Method and apparatus for compensating for device dynamics in inverter based control systems

[0002] Not applicable.

[0003] Not applicable.

[0004] The present invention relates to motor controllers and more particularly to a method and an apparatus for compensating for switching device drops and switching device dynamics in inverter systems.

[0005] One type of commonly designed motor is a three phase motor having three Y-connected stator windings. In this type of motor, each stator winding is connected to an AC voltage source by a separate supply line, the source providing time varying voltages across the stator windings. Often, an adjustable speed drive (ASD) will be positioned between the voltage source and the motor to control motor speed by controlling the stator voltages and frequency.

[0006] Many ASD configurations include a pulse width modulated (PWM) inverter consisting of a plurality of switching devices. Referring to FIG. 1, an exemplary PWM inverter leg 10 corresponding to one of three motor phases includes two series connected switches 12, 13 between positive and negative DC rails 18, 19 and two diodes 16, 17, a separate diode in inverse parallel relationship with each switch 12, 13. By turning the switches 12, 13 ON and OFF in a repetitive sequence, leg 10 receives DC voltage via rails 18 and 19 and provides high frequency voltage pulses to a motor terminal 22 connected to a stator winding 24. By firing the switching devices in a regulated sequence the PWM inverter can be used to control both the amplitude and frequency of voltage that is provided across winding 24.

[0007] Referring to FIG. 2, an exemplary sequence of high frequency voltage pulses 26 that an inverter might provide to a motor terminal can be observed along with an exemplary low frequency alternating fundamental or terminal voltage 28 and related alternating current 30. By varying the widths of the positive portions 32 of each high frequency pulse relative to the widths of the negative portions 34 over a series of high frequency voltage pulses 26, a changing average voltage which alternates sinusoidally can be generated. The changing average voltage defines the terminal voltage 28 that drives the motor. The terminal voltage 28 in turn produces a low frequency alternating current 30 that lags the voltage by a phase angle φ.

[0008] The hardware that provides the firing pulses to the PWM inverter includes both a control board and a power board. The control board includes, among other components, a processor that receives a frequency command signal and various feedback signals and, in response thereto, provides command signals to the power board. The power board converts the command signals to modulating signals. Among other things, the power board typically includes a comparator that receives each of at least one modulating signal and a carrier signal for comparison. Referring to FIG. 3(a), exemplary modulating and carrier waveforms used by a power board to generate the firing pulses for leg 10 are illustrated. As well known in the art, a carrier waveform 36 is perfectly periodic and operates at what is known as a carrier frequency. A modulating voltage waveform 38 generally and ideally is sinusoidal, having a much greater period than carrier waveform 36.

[0009] Referring also to FIGS. 3(b) and 3(c), an ideal upper signal 40 and an ideal lower signal 42 generated by a PWM comparator comparing the signals of FIG. 3(a) and that may control the upper and lower switches 12, 13 respectively, are illustrated. The turn-on times tu1, tu2 and turn-off times to 1, to 2 of the upper and lower signals 40, 42, respectively, come from the intersections of the modulating waveform 38 and the carrier waveform 36.

[0010] When the modulating waveform 38 intersects the carrier waveform 36 while the carrier waveform has a positive slope, the upper signal 40 goes OFF and lower signal 42 goes ON. On the other hand, when the modulating waveform 38 intersects the carrier waveform 36 while the carrier waveform has a negative slope, the upper signal 40 goes ON and the lower signal 42 goes OFF. Thus, by comparing the carrier waveform 36 to the modulating waveform 38, the states of the upper and lower signals 40, 42, respectively, can be determined.

[0011] While the modulating and carrier signals are referred to as waveforms in order to simplify understanding of this explanation, in reality, each of the waveforms is a digital count that represents a corresponding waveform. For instance, the modulating waveform may be converted into a count that oscillates within a range between a first or minimum modulating count equal to or greater than zero and a second or maximum modulating count. Where the modulating waveform is sinusoidal, the modulating count changes in a sinusoidal time-varying fashion indicative of the waveform. In this case, a carrier count oscillates in a linear time varying fashion from zero to a maximum carrier count T_cmaxthat is equal to or greater than the maximum modulating count and back to zero during each carrier cycle. The comparator compares the modulating count with the carrier count and when the modulating count is greater than the carrier count, causes the corresponding upper and lower switches to be turned ON and OFF, respectively, and when the modulating count is less than the carrier count, causes corresponding upper and lower switches to be turned OFF and ON, respectively. Because the modulating count value determines the ratio of switch ON times to PWM period duration (i.e., carrier cycle period), the modulating count is also referred to as a duty cycle Ts.

[0012] When the duty cycle count is less than one-half the maximum carrier count, the resulting terminal voltage is negative because the corresponding lower switch is ON for more than half the PWM period. Similarly, when the duty cycle count is greater than one-half the maximum carrier count, the resulting terminal voltage is positive because the corresponding upper switch is ON for more than half the PWM period.

[0013] Referring also to FIGS. 2 and 3(d), an ideal high frequency voltage pulse 26 resulting from the ideal upper and lower signals 40, 42 in FIGS. 3(b) and 3(c) that might be provided at terminal 22 can be observed. When the upper signal 40 is ON and the lower signal 42 is OFF, switch 12 allows current to flow from the high voltage rail 18 to motor terminal 22 thus producing the positive phase 44 of pulse 26 at motor terminal 22. Ideally, when the upper signal 40 goes OFF and the lower signal 42 goes ON, switch 12 immediately turns OFF and switch 13 immediately turns ON connecting motor terminal 22 and the low voltage rail 19 producing the negative phase 46 of pulse 26 at terminal 22. Thus, the ideal high frequency voltage pulse 26 is positive when the upper signal 40 is ON and is negative when the lower signal 42 is ON. Also, ideally, the low frequency terminal voltage and corresponding current (see FIG. 2) should completely mirror the modulating waveforms.

[0014] Unfortunately, these ideal switch operating conditions do not occur as there are several switch and inverter operating phenomenon that cause terminal voltage distortions. For example, one problem with PWM inverters has been that the high frequency terminal voltage pulses (see 26 in FIG. 2) cause ripple in the resulting low frequency phase voltages and currents. This ripple distortion has generally been addressed by either providing line filters that tend to smooth the ripple or by adopting faster switching technology. Elaborate filters are bulky and expensive and therefore are not preferred. Current inverter switching technology has advanced rapidly and modern switches are now capable of changing state in as little as several tens of nano-seconds (e.g., 50 nsec.). For this reason, recent inverter designs have typically adopted high speed switching configurations to reduce ripple distortion.

[0015] One other relatively well understood and therefore, not surprisingly, generally well compensated distortion phenomenon, is referred to as inverter switch delay. Control schemes for compensating for switch delays are generally referred to as dead time compensation (DTC) schemes. Exemplary DTC schemes are described in U.S. Pat. No. 5,811,949 and U.S. Pat. No. 5,917,721.

[0016] While switch delays and ripple distortion are well understood and DTC and other schemes have been developed that generally minimize terminal current and voltage distortion due to turn on delays and ripple, unfortunately, experience has shown that even after DTC has been implemented and fast switching technology has been adopted, terminal voltages and currents still include appreciable distortion. To this end, referring to FIG. 4, two phase terminal currents are illustrated that were generated with a 10 hp industrial drive with full DTC, with a 3 Hz modulating frequency and a 4 kHz carrier frequency. Clearly, under these conditions, despite full dead time compensation, terminal current distortion is relatively large and would be unacceptable for many applications.

[0017] Several systems have been developed that employ several feedback loops and complex processor algorithms to compensate for the distortion illustrated in FIG. 4. While suitable for certain applications, unfortunately, the processors required to perform the compensation algorithms are often relatively expensive and therefore are not useable in low end control systems.

[0018] Thus it would be advantageous to have a control methodology and system that is relatively simple and inexpensive to implement that could mitigate at least the lion's share of distortion illustrated in FIG. 4.

[0019] In order to best mitigate the affects of a distortion source first the source and the magnitude of the distortion has to be identified. To this end, control board error has been studied in great detail and, while control board error occurs, the magnitude of control board error is to small to account for the distortion illustrated in FIG. 4.

[0020] To gain a better understanding of the source of the disturbances illustrated in FIG. 4 experiments were performed using the system employed to generate the waveforms of FIG. 4 with different operating frequencies. To this end, FIGS. 5 and 6 illustrate two phase current waveforms with 10 and 16 Hz operating frequencies, respectively. Clearly, current distortion decreases as the operating frequency is increased. When distortion changes as a function of operating frequency experience has shown that likely sources of distortion include the power board and or the power structure.

[0021] More specifically, it has been recognized that, despite seemingly identical upper and lower inverter components in a typical inverter, the upper and lower components operate differently during switching and therefore have slightly different propagation delays and device dynamics (i.e., switching ramp ON and OFF times). At low operating frequencies (e.g., 2 Hz), the proportion of applied voltage attributed to propagation delays and device dynamics is relatively large when compared to the proportion at high operating frequencies—hence the varying levels of distortion illustrated in FIGS. 4, 5 and 6.

[0022] To this end, referring to FIG. 7, a carrier signal 36, an exemplary high frequency modulating signal 38_Hand an exemplary low frequency modulating signal 38_Lare illustrated. For the purposes of this explanation it should be assumed that each time a modulating signal (e.g., 38_Hor 38_L) and the carrier waveform 36 intersect, an inverter switching sequence or state change occurs. Thus, at each of times T1 and T4, switching sequences occur corresponding to signal 38_L. Similarly, at each of times T2 and T3 switching sequences occur corresponding to signal 38_H, etc. It should also be assumed that each time a switching sequence occurs some device dynamics error due to imperfect switching characteristics occurs. During period T_A, while each modulating signal results in two switching sequences and hence errors of similar magnitudes, because the period during which high frequency modulating signal 38_His below carrier waveform 36 is shorter than the period during which low frequency modulating signal 38_Lis below carrier waveform 36 and, because resulting pulses (see again FIG. 3d) are positive when modulating waveforms are above carrier waveform 36, the magnitude of the voltage resulting from high frequency waveform 38_His greater than the voltage magnitude resulting from low frequency waveform 38_L. Thus, during period T_A, the ratio of switching errors to voltage magnitude corresponding to high frequency waveform 38_His less than the ratio of switching errors to voltage magnitude corresponding to low frequency waveform 38_L. This ratio difference partially explains the varying levels of distortion at different operating frequencies as illustrated in FIGS. 4 through 6.

[0023] To mitigate the distortion illustrated in FIGS. 4, 5 and 6, the present invention includes a device dynamics compensator that is useable to roughly adjust the modulating signals. More specifically, the present invention includes a device dynamics compensator including a polarity discriminator, several two pole switches (one switch per phase) and a memory. Two separate values are stored in the memory including upper and lower compensation values where the upper and lower values are typically different. The upper and lower values are linked to separate inputs of the switch. The discriminator receives current feedback signals from each of three load phases and, for each phase, determines the current polarity (i.e., positive or negative) thereby generating polarity signals. A separate polarity signal controls each switch such that, when a polarity is positive, a corresponding switch provides the upper compensation value to the switch output and, when the polarity is negative, a corresponding switch provides the lower compensation value.

[0024] The output compensation values are provided to summers which mathematically combine the compensation values with command modulating signals that have already been compensated in a conventional fashion for deadtime. More specifically, each summer adds the received values.

[0025] With respect to the upper and lower compensation values, the upper value is generally associated with the upper half of the PWM inverter while the lower value is generally associated with the lower half of the PWM inverter thereby reflecting disparate propagation delays and device dynamics attributable to the upper and lower inverter devices, hence the “upper” and “lower” labels for the compensation values.

[0026] To identify the compensation values, the invention also includes a commissioning procedure described below.

[0027] FIG. 1 is a schematic diagram illustrating switching components in one leg of a PWM inverter;

[0028] FIG. 2 is a graph illustrating exemplary high frequency PWM voltage pulses, an exemplary low frequency alternating voltage and a related low frequency alternating current;

[0029] FIG. 3ais a graph illustrating an exemplary modulating signal and an exemplary carrier signal,

[0030] FIGS. 3band 3cillustrate firing pulses provided to a PWM inverter, and corresponding to FIG. 3aand

[0031] FIG. 3dis a graph illustrating a high frequency voltage pulse similar to the high frequency pulses of FIG. 2;

[0032] FIG. 4 is a graph illustrating two feedback load currents where an operating frequency was set to 3 Hz;

[0033] FIG. 5 is a similar to FIG. 4, albeit where the currents were generated with a 10 Hz operating frequency;

[0034] FIG. 6 is similar to FIG. 4, albeit where the operating frequency was set to 16 Hz;

[0035] FIG. 7 is a graph similar to the graph of FIG. 3a, albeit illustrating two modulating signals of different frequencies;

[0036] FIG. 8 is a schematic diagram illustrating a control system according to a first embodiment of the invention;

[0037] FIG. 9 is a detailed schematic diagram of the device dynamic compensator of FIG. 8;

[0038] FIG. 10 is similar to FIG. 8, albeit illustrating a second inventive control system;

[0039] FIG. 11 is similar to FIG. 9, albeit illustrating an exemplary device dynamic compensator used in the system of FIG. 10;

[0040] FIG. 12 is similar to FIG. 4, albeit illustrating currents generated with the present invention-employed; and

[0041] FIG. 13 is a flow chart illustrating a commissioning method according to the present invention.

[0042] FIG. 8 shows a block diagram of an exemplary inventive drive system 100 including a controller 102, a modulator 104, a duty cycle converter 106, a deadtime compensator 108, a device dynamic compensator 110, a PWM generator 111, three current sensors collectively identified by numeral 116 and an exemplary load 112. Controller 102 receives a command signal f_e^*and uses the command signal to generate command voltage and angle signals V* and θ*, respectively. The voltage and angle command signals are provided to modulator 104 which uses those signals to generate three-phase voltage modulating signals V_u^**,V_v^**and V_w^**. The three-phase voltage modulating signals are provided to duty cycle converter 106 which generates three-phase command duty cycle count values T_u^**,T_v^**and T_w^**, a separate duty cycle count value corresponding to each of the three received voltage modulating signals. The command duty cycle counts T_u^**,T_v^**and T_w^**are provided to deadtime compensator 108.

[0043] In addition to receiving the duty cycle counts, compensator 108 also receives three current feedback signals I_uf, I_vfand I_wffrom the three current sensors 116, a separate current sensor 116 linked to each of three supply lines that provide power to load 112. Deadtime compensator 108 uses the current feedback signals to determine how to modify the command duty cycle signals T_u^**,T_v^**and T_w^**to compensate for deadtime effects in any manner well known in the art. Deadtime compensator 108 generates modified duty cycle signals T*_u, T*_vand T*_wwhich are provided to device dynamic compensator 110.

[0044] Referring still to FIG. 8, device dynamic compensator 110 receives the modified duty cycle signals T*_u, T*_vand T*_wand modifies those signals to provide compensated duty cycle signals T_u, T_vand T_w. Compensated duty cycle signals T_u, T_vand T_ware provided to PWM generator 111.

[0045] Generator 111, as described above, compares the compensated duty cycle signals T_u, T_v, T_wto a carrier signal and generates firing pulses that cause an inverter to generate high frequency voltage pulses on the three lines linked to load 112. The high frequency voltage pulses cause a fundamental low frequency alternating voltage at the frequency of the modulating waveforms and hence cause currents to pass through the load lines. The currents through the load lines are sensed, as indicated above, by sensors 116 and are provided back to compensators 108 and 110. As well known in the art, while three sensors 116 are illustrated, some configurations may only include two sensors for sensing the current in two of the three system phases and the two signals generated may be used to identify the third phase signal.

[0046] Referring to FIG. 9, an exemplary device dynamics compensator 110 is illustrated in more detail. Referring also to FIG. 8, compensator 110 includes three separator modules 120, 122 and 124, a separate compensator module 120, 122, 124 for each of the three phases U, V and W of the control system illustrated in FIG. 8. Each of the compensator modules 120, 122 and 124 is essentially identical to the other compensator modules and therefore only compensator module 120 will be described here in detail. Compensator module 120 includes a memory 128, a polarity discriminator 130, a single pole double throw switch 132 and a summer 134. Memory 128 is programmed to include first and second compensation values also referred to as upper and lower compensation values T_αUand T_αL, respectively. Upper value T_αUis negated before storing. Memory 128 is linked to switch 132 such that each of the upper and lower compensation values −T_αUand T_αL, respectively, are linked to separate ones of the inputs to switch 132. The output of switch 132 is provided as an input to summer 134. In addition, summer 134 also receives the U-phase modified duty cycle signal T_u^*and adds the modified duty cycle signal T_u^*and the value received from switch 132 to provide the compensated duty cycle signal T_u.

[0047] Referring still to FIG. 9, polarity discriminator 130 receives the U-phase current feedback signal I_ufand determines the polarity of the received signal. Thus, discriminator 130 provides an output signal that indicates that the received current feedback signal I_ufis either positive or negative. Where the feedback signal I_ufis positive, discriminator 130 links the output of switch 132 to a first of the switch inputs thereby passing the upper compensation value −T_αUto the output of switch 132 as the compensation value T_α. Similarly, where the feedback current I_ufis negative, discriminator 130 causes switch 132 to link the output of switch 132 to the second switch input thereby passing lower compensation value T_αLto summer 134. Thus, either upper compensation value −T_αuor lower compensation value T_αLis provided to summer 134 as a compensation signal and added to the modified duty cycle signal T_u^*to generate the compensated duty cycle signal T_u. As indicated above, each of modulators 122 and 124 operates in a fashion similar to that described above with respect to module 120, albeit using signals corresponding to the other two system phases.

[0048] Prior to explaining a commissioning procedure for identifying the T_αuand T_αLvalues a bit of theory is instructive. In the case of an ideal inverter driven by a commissioning current regulator having a current feedback loop, when excited using two different current commands I*_Land I*_H, the ratio of the current regulator output voltages V_H^*V_L^*be expressed as: $\begin{array}{cc}\frac{V_H^{*}}{V_L^{*}}=\frac{I_H}{I_L}& \text{Eq. 1}\end{array}$

[0049] As well known in the art, in reality, when a large current value is commanded, the resulting current regulator output voltage is very close to the ideal output voltage. Thus, in Equation 1, because both the high and low current values I*_Land I*_Hcan be commanded during a commissioning process and therefore are known and the high voltage V_H^*resulting from a high current is essentially ideal and measurable, the only unknown in Equation 1 is the ideal low voltage value V_L. Therefore Equation 1 can be reshuffled to yield Equation 2: $\begin{array}{cc}{V}_L^{*}=V_H^{*}\frac{I_L}{I_H}& \text{Eq. 2}\end{array}$

[0050] It is also true that, in reality, at low command current levels, the current regulator output voltage V_Ldistexceeds the ideal terminal voltage V_L^*by an amount equal to the parasitic distortion ΔV_Ldistthat occurs within the system such that:

[0051] V_Ldist=V_L^*+ΔV_Ldist  Eq. 3

[0052] Therefore, Equations 2 and 3 can be combined to provide equation 4 that can be solved to identify a parasitic distortion value ΔV_Ldist: $\begin{array}{cc}\Delta V_{\mathrm{Ldist}}=V_{\mathrm{Ldist}}-V_H^{*}\frac{I_L}{I_H}& \text{Eq. 4}\end{array}$

[0053] The distortion value ΔV_Ldistcan be converted into a duty cycle T_eby solving the following equation: $\begin{array}{cc}\frac{T_e}{T_s}=\frac{V_{\mathrm{Ldist}}}{V_{\mathrm{dc}}}& \text{Eq. 5}\end{array}$

[0054] where Ts is a carrier period and Vdc is a DC bus voltage.

[0055] In order to identify distinct T_αUand T_αLvalues and thereby account for asymmetries in the upper and lower portions of the inverter, the process described above is ideally performed twice, a first time with positive command currents and a second time with negative command currents. Where positive command currents are used to drive the system, the resulting T_evalue is the T_αUvalue and where negative currents are used the T_αLvalue results.

[0056] According to even a simpler commissioning procedure, the process described above may only be performed once using either positive or negative command currents and the resulting value may be used as both the T_αUand T_αLvalues. In this case the assumption is made that the variances between the upper and lower halves of the inverter are negligible.

[0057] Referring now to FIG. 13, an exemplary method 200 consistent with the discussion above for determining the upper and lower compensation values T_αUand T_αL, respectively, is illustrated. The method of FIG. 13 is an automated method that can be performed by a processor essentially without user input. However, it should be appreciated that a semi-automated method may also be employed at the factory or by the user where system inputs are adjusted and upper and lower compensation values identified when compensation results are sufficient for the user's purposes. Prior to performing the process illustrated in FIG. 13, a commissioning current regulator having a current feedback loop is linked to and for driving a system inverter during the commissioning process.

[0058] Referring also to FIGS. 8 and 9, at block 202 a counter N is set which enables a system processor to distinguish between commissioning steps corresponding identification of the upper compensation value T_αUand the lower compensation value T_αL. To this end, counter N is initially set equal to one.

[0059] Continuing, at block 204, counter N is compared to 1 and, where N is equal to 1 control passes to block 206 and blocks thereafter to determine upper value T_αU. Thus, because N is initially 1, control first passes to block 206. At block 206 a current regulator is commanded with a positive high DC current I*_H. For example, high current value I*_Hmay be anywhere within the range of 70 to 100% of the rated current value.

[0060] At block 208 the resulting high voltage V*_Hgenerated by the current regulator is sensed and the value V*_His stored. Next, at block 210 the regulator is commanded with a relatively low positive current I*_L. For example, the low current value may be anywhere within the range of 5 to 30% of the rated current value. At block 212 the regulator output voltage V_Ldistis sensed.

[0061] Continuing, at block 214 the V_Ldistvalue and the stored high voltage V*_Hvalue as well as the high and low current values I*_Hand I*_L, respectively, are plugged into Equation 4 above to identify the ΔV_Ldistvalue. Thereafter, at block 216 the ΔV_Ldistvalue is plugged into Equation 5 to identify the upper compensation value T_αU. Also, at block 216, the negative of T_αUvalue is stored for use during subsequent normal operations.

[0062] At block 218, counter N is incremented by 1 indicating that, from that point on, the commissioning steps are carried out to determine the lower compensation value T_αL. Thereafter control passes back up to block 204 where counter N is again compared to value 1. Here, because counter N has been incremented to 2, control passes to block 220 and blocks thereafter to determine lower value T_αL. At block 220 the regulator is driven with a negative high DC current I*_H. Again, the high current magnitude may be anywhere within the range of 70 to 100% of the rated current value.

[0063] At block 222 the resulting high voltage V*_Hgenerated by the current regulator is sensed and the value V*_His stored. Next, at block 224 the inverter is driven with a relatively low negative current I*_L. For example, the low current value may be anywhere within the range of 5 to 30% of the rated current value. At block 226 the regulator output voltage V_Ldistis sensed.

[0064] Continuing, at block 228 the V_Ldistvalue and the stored high voltage V*_Hvalue as well as the high and low current values I*_Hand I*_L, respectively, are plugged into Equation 4 above to identify the ΔV_Ldistvalue. Thereafter, at block 230 the ΔV_Ldistvalue is plugged into Equation 5 to identify the lower compensation value T_αL. Also, at block 230, the T_αLvalue is stored for use during subsequent normal operations. This completes the exemplary commissioning process.

[0065] Referring now to FIG. 12, a plot of U-phase and W-phase feedback currents I_ufand I_wfis illustrated that were created using the same system used to create the waveforms in FIG. 4, except that the device dynamic compensator 110 illustrated and described above was employed after a suitable commissioning procedure had been completed. Comparing FIGS. 4 and 12, it should be appreciated that the device dynamic compensator 110 reduced load current distortion appreciably.

[0066] Referring now to FIG. 10, a second embodiment 100aof the invention is illustrated. In FIG. 10, many of the components are essentially identical to those described with respect to FIG. 8, are identified by similar numbers and will not be described again here in detail. To this end, controller 102, modulator 104, sensors 116 and load 112 are similar to components described above. Deadtime compensator 108ais different than the deadtime compensator 108 described above in that compensator 108amodifies modulating signals as opposed to modifying duty cycles and generates modified modulating signals V_u^*,V_v^*and V_w^*that are provided to device dynamic compensator 110a. Similarly, generator 111ais different than generator 111 described above in that generator 111areceives and employs compensated modulating signals V_u, V_vand V_winstead of employing duty cycle signals to generate voltages and currents on the load supply lines.

[0067] Referring still to FIG. 10 and also to FIG. 11, device dynamic compensator 110ain FIG. 10 is illustrated in greater detail in FIG. 11. Compensator 110aincludes three compensator modules 250, 252 and 254. Again, each of modules 250, 252 and 254 is essentially identical in construction and operation and therefore, only module 250 will be described here in detail. Suffice it to say that a separate module 250, 252, and 254 is provide for each of the three separate phases in system 100ain FIG. 10.

[0068] Referring still to FIGS. 10 and 11, module 250 includes a memory 256, a single pole double throw switch 258, a gain module 260, a summer 262 and a polarity discriminator 264. Memory 256 is programmed with upper and lower device dynamic factors or compensation values −DDF_Uand DDF_L, respectively, the upper and lower values provided at the first and second inputs of switch 258, respectively. The output of switch 258 provides one of the upper or lower values DDF_Uor DDF_L, depending on switch 258 state, as a compensation value or a device dynamic factor DDF to gain module 260. Gain module 260 multiplies the current PWM frequency PWM_fby the DC bus voltage value V_dcand multiplies the result by the compensation value DDF and provides its output as a compensation voltage V_α to summer 262. Summer 262 also receives the modified U-phase modulating voltage V_u^*and adds the two received signals to provide the compensated U-phase modulating signal V_u.

[0069] As above, the polarity discriminator 264 receives the U-phase current feedback I_ufand controls switch 258 and a function of the polarity of that feedback signal. Where the feedback current signal I_ufis positive, discriminator 264 causes switch 258 to provide the upper value −DDF_Uat the switch output. Where the polarity of feedback current I_ufis negative, discriminator causes switch 258 to provide the low value DDF_Lat its output. Once again, modules 252 and 254 operate in a similar fashion to that described above with respect to module 250 except that modules 252 and 254 operate on the V and W phases, respectively, to generated compensated V and U-phase modulating signals V_vand V_w, respectively.

[0070] Referring again to FIG. 13, a portion of the commissioning procedure in FIG. 13 can be used to determine DDF_Uand DDF_Lvalues. To this end, at block 214, after value ΔV_Ldistis identified, the following equation can be solved to convert value ΔV_Ldistto a compensation value DDF_U: $\begin{array}{cc}{\mathrm{DDF}}_U=\frac{\Delta V_{\mathrm{Ldist}}}{\left({\mathrm{PWM}}_f\right)\left(V_{\mathrm{dc}}\right)}& \text{Eq. 6}\end{array}$

[0071] Thereafter, at block 216, value DDF_Uis stored and N is again incremented at block 218. Similarly, at block 228 Equation 6can be solved for value DDF_Land the resulting value stored for subsequent use. Experiments have shown that the results of a properly commissioned system constructed in accordance with FIGS. 10 and 11 are similar to that illustrated in FIG. 12.

[0072] It should be understood that the methods and apparatuses described above are only exemplary and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall under the scope of the invention. For example, in cases where precise compensation is not necessary the commissioning procedures described above could be performed at a factor for each type of system once and the resulting compensation values could then be programmed for all systems of the particular type. However, where precision is relatively important, commissioning should be performed within the field after a specific load has been linked to the system.

[0073] To apprise the public of the scope of this invention, the following claims are made.