Abstract:
A method for shorting rotor windings in a wound rotor induction machine is disclosed. The method includes the steps of monitoring rotor current for a frequency indicative of a desired steady-state operating condition and electronically shorting the rotor windings when the monitored rotor current frequency reaches a defined threshold indicative of the desired steady-state operating condition.

Description:
BACKGROUND OF INVENTION 
     This invention relates generally to rotor winding shorting on a wound rotor type induction machine and, more particularly, to methods and apparatus employing electronic circuits to short-circuit the rotor winding. 
     In a wound rotor motor, to provide a balanced simple winding which will result in low reactance and good performance characteristics, the rotor slots will be multiples of the poles and the phases. With the stator arranged the same way, the result gives rise to permeance locking torques at standstill or zero motor speed. Typically, to initially rotate the rotor, a high resistance is inserted into a rotor circuit to produce torque and limit current. As the angular speed of the rotor increases, the resistance is decreased. Typically, the external rotor resistance circuit is electrically connected to the rotor winding via collector (slip) rings and brushes. 
     As the speed approaches rated values, the windings are shorted so that a sufficient magnetic field can be induced into the rotor windings from the stator winding to produce the required torque. However, supplying a short circuit to the rotor windings through the collector rings and brushes is inefficient because of the brush wear caused by a friction between the rings and brushes. Additionally, since most brushes are carbon based, carbon dust typically accumulates in the motor from the brush wear. 
     Originally, shorting the collector rings was done by manual operations, such as, for example, a knife switch across the collector rings supply circuit shorts across the rotor windings, and then the brushes are manually lifted from the rings. This type of solution is not desirable, for safety purposes, when applied to high voltage, high horsepower machines. 
     Other methods use a motor driven plate with shorting studs which were moved into place to short the collector rings using a worm gear and an electrically driven brush lifting gear, or alternatively a plate with shorting studs is hydraulically driven into place to short the slip rings. 
     It is desirable to employ a method of shorting out the windings of the rotor which did not employ moving parts, thereby, enhancing the safety and reliability of such an operation. It is also desirable that such a method operates independent of the slip rings and brushes used in known machines. 
     SUMMARY OF INVENTION 
     The present invention, in one aspect, is a method for shorting rotor windings in a wound rotor induction machine, the method including monitoring rotor current for a frequency indicative of a desired steady-state operating condition and electronically shorting the rotor windings when the monitored rotor current frequency reaches a defined threshold indicative of the desired steady-state operating condition. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is an electrical diagram of a wound rotor including a shorting circuit. 
     FIG. 2 is a top level schematic view of a shorting circuit. 
     FIG. 3 is a detailed schematic view of the shorting circuit shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is an electrical diagram of a wound rotor  10  including a shorting circuit for a wound rotor electrical machine according to one embodiment of the present invention. Referring specifically to FIG. 1, mounted along a rotor shaft  12 , are a plurality of collector rings  14  and a rotor circuit  16 . In electrical contact with collector rings  14  are a plurality of brushes  18 , which, in the embodiment shown in FIG. 1, provide electrical contact between the collector rings  14  and a network of resistors  20  connected at a common point  22 . 
     Rotor circuit  16  is also in electrical contact with collector rings  14 . Collector rings  14  provide for electrical contact with rotor windings  24 . In the embodiment shown, rotor windings  24  illustrate a three-phase winding. As shown in FIG. 1, each rotor winding  24  includes a first contact  26  and a second contact  28 . First contacts  26  of windings  24  are each electrically connected to one of collector rings  14 . Second contacts  28  of windings  24  are electrically connected together at a common node  30  thereby configuring the windings  24  into what is commonly known in the art as a Wye or Y configuration. 
     As explained previously, if collector rings  14  are shorted at stand-still in a wound rotor induction motor, permeance locking can occur when power is applied to a stator (not shown), and, therefore, rotor shaft  12  may not rotate. To initiate rotor shaft  12  rotation, resistors  20  are switched into rotor circuit  16  via collector rings  14  and brushes  18  to generate torque to rotate shaft  12 . The torque is caused by the magnetic field from a plurality of stator windings (not shown) being induced into rotor windings  24 . As the speed of rotor shaft  12  increases, sections of resistors  20  are removed from rotor circuit  16 . To reduce brush  18  and collector ring  14  wear and reduce the amount of dust within the motor, a mechanism (not shown) is used to position collector rings  14  such that brushes  18  do not contact collector rings  14 . 
     If resistors  20  are variable in resistance, the resistance in the circuit can be varied to control the speed of the motor. To operate the motor at a constant speed, resistors  20  are removed from rotor circuit  16  and rotor windings  24  are shorted. Known methods of shorting rotor windings  24  mechanically through collector rings  14  present safety issues due to the higher rotor currents in larger motors which exceed 1000 amperes. 
     Rotor circuit  16  includes an electronic circuit used to short rotor windings  24  when rotor shaft  12  rotates at rated speed. The circuit includes a switching circuit  32  and a plurality of gated silicon controlled rectifier circuits  34  arranged in parallel. Gated silicon controlled rectifier circuits  34 , when turned on, tend to act as electrical shorts. Since the motor is an alternating current machine, gated silicon controlled rectifier circuits  34  are configured as a double three phase bridge because of the reversing alternating current in the rotor. Each phase of the bridge includes two silicon controlled rectifiers electrically connected. The silicon controlled rectifiers are connected so that there is a short circuit for both directions of the alternating current. Gated silicon controlled rectifier circuits  34  are enabled, or turned on, by switching circuit  32 . 
     In an exemplary embodiment, switching circuit  32  is configured to recognize a frequency of the rotor current. As rotor shaft  12  gains speed upon application of power to the stator and applied resistance to rotor windings  24 , the rotor current frequency decreases. The rotor frequency decreases since the rotating magnetic fields of the stator windings are cutting through rotor windings  24  at a lower rate due to rotor  12  accelerating to the speed of the revolving magnetic field. At rated speed for certain motors, the rotor frequency is as low as or less than one Hertz. 
     Typical motors have a rotor current frequency of less than 0.5 Hertz. Switching circuit  32  is configured to recognize rotor frequency, as stated above, and further configured to provide a signal to turn on gated silicon controlled rectifier circuits  34  to short rotor windings  24 . Shorting of rotor windings  24  results in rotor  12  rotating at a constant speed near synchronous speed. 
     In the embodiment shown in FIG. 1, rotor current frequency is used to select a motor operating speed using switching circuit  32  and gated silicon controlled rectifier circuits  34 . Although the embodiment described uses gated silicon controlled rectifiers as the electronic mechanism for shorting rotor windings  24 , other electronic circuit choices are contemplated, such as, for example, but not limited to, diodes, rectifiers and power transistors. 
     FIG. 2 is a top level schematic view of shorting circuit  36  including a wound rotor motor  38 , a rectifier circuit  40 , a commutator circuit  42 , a shorting sub-circuit  44 , a resistor circuit  46 , and a power source circuit  48 . Resistor circuit  46  provides an external resistance to start motor  38 . Once motor  38  attains rated speed, the external resistance is removed and rectifier circuit  40  and shorting sub-circuit  44  are utilized to short a plurality of rotor windings (not shown in FIG. 2) of motor  38 . Power source circuit  48  provides power to commutator circuit  42  and shorting sub-circuit  44 . In an exemplary embodiment, power source circuit  48  controls commutator circuit  42  and shorting sub-circuit. 
     FIG. 3 is a detailed schematic view of the shorting circuit shown in FIG.  2 . Wound rotor motor  38  includes a plurality of stator windings  50  and a plurality of rotor windings  52 . Rectifier circuit  40  includes a plurality of silicon controlled rectifiers  54 , a first third-phase silicon controlled rectifier  56 , and a second third-phase silicon controlled rectifier  58 . Commutator circuit  42  includes a plurality of resistors  60  forming a commutator resistor group  62 . Commutator circuit  42  further includes a first gating circuit  64  connected to a gated silicon controlled rectifier  66  and a current monitor  68 . Shorting sub-circuit  44  includes a resistor  60 , a capacitor  70 , and a second gating circuit  72  connected to a gated silicon controlled rectifier  74  and a current sensor  76 . Resistor circuit  46  includes a plurality of resistors  60  and a plurality of slip rings  78 . More specifically, resistor circuit  46  includes a first resistor group  80  and a second resistor group  82 . Resistor circuit  46  further includes a plurality of connectors  84  arranged to form a first connector group  86 , a second connector group  88 , and a third connector group  90 . Power source circuit  48  includes a diode bridge  92  and a first three-phase winding  94 . A second three-phase winding  96  is connected to a stationary circuit (not shown). 
     During a start up of motor  38 , first resistor group  80  is electrically connected to slip rings  78  by closing first connector group  86 . Slip rings  78  are electrically connected to rotor windings  52 . When stator windings  50  are energized, a voltage is induced within rotor windings  52  and a slip of 1 (one) allows electrical current to flow through resistors  60  of first resistor group  80  thereby rotating a rotor shaft (not shown) having a rated speed. When the rotor shaft approaches approximately 50% of the rated speed, second connector group  88  is closed to allow current flow through second resistor group  82 . Since second resistor group  82  is connected in parallel to first resistor group  80 , current flow through rotor windings  52  is increased enabling the rotor to obtain a speed approximately equal to the rated speed. When the rotor approximates the rated speed third connector group  90  is closed to short rotor windings  52 . Accordingly, the rotor attains rated speed. For simplicity, the embodiment shown includes three groupings ( 80 ,  82 , and  90 ). However, it is contemplated that the benefits accrue to circuits having more than three groupings. In one embodiment, there are between four and seven resistance groupings. 
     Since the rotor is at rated speed, the slip is between 0.5% and 1.5% and, therefore, the frequency of the current through the rotor windings  52  is less than approximately 1.0 Hertz. Current sensor  76  is configured to provide a signal to second gating circuit  72  when the rotor current frequency falls to approximately 1.0 Hertz. Second gating circuit  72  then gates gated silicon controlled rectifier  74  going backwards to the plurality of silicon controlled rectifiers  54 . Accordingly, silicon controlled rectifiers  54 ,  56 ,  58 , and gated silicon controlled rectifier  74  constitute a full wave diode bridge which takes three conductors (not shown) from slip rings  78  and rectifies the conductors to a DC voltage and, thus, shorting rotor windings  52 . Prior to being gated, gated silicon controlled rectifier  74  was open preventing current flow therethrough. Resistor  60  and capacitor  70  form a resistor compactor circuit  98  preventing a false firing of gated silicon controlled rectifier  74  by limiting the rate of rise of voltage across gated silicon controlled rectifier  74 . A mechanism (not shown) positions slip rings  78  such that the brushes (not shown) are lifted to reduce wear and dust. 
     In an exemplary embodiment, first gating circuit  64  detects the DC voltage through current monitor  68  and sends a signal to power source circuit  48 . Power source  48  then opens one phase of diode bridge  92  producing a nonsymmetrical loading on second three phase winding  96 . A stationary circuit (not shown) interprets the nonsymmetrical loading and positions slip rings  78  such that the brushes are lifted. 
     Additionally, the short circuit through gated silicon controlled rectifier  74  is removable. After slip rings  78  are repositioned such that the brushes are in electrical contact with the rotor (not shown), and connector  84  reconnects group  90  to the brushes, the stationary circuit reverses power to second three phase winding  96  providing a change in frequency detectable by power source circuit  48 . The change in frequency is interpreted by power source circuit  48  that slip rings  78  are repositioned and group  90  is engaged. Power source circuit  48  sends a signal to first gating circuit  64 . First gating circuit  64  gates gated silicon controlled rectifier  66  such that gated silicon controlled rectifier  66  acts as a short. Accordingly, the voltage across first third-phase silicon controlled rectifier  56  is lower than the forward voltage drop across second third-phase silicon controlled rectifier  58  and silicon controlled rectifier  74 . Current flow through gated silicon controlled rectifier  74  is removed and the gate of silicon controlled rectifier  74  shuts off. Gate controlled rectifier  66  self commutates off when reversed biased by voltage from resistor group  62  and rectifier  54  when the third phase voltage is the lowest of the three phases. 
     Accordingly, a wound rotor type induction machine is started with brushes engaged, the brushes are disengaged to reduce wear and extend the life of the machine, and then the brushes are re-engaged if desired. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.