Abstract:
An electric power supply system has a power bus for providing DC power, and a control unit for a source of power to supply the power bus. The power unit includes a damping algorithm to provide damping to power supplied on the power bus. A motor and a motor control include a compensation block for tapping power from the bus, and identifying a portion of a supplied signal due to the damping. The compensation block provides a signal to a summing block that addresses the damping on the power bus prior to the power being supplied to the motor. A method of utilizing such a system is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     This application relates to a damping circuit associated with a motor drive for a motor to be powered by a DC power bus. 
     Many electrical systems include a power bus that passes electric power among a number of components associated with the system. One example is an aircraft electrical power system. In such a system, a DC power bus supplies power to many components mounted around the aircraft. 
     Such systems typically include electric generators which are driven by prime movers, such as gas turbine engines, to generate AC power. A gear box is typically included between the prime mover and the generator. The AC power is converted to DC and passed to the power bus. 
     There are a number of torsional and vibratory challenges in such systems. Mechanical dampening systems have been incorporated, however these are complex and require a good deal of space and weight. 
     More recently, generator control units have been provided with damping algorithms that modulate the DC bus voltage. The bus voltage modulation, in combination with a resistive load on the bus, is used as a means of damping mechanical oscillations in the prime mover/generator system. 
     However, when the bus loading is not resistive, these damping algorithms are ineffective. Such is the case when a motor drive with a constant power algorithm is connected to the bus. The constant power algorithm in the motor drive makes it appear as a negative impedance load on the bus. 
     SUMMARY OF THE INVENTION 
     An electric power supply system has a power bus for providing DC power, and a control unit for a source of power to supply the power bus. The power unit includes a damping algorithm to provide damping of standing vibrations in the generator/prime mover system. A motor and a motor control include a compensation block for tapping power from the bus, and identifying a portion of a supplied signal due to the damping. The compensation block provides a signal to a summing block that addresses the damping on the power bus prior to the power being supplied to the motor. A method of utilizing such a system is also disclosed. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows an aircraft electric system. 
         FIG. 2  shows a power circuit for powering a single motor. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An electric system  20  for an aircraft is illustrated in  FIG. 1 . Although the invention is illustrated on an aircraft, other applications utilizing an electrical generator to supply power to a bus would benefit from this invention. A gas turbine engine  22  powers an electric generator  24  to supply AC current to an AC to DC converter  28 . A gear box  23  is positioned between the engine  22  and generator  24 . A generator control unit (GCU)  26  controls the AC power supply, to increase or decrease generator power output in response to changing speeds from the engine  22  to the generator  24 . The generator control unit employs an active damping algorithm which uses phase compensation to effectively dampen torsional oscillation. 
     Downstream of the AC to DC converter  28  is a DC power bus  30 . A number of components draw power through taps  32  from the bus  30 . Motor controls  34  draw from the power bus through taps  32 . In particular, motor controls  34  are used in applications aboard aircraft to run a variety of mechanical loads such as engine starters, hydraulic pumps, centrifugal compressors, fans, etc. 
     As mentioned, engine  22  generates mechanical energy that is provided through gearbox  23  to synchronous generator  24  (or to multiple synchronous generators). Generator  24  produces AC electric power that may be transmitted to a number of AC electric loads, including AC to DC and AC to AC power converters. The  FIG. 1  schematic is simplified, but it should be understood that power can be delivered to more than bus  30 . AC to DC converter  28  provides DC power as input to bus  30 . 
     A shaft transmits the mechanical energy from the gearbox to the synchronous generator. Due to a multitude of competing mechanical design considerations, the shaft may be relatively long and mechanically compliant. Inertias associated with engine  22 , gearbox  23 , generator  24 , and other gearbox driven accessories, in combination with the mechanical compliance or spring rates of the mechanical drive train, including the generator shaft, create a distributed mechanical spring-mass system that has associated torsional resonances. There are multiple torsional modes and associated resonances that involve the generators when multiple direct-driven generators are driven from a common gearbox. Engine gearboxes typically exhibit very lightly damped characteristics, and because the generator is controlled by control  26  to maintain an AC voltage at its output, it presents a near constant power load characteristic to the mechanical drive train that results in negative damping for disturbance frequencies that are within the generator&#39;s voltage regulation bandwidth. In certain situations, depending on the generator speed, the generator electrical load characteristics, and the net effective damping in the overall mechanical drive train, the torsional resonance of the spring-mass system involving the generator or generators can lead to large, undesirable torsional oscillations. 
     Mechanical damping may be used to offset the negative damping characteristic of the synchronous generator and thus dampen the torsional oscillations in the spring-mass system, but, as mentioned above, mechanical damping requires additional parts that increase the weight and cost of the system. 
     Thus, a GCU  26  is provided with an active damping algorithm to provide positive mechanical damping to the generator rotor over a limited torsional oscillation frequency range. It does this by increasing/decreasing the generator power output in response to generator speeds that are increasing/decreasing. This GCU active damping algorithm can be tuned through selective use of phase compensation to effectively dampen the aforementioned torsional oscillations for a certain class of electrical loads. As an example, resistive loads whose DC load impedance characteristics define a phase relationship between a sinusoidal input DC voltage perturbation and the corresponding sinusoidal DC current response to the DC voltage perturbation at the same frequency as the voltage perturbation can be used. 
     High performance motor controls  34  have a number of competing requirements including the steady state and dynamic performance requirements of the motor drive loads and the DC load impedance requirements dictated by DC source. DC load interactions determine electric power quality and electric system stability. The addition of a DC load impedance phase relationship related to the damping effectiveness of the GCU  26  active damping algorithm in the generator controls of the synchronous generator supplying the electric loads could lead to competing requirements. 
     Modern motor controls  34  incorporate a variety of means to modulate or control the mechanical power delivered to the motor load. These techniques include modern closed loop control for both synchronous and induction motors and simple open loop V/Hz control for induction motors, as two examples among many. 
     Depending on the dynamic response requirements of the motor load, the DC load impedance of these motor controls as electrical loads on the DC bus may approximate constant power or negative impedance load characteristics in the frequency range of the fundamental mechanical drive line torsional resonance frequencies, which would typically be 20 to 60 Hz. For the more active of these motor control loads, the DC load impedance phase angle may be as negative as −135° at the lower limit of the aforementioned torsional resonance frequency range. As a reference a pure constant power load has a DC load impedance phase angle of −180°. When included as the loads that must be supplied by the generators powering the electric system, these large negative DC load impedance angles can impose severe restrictions on the effectiveness of the GCU active damping algorithms in providing positive mechanical damping to the mechanical drive line of the engine powering the synchronous generator or generators. 
     An overall control for the system  36  provides a motor speed control signal  38  to the control  34 . As shown in  FIG. 2 , control  34  includes a compensation block with additional components to compensate for the damping from the GCU  26 . 
     The compensation block  234  in control  34  as shown in  FIG. 2  actually includes two taps to the power bus  30  including a first control tap  132 , and a second power tap  232 , rather than the single tap  32  shown schematically in  FIG. 1 . The tap  132  extends to compensation block  234  which includes a band pass filter  240 , a gain and compensation block  242 , and a limit block  244 . 
     Band pass filter (BPF) block  240  isolates the DC voltage disturbance caused by the torsional oscillation and the GCU active damping response. Gain &amp; compensation block  242  converts the voltage disturbance to a motor drive control variable modulation, or a frequency modulation at the correct phase relationship to the DC voltage disturbance. Limit block  244  limits the output range of the motor drive control variable modulation. 
     As can be seen in  FIG. 2 , the signals leave the compensation block  234  and extend through connection  238  to a first summation block  236 . First summation block  236  also sees the speed demand signal  38 . Downstream of the first summation block  236  is an inverter VHz component  246 , which extends to a second summation block  248 . An inverter  250  to supply power to motor  260  is downstream, and also receives power  232  from the bus  30 . As can be seen, a feedback signal  252  of the actual power supplied downstream of the inverter  250  extends back to the second summation block  248 . The closed loop feedback control provided by summation block  248  may be generally as known in the art. 
     When the DC link voltage disturbance is positive, the frequency modulation command is positive. Conversely, when the DC link voltage disturbance is negative, the frequency modulation command is negative. The frequency modulation command is limited and then added to the controller command frequency. This controller command frequency is then fed to the motor drive V/Hz algorithm controlling the inverter. 
     It is recognized by one skilled in the motor drive art that the V/Hz example described above is but one of many implementations. In the V/Hz example the inverter frequency has a strong, direct influence on the induction motor torque and thus power. An instantaneous increase in inverter frequency produces a near instantaneous increase in motor torque and thus power. For a high bandwidth vector control motor drive, the variable relating to the motor torque and power is the torque producing current component or q-axis current in the vernacular. For a low bandwidth vector control motor drive controlling a synchronous motor, the phase angle of the current relative to the rotor position plays the equivalent role to the inverter frequency. Thus for the many possible motor drive implementations and/or types, the algorithm will include function blocks  240 ,  242 , and  242 , although it is possible that an algorithm that omits one or more of the blocks could come within the scope of this application. 
     Relative to these functions, the motor drive control variable signal is the inverter frequency for a V/Hz controlled induction motor, the torque producing current for a high bandwidth vector control and the phase angle of the current vector relative to the motor rotor in a low bandwidth vector control. 
     Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.