Abstract:
A permanent magnet alternator (PMA) includes a rotatable shaft, windings, a shunt regulator circuit, and a speed detection circuit. The rotatable shaft is connected electromagnetically to the windings. The shunt regulator circuit is electrically connected to the windings. A current sense transformer with a primary coil is electrically connected to the shunt regulator circuit. A secondary coil is electrically connected to a comparator circuit with reference voltage and generates voltage pulse indicating PMA speed. The voltage pulses form an output corresponding to and indicative of rotation speed of the shaft suitable for processing by a processor to present a PMA speed indication for use in the overall system architecture as a measurement parameter.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present disclosure relates to speed detection for rotating machines, and more particularly to permanent magnet alternator speed detection using current sense transformers. 
         [0003]    2. Description of Related Art 
         [0004]    Permanent magnet alternators are used in mechanical systems with moving components that can be manipulated to generate electrical power. For example, a three-phase electrical system can be derived from a rotating engine component or shaft to which a rotor of a permanent magnet alternator (PMA) is attached. The three-phase voltage source can be rectified and filtered to create a useable voltage bus for use by on board electronics. 
         [0005]    In some applications, there is a need to monitor the speed of the associated mechanical component used to generate the electrical power. For example, the required speed to be monitored could be engine speed on an aircraft. Conventional speed detection for permanent magnet alternators has been done using diode detection circuitry. Diode detection circuitry generally provides output with a relatively low signal to noise ratio and a relatively low voltage detection level (e.g. less than about 1 volt). This can make such circuitry difficult to reliably implement in high current environments. Moreover, in some systems, the need for electrical isolation may require additional electrical components, such as optical isolators for example. 
         [0006]    Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is a need in the art for speed detection devices for permanent magnet alternators with improved reliability. The present disclosure provides solutions to this need. 
       SUMMARY OF THE INVENTION 
       [0007]    A permanent magnet alternator includes a shaft, alternator windings, a shunt regulator circuit, a current sense transformer, and a speed detection circuit. A permanent magnet is connected to the shaft and is rotatable with respect to the alternator windings. The alternator windings are electrically connected to the shunt regulator circuit. The shunt regulator circuit is connected to the speed detection circuit through primary and secondary coils of the current sense transformer. The coils of the current sense transformer connect the shunt regulator circuit and the speed detection circuit such that a voltage indicative of shaft rotational speed is created in the speed detection circuit from current returning to a winding of the permanent magnet alternator in the shunt regulator circuit. 
         [0008]    In certain embodiments, the connection between the speed detection circuit and the shunt regulator circuit includes an electromagnetic coupling. The shunt regulator circuit can include a current reverse flow leg electrically connected to a winding of the permanent magnet alternator for returning current to the winding as rotation of the shaft successively induces current in other alternator windings. The current sense transformer primary coil can be electrically connected to the reverse flow leg, the current sense transformer secondary coil can be electrically connected to the speed detection circuit, and electromagnetic coupling can couple the primary coil to the secondary coil such that current flow through the reverse flow leg induces current flow in the speed detection circuit. It is contemplated that secondary coil can have a greater number of turns than the primary coil. The primary coil can have a single turn. 
         [0009]    In accordance with certain embodiments, the speed detection circuit includes a comparator. A first input of the comparator can be electrically connected to the current sense transformer secondary coil. A reference voltage source can be electrically connected to a second input of the comparator. The comparator can be configured such that, when voltage across the sense resistor exceeds the reference source voltage, an output of the comparator forms a voltage edge indicative of shaft speed. It is contemplated that the speed detection circuit can include a sense resistor with a first end electrically connected between the comparator and the current sense transformer secondary coil. A second end of the sense resistor can be electrically connected to a ground terminal. The sense resistor can have a resistance of about 25 ohms, for example. 
         [0010]    It is also contemplated that in certain embodiments the speed detection circuit includes a capacitor. A first end of the capacitor can be electrically connected between the sense resistor first end and current sense transformer secondary coil. A second end of the capacitor can be electrically connected to the ground terminal. A diode can be electrically connected between the capacitor first end and current sense transformer secondary coil for opposing current flow towards the secondary coil through the diode. 
         [0011]    These and other features of the systems and methods of the subject disclosure will become more readily apparent to those skilled in the art from the following detailed description of the preferred embodiments taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    So that those skilled in the art to which the subject disclosure appertains will readily understand how to make and use the devices and methods of the subject disclosure without undue experimentation, preferred embodiments thereof will be described in detail herein below with reference to certain figures, wherein: 
           [0013]      FIG. 1  is a schematic view of an exemplary embodiment of a power bus in accordance with the present disclosure, showing a permanent magnet alternator connected to a shunt regulator and a speed detection circuit; and 
           [0014]      FIG. 2  is a circuit diagram of the power bus of  FIG. 1 , showing circuitry of the shunt regulator and the speed detection circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, a schematic view of an exemplary embodiment of a speed detection circuit in accordance with the disclosure is shown in  FIG. 1  and is designated generally by reference character  100 . Other embodiments of speed detection circuits in accordance with the disclosure, or aspects thereof, are provided in  FIG. 2 , as will be described. The systems and methods described herein can be used for measuring rotor speed in permanent magnet alternators, such as in aircraft electrical systems. 
         [0016]    With reference to  FIG. 1 , an electrical system  10  is shown. Electrical system  10  includes a permanent magnet alternator  20 , a shunt regulator circuit  30 , and an electrical load  40 . A prime mover  50  is operatively associated with permanent magnet alternator  20  through a shaft  52 . Shaft  52  is operatively associated a rotor with a permanent magnet of permanent magnet alternator  20  and configured for rotating the magnet in relation to first, second, and third phase windings  24 A,  24 B, and  24 C (shown in  FIG. 2 ). This induces current flow in phase windings  24  that varies as a function of rotational speed and position of shaft  52 . 
         [0017]    First, second, and third phase leads  22 A,  22 B, and  22 C electrically connect first, second, and third phase windings  24 A,  24 B, and  24 C (shown in  FIG. 2 ) to shunt regulator circuit  30 . Shunt regulator circuit  30  is configured and operative for converting three-phase alternating current received through first, second, and third phase leads  22 A,  22 B, and  22 C into single-phase current suitable for powering electrical load  40 . 
         [0018]    Speed detection circuit  100  connects to shunt regulator circuit  30  through an electromagnetic coupling  102  and is configured to generate a voltage having information indicative of rotation of a rotary portion of permanent magnet alternator  20 . An analog to digital converter  60  is electrically connected to speed detection circuit  100  through a comparator output lead  62  and is configured to convert voltage received from speed detection circuit  100  having information indicative of rotation of the rotary portion of permanent magnet alternator  20  into input suitable for use by an aircraft control architecture. 
         [0019]    With reference to  FIG. 2 , a circuit diagram of electrical system  10  is shown. Permanent magnet alternator  20  includes a first phase winding  24 A, a second phase winding  24 B, and a third phase winding  24 C. First phase lead  22 A is electrically connected to first phase winding  24 A, second phase lead  22 B is electrically connected to second phase winding  24 B, and third phase lead  22 C is electrically connected to third phase winding  24 C. 
         [0020]    Shunt regulator circuit  30  includes a first, second, and third phase diode  32 A,  32 B, and  32 C. Shunt regulator circuit also includes a first, second, and third MOSFET  34 A,  34 B, and  34 C. First, second, and third phase diodes  32 A,  32 B, and  32 C are electrically connected between phase windings  24 A,  24 B and  24 C, respectively, and electrical load  40 . First, second, and third phase diodes  32 A,  32 B, and  32 C are configured for allowing current flow from respective phase windings to electrical load  40  and opposing current flow from electrical load  40  to first, second, and third phase windings  24 A,  24 B, and  24 C, respectively. 
         [0021]    As illustrated, first, second, and third MOSFETs  34 A,  34 B, and  34 C are n-channel MOSFETs that respectively include a source terminal, a drain terminal, and a control terminal. The drain terminal of first MOSFET  34 A connects to first winding  24 A through first phase lead  22 A and the source terminal of first MOSFET  34 A connects to a first ground terminal  38 . The drain terminal of second MOSFET  34 B connects to second winding  24 B through second phase lead  22 B and the source terminal of second MOSFET  34 B connects to first ground terminal  38 . The drain terminal of third MOSFET  34 C connects to third winding  24 C through third phase lead  22 C and the source terminal of third MOSFET  34 C connects to first ground terminal  38 . As will appreciated, embodiments of shunt regulator  30  can include one or more p-channel MOSFETs and remain within the scope of the present disclosure. 
         [0022]    A pulse width modulation controller  70  is electrically connected between bus segment  26  and control terminals of first, second, and third MOSFETs  34 A,  34 B, and  34 C. Pulse width modulation controller  70  is configured for applying a control voltage to the control terminals of the first, second, and third MOSFETs  34 A,  34 B, and  34 C based on current flow (draw) through bus segment  26 . As current flow varies through bus segment  26  in response to the needs of electrical load  40 , pulse width modulation controller  70  alters current applied to the control terminals of first, second and third MOSFETs  34 A,  34 B, and  34 C. This regulates current flow through bus segment  26  by shunting to first ground terminal  38  current generated by permanent magnet alternator  20  that is not required by electrical load  40 . 
         [0023]    A reverse flow leg  33  electrically connects to third phase winding  24 C on a first end, and source terminals of first, second, and third MOSFETs  34 A,  34 B, and  34 C as well as first ground terminal  38  on an opposite end. A first blocking diode  35  is electrically connected to reverse flow leg  33  and is configured for opposing current flow from third phase winding  24 C through reverse flow leg  33  (toward first ground terminal  38 ). First blocking diode  35  also allows reverse flow current to return (indicated with arrow i) from first, second, and third phase leads  22 A,  22 B, and  22 C to third phase winding  24 C through reverse flow leg  33 . A second blocking diode  37  is electrically connected between third MOSFET  34 C and third phase lead  22 C, and is configured for opposing current flow from first and second phase leads  22 A and  22 B flowing to third phase lead  22 C through third MOSFET  34 C. 
         [0024]    Speed detection circuit  100  includes a current sense transformer  110  (shown in dotted outline in  FIG. 2 ) with a primary coil  112  and a secondary coil  114 , a diode  120 , a sense resistor  130 , a capacitor  140 , and a comparator  150 . Primary coil  112  is electrically connected to reverse flow leg  33  between first ground terminal  38  and first blocking diode  35 . Secondary coil  114  is electrically isolated from primary coil  112  and is electrically connected to a second ground terminal  39 . Sense resistor  130  includes a first end electrically connected between comparator  150  and secondary coil  114 , and a second end electrically connected to second ground terminal  39 . Capacitor  140  includes a first end electrically connected to between the first end of sense resistor  130  and secondary coil  114 , and a second end electrically connected to second ground terminal  39 . Diode  120  is arranged between the first end of capacitor  140  and secondary coil  114 , and is configured for opposing current flow through diode  140  towards secondary coil  114 . 
         [0025]    Comparator  150  includes first and second inputs and an output and is configured for providing to comparator output lead  62  the higher of voltages applied to a first and second comparator inputs. The first comparator input is electrically connected to secondary coil  114  through diode  120 . The second comparator input is connected to a reference voltage source. The comparator output is connected to comparator output lead  62 . When voltage associated with current induced in secondary coil  114  exceeds that of the reference voltage, comparator  150  applies voltage applied to the first comparator input to comparator output lead  62 . Otherwise comparator  150  applies the reference voltage to output lead  62 . 
         [0026]    At intervals during rotation of shaft  52 , current flows from first ground terminal  38  through reverse flow leg  33  to third lead  22 C. This returning current (indicated with current arrow ‘i’ in  FIG. 2 ) traverses primary coil  112  and alters the strength of electromagnetic coupling  102  extending between primary and secondary coils  112  and  114 . This induces a corresponding current flow in secondary coil  114  that is commensurate with current flow in reverse flow leg  33  and which is scaled by the ratio turns in secondary coil  114  to primary coil  112 . Since current flow through reverse flow leg  33  is a function of the rotational position of shaft  52 , and the rate of flow and ebb in the current in reverse flow leg  33  is indicative of rotational speed of permanent magnet alternator  20 , voltage applied to the first input flow of comparator  150  increases and decreases according to current flow and ebb in reverse flow leg  33 . In this respect primary coil  112  couples the phase current, i.e. current flowing through reverse flow leg  33 , to secondary coil  114  through electromagnetic coupling  102 , thereby inducing corresponding current flow in secondary coil  114 . 
         [0027]    The induced current flows to second ground terminal  39  through sense resistor  130  and applies voltage to the first comparator input corresponding to the magnitude of the induced current flow. Comparator  150  receives the corresponding voltage and compares it to the reference voltage. When the voltage exceeds that of the reference voltage, comparator  150  trips high and applies the higher input voltage to the comparator output. This forms a pulse edge in the voltage applied by comparator  150  to comparator output lead  62  indicative of the rotational speed of permanent magnet alternator rotor. In this way current sense circuit  100  picks up a given phase&#39;s return current using current sense transformer  110  and conditions the event into a digital level pulse applied to comparator output lead  62 , thereby providing output suitable for by speed analysis circuitry to make system level computations needed for a given operational plant, such as for an aircraft engine controller for example. As will be appreciated, secondary winding  114  is isolated from primary coil  112  (carrying phase current). This allows for operation in an isolated ground system without the need for specialized isolation equipment, such as optical isolators for example. 
         [0028]    Secondary coil  114  is scaled for comparator  150  through the ratio of turns of secondary coil  114  to turns of primary coil  112  such that voltage in speed detection circuit  100  is linear over the operating frequency range of PMA  20 . Sense resistor  130  is correspondingly scaled with the turn ratio of the current sense transformer to achieve sensing levels high enough not to be affected by background noise. As will be appreciated, the accuracy of the current measurement in the phase is not important (the usual use for current sense transformers), just the detection of the current passing back through the phase selected for speed detection. In embodiments, the ratio of secondary coil turns to primary coil turns is about 150:1, the secondary coil has 114 turns, primary coil  112  has a single turn, and sense resistor is a low power resistor with a resistance of about 25 ohms. 
         [0029]    The methods and systems of the present disclosure, as described above and shown in the drawings, provide for speed detection circuits with superior properties including improved accuracy and/or reduced sensitivity to noise. The circuit topology illustrated can also provide primary secondary transformer isolation as reverse flow leg  33  is not electrically connected to secondary coil  114 . While the apparatus and methods of the subject disclosure have been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the spirit and scope of the subject disclosure.