and not restrictive.