Abstract:
The present invention relates to a protective circuit to provide over voltage protection for an ASD. The protective circuit provides the benefits of fewer components with lower power ratings than existing protective circuits. The protective circuit may be incorporated directly in the ASD for continuous protection or mounted externally and connected to the ASD under operating conditions that require the circuit. This flexibility for mounting the protective circuit allows the capacitor of the protective circuit to be sized either in relation to capacitive elements on the DC bus within the ASD or according to external capacitance observed at the output of the ASD. In addition, the circuit is only operative during an overvoltage condition allowing for power ratings lower than would be required for continuous operation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to protective circuits for adjustable speed drives for AC electric motors, and in particular to a DC bus clamping circuit providing protection against component failure during an over voltage fault or adverse operating conditions. 
         [0002]    Common AC motors use three-phase electrical power connected to the stator windings of a motor to run the motor. Each stator winding is connected to a different conductor from a three-phase power source, in which each conductor delivers a different phase of the electrical power to the motor. The three-phase power source may be a direct connection to line power, but more commonly, the motor is connected to an adjustable speed motor drive (ASD). The ASD allows for speed control of the motor not available by connecting the motor directly to line power. 
         [0003]    As is known in the art, there are many electrical topologies for ASDs used to convert the fixed voltage and frequency from the line input into a controlled voltage and frequency output for a three-phase motor. One common topology includes a rectifier section which converts the line power into a DC voltage used to charge a DC bus section of the ASD. An inverter section then uses a switching algorithm, typically pulse width modulation (PWM), to convert the DC voltage from the DC bus into a variable voltage and frequency output to the motor. Controlling the variable voltage and frequency output to the motor controls the speed at which the motor rotates. 
         [0004]    While an ASD provides the benefit of speed control for a motor, it can also introduce negative side effects. PWM algorithms utilize high frequency switching to alternate between short pulses of DC voltage from the DC Bus and no voltage being output to the motor. The duration of these pulses vary such that averaging the DC voltage of the pulse along with the periods during which no voltage is being output results in a lower fundamental AC voltage and frequency seen at the motor, providing a three phase AC waveform suitable for speed control of a motor. However, the high frequency voltage pulses also introduce high frequency electrical transients at the motor output. The motor is connected to the ASD by a power cable. The power cable may introduce a common mode capacitance to the system. In systems where a high capacitance may be present, such as with long power cables, multiple motors operating from a common DC bus, or a high horsepower motor, the high capacitance output creates an impedance that interacts with the high frequency voltage pulses to establish reflected voltage and current waveforms at the output. These reflected waveforms produce voltage and current surges that can create an over voltage condition for both the motor and the drive components. If the over voltage condition is generated in the motor windings, the life of the motor may be shortened. If the over voltage condition occurs at the drive, the voltage seen between the DC bus and ground may be much higher than normal, creating a voltage stress on the insulation of the drive components (e.g. switching mode power supply transformer, IGBT gate opto-coupler, DC link chokes, and common mode capacitors). This voltage stress may similarly result in a shortened life of the drive components. 
         [0005]    The inventors have identified a second condition in which an overvoltage condition for the drive may be generated. If multiple motors are operating from a common DC bus, it is possible that a single motor may fail while the other motors continue operation. It is also possible that a single motor may be damaged yet continue to operate. For example, if one winding in a motor has a ground fault condition, it may draw substantial currents while that phase is switched on by the inverter and substantial voltage or current spikes can be generated. Because each of the multiple motors operates from a common DC bus, the other motors may similarly be affected by the voltage or current spikes. Further, the multiple inverter sections may allow for conductive paths not present in single inverter systems that allow for voltage or current surges that can create an over voltage condition. Therefore, if a single motor fails, or begins to fail, it may affect other motors or drives on the common DC bus and cause subsequent motor or drive failures. 
         [0006]    Prior attempts to protect against over voltage conditions include detecting rising voltage levels on the DC bus and shutting off the inverter section in response to the over voltage condition. This approach may result in undesirable down time of a motor or even an entire process line on which the motor is installed. 
         [0007]    Other attempts to protect against over voltage conditions have included snubber circuits at the motor terminals or across the DC bus. Such circuits have been designed to handle continuous operation and are, therefore, required to handle significant voltages and currents. This requirement results in high voltage and power ratings for the components or alternatively multiple components designed to share the load. The increased power ratings or number of components results in an increased cost of the circuit. 
       SUMMARY OF THE INVENTION 
       [0008]    The present inventors have identified a protective circuit that may be used to provide over voltage protection for an ASD. The protective circuit provides the benefits of fewer components with lower power ratings than existing protective circuits. The protective circuit may be incorporated directly in the ASD for continuous protection or mounted externally and connected to the ASD under operating conditions that require the circuit. This flexibility for mounting the protective circuit allows the capacitor of the protective circuit to be sized either in relation to capacitive elements on the DC bus within the ASD or according to external capacitance observed at the output of the ASD. In addition, the circuit is only operative during an overvoltage condition, allowing for power ratings lower than would be required for continuous operation. 
         [0009]    Specifically then, the present invention applies to a power supply having a DC bus providing a DC voltage across a positive and negative bus rail. The DC bus is connected to one or more inverters which provide power to windings of an electric motor. The DC bus has at least one capacitive element connected across the positive and negative bus rails. The present invention is an apparatus for reducing electrical transients on the DC bus including at least one damping element having a resistor and a damping capacitor connected in parallel with the resistor. A first and a second diode are each connected in series with a damping element between the common point and the positive and negative bus rails, respectively. The direction of conductance of the first diode is from the common point to the positive rail, and the direction of conductance of the second diode is from the negative rail to the common point. 
         [0010]    Thus, it is one object of the invention to provide a damping circuit to reduce electrical transients across the capacitive elements of a DC bus. The diode connected in series with the damping element acts as a switch to engage the damping circuit only when it is needed, allowing for lower power ratings on the damping circuit components. 
         [0011]    A first end of one of the damping elements may connect to the common point. A first diode is then connected in series between the second end of the damping element and the positive bus rail, and a second diode is connected in series between the negative bus rail and the second end of the damping element. 
         [0012]    Consequently, a single damping element is connected in series with each of the diodes to complete the protective circuit. Thus, it is one object of the invention to provide a protective circuit with the fewest number of additional components. 
         [0013]    Alternatively, a first damping element may connect in series with a first diode between the common point and the positive bus rail, and a second damping element may connect in series with a second diode between the negative bus rail and the common point. 
         [0014]    Consequently, each of the first and the second damping elements will typically have a portion of the over voltage conditions occur across the damping element. The duration of time for which either one of the damping elements is conducting is reduced in comparison to using a single damping element. Thus, it is another object of the invention to provide a protective circuit with the lowest power ratings for the additional components. 
         [0015]    The apparatus may be contained within the adjustable speed drive. Thus it is one object of the invention to provide inherent protection against certain fault and adverse operating conditions. 
         [0016]    Alternatively, the apparatus may further include a frame supporting each damping element and diode and a connector allowing the apparatus to be externally connected to the positive bus rail, the negative bus rail, and the common point of an adjustable speed drive. 
         [0017]    Thus it is one object of the invention to provide a separate module that may be added to an adjustable speed drive as needed. 
         [0018]    Multiple adjustable speed drives may be configured to operate from a common DC bus and the apparatus may be externally connected to the common DC bus. 
         [0019]    Thus it is another object of the invention to provide a single protection circuit to protect multiple inverters configured to operate from a common DC bus. 
         [0020]    These and other features, advantages, and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Preferred exemplary embodiments of the invention are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
           [0022]      FIG. 1  is a schematic representation of a standard ASD illustrating a rectifier, a DC bus, and an inverter section; 
           [0023]      FIG. 2  is a schematic representation of one embodiment of an inverter section for use in the ASD of  FIG. 1 ; 
           [0024]      FIG. 3  is a schematic representation of multiple inverter sections connected to a single rectifier and DC bus section; 
           [0025]      FIG. 4  is a schematic representation of a first embodiment of the present invention, shown in a standard ASD including common mode capacitors; 
           [0026]      FIG. 5  is a schematic representation of a first embodiment of the present invention, shown in a standard ASD without common mode capacitors; 
           [0027]      FIG. 6  is a schematic representation of a second embodiment of the present invention, shown in a standard ASD; 
           [0028]      FIG. 7  is a schematic representation of a standard ASD with an embodiment of the present invention mounted externally to the ASD 
           [0029]      FIG. 8  is a schematic representation of a third embodiment of the present invention, shown in a standard ASD. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0030]    Referring to  FIG. 1 , a common topology for an Adjustable Speed Drive  10  is illustrated. The ASD  10  includes a rectifier section  12 , a DC bus section  14 , and an inverter section  16 . The ASD  10  receives three-phase AC input voltage  18  into the rectifier section  12 . The rectifier section  12  may include passive or active rectification, for example diodes, thyristors, silicon controlled rectifiers, or transistors as is known in the art, to convert the three-phase AC input voltages into DC voltages. These DC voltages are present on the positive and negative DC bus rails  20  and  22  of the DC bus section  14 . Typical DC voltages may be a positive or negative 650 volts for a common 460 volt, three-phase AC input voltage. To maintain a stiff DC voltage on each of the positive and negative bus rails,  20  and  22 , a DC bus capacitor  25  is included between the rails  20  and  22 , wherein a stiff DC voltage remains approximately equal to the peak value of the AC input voltage  18  despite transient voltages and currents present on the DC bus, for example when there is a change in the load on the output of the ASD  10 . 
         [0031]    At least one DC bus capacitor  25  is included between the positive rail  20  and the negative rail  22 . Two common mode capacitors may also be included in series between the positive rail  20  and the negative rail  22  to reduce the common mode effects generated by the inverter switching. The common mode capacitors  26  and  28  typically have a low capacitance, for example about 0.1 microfarads, to keep ground currents low and to reduce costs. The first common mode capacitor  26  is connected between the positive rail  20  and a common point  24 , the common point preferably being a ground point and, in particular, the same ground point at which most of the control circuitry is connected. The second common mode capacitor  28  is similarly connected between the negative rail  22  and the common point  24 , or ground. The DC bus capacitor  25  and each common mode capacitor,  26  and  28 , may alternately consist of one or more capacitors connected in series, parallel, or any combination thereof, as is known in the art. 
         [0032]    The DC bus section  14  is then connected to the inverter section  16 . The inverter section  16  consists of switching elements, such as transistors or thyristors as is known in the art.  FIG. 2  illustrates an exemplary embodiment of an inverter section wherein a transistor  15  and a diode  17  are connected in pairs between the positive rail  20  and each phase of the output voltage  30  as well as between the negative rail  20  and each phase of the output voltage  30 . The inverter section  16  converts the DC voltages from the positive and negative rails  20  and  22  into a controlled three phase output voltage  30  to the motor  32 . Alternatively, multiple inverter sections  16  may be connected to a single DC bus section  14  as shown in  FIG. 3 . 
         [0033]    Referring next to  FIG. 4 , a first embodiment of the protective circuit  34  is illustrated. The protective circuit  34  includes a damping element, consisting of a resistor  36  and a damping capacitor  38  connected in parallel with each other. The protective circuit  34  further includes a first diode  40  connected in series with the damping element between the common point  24  and the positive DC bus rail  20 . A diode conducts from the anode to the cathode, and the first diode  40  is oriented such that the direction of conductance is from the common point  24  to the positive rail  20 . The protective circuit  34  also includes a second diode  42  connected in series with the damping element between the negative rail  22  and the common point  24 . The second diode  42  is oriented such that the direction of conductance is from the negative rail  22  to the common point  24 . Generally, the resistor  36  and capacitor  38  may be implemented through multiple series or parallel-connected resistors and capacitors as is known in the art. While  FIG. 4  illustrates the protective circuit connected across a DC bus section  14  that includes the common mode capacitors,  26  and  28 , the circuit may similarly be applied to a DC bus section  14  that does not include common mode capacitors, as shown in  FIG. 5 . 
         [0034]    Referring next to  FIG. 6 , a second embodiment of the protective circuit  34  is illustrated providing an alternative topology for but an equivalent circuit with the first embodiment. The protective circuit  34  includes two damping elements, each consisting of a resistor  36  and a damping capacitor  38  connected in parallel with the resistor  36 . A first diode  40  is connected in series with the first damping element between the common point  24  and the positive rail  20 . The first diode  40  is oriented such that the direction of conductance is from the common point  24  to the positive rail  20 . A second diode  42  is connected in series with the second damping element between the negative rail  22  and the common point  24 . The second diode  42  is oriented such that the direction of conductance is from the negative rail  22  to the common point  24 . 
         [0035]    As illustrated in  FIGS. 4-6 , the protective circuit  34  may be mounted internal to the ASD  10 . Such a configuration provides protection for each drive and is especially suited for applications in which a separate DC bus section  14  is used for each ASD  10  and motor  32  combination. It is also contemplated that the protective circuit  34  be mounted external to the ASD  10 , as shown in  FIG. 7 . It is common for an ASD  10  to have a terminal block  44  including connections for each of the positive rail  20 , negative rail  22 , and common point  24  in a DC bus section  14 . The components of the protective circuit  34  can be mounted on any suitable frame  46 , as would be known in the art, along with a connector  48 , by which the protective circuit  34  may be connected to the ASD  10  as previously described. The connector  48  is preferably a terminal block of any suitable type as known in the art; however, the connector  48  may also be lead wires to tie the protective circuit  34  directly to the ASD  10  or any other means known in the art to connect the protective circuit  34  to the ASD  10 . Such a configuration is especially suited for applications in which multiple inverter sections  16  and motors  32  are operating from a common DC bus section  14 , allowing a single protective circuit  34  to protect multiple inverters  16  and motors  32 . 
         [0036]    In operation, the protective circuit  34  reduces peak transient values of voltage during over voltage conditions. During normal operating conditions, the common point  24  is at a voltage level between the positive and negative DC bus rails  20  and  22  and is preferably electrically grounded. Under these conditions, both the first diode  40  and the second diode  42  are reverse biased and neither conducts. The protective circuit  34  is, therefore, not active under normal operating conditions. During an overvoltage condition, a transient voltage exists in the ASD such that the voltage potential at the common point  24  is either greater than the voltage potential on the positive DC bus rail  20  or less than the voltage potential on the negative DC bus rail  22 . If the voltage potential at the common point  24  is greater than the voltage potential on the positive rail  20 , the first diode  40  becomes forward biased and begins to conduct. Similarly, if the voltage potential at the common point  24  is less than the voltage potential on the negative rail  20 , the second diode  42  becomes forward biased and begins to conduct. 
         [0037]    In order for the protective circuit  34  to effectively reduce the peak transient values of voltage during the over voltage condition, the value of the damping capacitor  38  should be greater than the common mode capacitor,  26  or  28 , across which it is connected. Preferably, the value of the damping capacitor  38  is about ten times greater than the value of the common mode capacitor  26  or  28 . For example, a typical ASD may have a common mode capacitor of about 0.1 microfarad. The damping capacitor  38  should then have a value of at least 1 microfarad, and preferably a value between about 1 and 5 microfarads. 
         [0038]    As mentioned above, the ASD  10  may be used on a system having high capacitance, such as with long power cables, multiple motors operating from a common DC bus, or a high horsepower motor. Under these conditions, it may be advantageous to use a protective circuit  34  mounted externally to the ASD  10 . The value of the damping capacitor  38  may further be selected in response to the capacitance of the system rather than the value of the common mode capacitor  26  or  28  in the drive. The value of the system capacitance, for example the common mode capacitance of long leads extending between an ASD  10  and a motor  32 , may be determined and the value of the damping capacitor  38  selected such that it is greater than, and preferably at least twice the value of, the system capacitance. 
         [0039]    While the damping element is conducting, the damping capacitor  38  begins storing energy. When the transient voltage has returned to a value between the voltages present on the positive rail  20  and the negative rail  22  such that the damping element is no longer conducting, the stored energy in the damping capacitor  38  is discharged through the resistor  36 . It is, therefore, desirable that the time constant of the damping element, determined by the values of the capacitor  38  and the resistor  36 , be small enough that the damping capacitor  38  can discharge across the resistor  36  prior to the next over voltage condition. Because many of the transients are a result of the electrical switching within the inverter section  16 , a time constant equal to or less than period of the electrical switching would be desirable. For example, if the ASD  10  is switching at 10 kHZ, the resultant period is 0.1 msec. Therefore, with a 1 microfarad damping capacitor  38 , a 100 ohm resistor  36  is preferred. 
         [0040]    One advantage of the protective circuit  34  disclosed herein is that the circuit  34  only conducts while an overvoltage condition exists and not under normal operating conditions where the ground voltage is between the voltages present at the positive rail  20  and the negative rail  22 . As a result, the power ratings of the resistor  36 , damping capacitor  38 , and diodes  40  and  42  may be selected based on intermittent operation rather than continuous operation. The resulting power ratings for the components in the protective circuit may be less than 100 watts, and preferably less than 50 watts. 
         [0041]    It is further contemplated that the protective circuit  34  may be used to detect ground faults or severe cable or motor operating conditions. A ground fault occurs when the insulation in one of the windings in the motor  32  fails and the winding shorts to the common, typically the chassis, of the motor  32 . Under such a condition, the inverter section  16  may continue to switch the DC bus voltages across the winding creating significant voltage transients to the common on the motor. Typically the common on the motor  32  is electrically connected to the common point  24  on the ASD  10 , usually over the ground wire in the motor cable. As a result, the voltage transients seen at the common point on the motor are also present at the common point  24  on the ASD  10 . These transients cause the protective circuit  34  to conduct. Therefore, a voltage sensor  50  detecting the voltage potential across the damping element or a current sensor  52  measuring the current flowing through the damping element may be used to generate a signal subsequently processed by the ASD  10  to determine whether a ground fault condition exists. 
         [0042]    It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention.