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3  BRIDGETEST SYSTEMSUMMARY

TestSystemSummary
Presentation

& Summary
4 meg PDF

TEST SYSTEM DEVELOPMENT PRESENTATION & SUMMARY

This presentation & summary is a more detailed view of the test system development outlined in this TDA.  The flexibility of this test system both in hardware and programmability allows the characterization of a variety of single and three phase bridge modules as well as substrate type power circuits. Included is a brief discussion on the equation sets developed and used for data collection. Included are graphical comparisons of the PWM techniques such as standard to grounded phase and triplen injection PWM schemes.  Digital generation of the equation sets for three phase bridge circuits allows more control of load variations via current and or voltage feedback monitoring techniques that are easily implemented. The initial intent her was to test a device that was capable of driving an inductive load at 70 amps rms to simulate a field coil of a PMB DC motor, however, test results show that a 90 to 100 amps rms and beyond are feasible with standard cooling.

I am available to discuss the development of test systems and special instrumentation required to perform these test. I can be reached through the contact form, just put Sal (JT) in the remarks section of the contact form and your e-mail address.

PWM WAVEFORM GENERATION USED IN THE TEST SYSTEM

STANDARD THREE PHASE SINE WAVE EQUATION SET

Three Phase Sinewave equation set used to generate the PWM waveforms. Note that   is the 120 phase shift. If we observe the waveform we see that the maximum amplitude we can achieve from this is about 86% efficiency at any one point on the wave form. This translates to any phase being turned on only 86%. This is a loss in efficiency and the power would be dissipated in the power module creating heat problems.

      


img4.jpg

STANDARD THREE PHASE EQUATIONS+3rd HARMONIC

This equation set improves the efficiency of the maximum amplitude to almost 100% efficiency by adding a third harmonic, (triplen injection) to the equation set.  This make the assumption that the phases are balanced and the triplen cancels out between phases. In summary this technique allows the full dynamic range vertically to be utilized. However it does require an offset and a gain factor as shown in order to normalize the equation set.

      

   
img5.jpg

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DUTY CYCLE & GROUNDED PHASE CONDITIONING

After normalizing and setting the gain factor of the above equation set, the duty cycle for each point of the wave form is ready to be calculated digitally. A physical number is generated representing the number of counts for each of the phases the high FETs should be ON based on the physical point of the waveform. Since this is an H Bridge configuration, the lower FET would be ON the remaining of the duty cycle for the selected point in the period. A a dead time is also required between switching the upper FET ON and lower FET ON. The IF-THEN condition shows the relationship to the actual voltage from the equation set to the FET-ON duration for each phase.  This number DCxh, DCxl  is then incorporated into the BYTE table and hence the PWM wave form is generated for the three phases.

The GROUNDED PHASE Technique is taking the output amplitudes generated by the equation set and normalizing it to the phase that has the lowest amplitude. The graph below is the actual wave form table generated by the application program for a 45 %  Duty Cycle.  The CASE statements to generate this waveform from a Sinewave PWM is shown on the left. The waveform shown below is a grounded phase PWM sinewave plus a 3rd harmonic content. This is explained in the PMB motor analysis section.

DUTY  CYCLE  ALGORITHM  FOR  EACH  PHASE

The duty cycle of each phase is referenced to the actual amplitude for the phase equations and then calculated to fit the number of points in the period. The default value is 1000 points per PWM period per waveform point. The default playback clock rate is 25.00MHz, 50 nanoseconds. This defaults to a 13 millisecond sine wave period for 260,000 byte playback memory size, (1000 pulses per vertical resolution per point 260 points per period). This allows for a 0.1% default duty cycle resolution per waveform point.  The number of points for the PWM period is programmable as well as the number of points for the waveform period.

    IF Va = 0 THEN
            DCah = 0
            DCal = INT(PERIOD)
    ELSE
            DCah = ABS(INT((Vab(PT) * PERIOD * DtyCyc) + .5))
            DCal = INT(PERIOD - DCah)
    END IF

    IF Vb = 0 THEN
            DCbh = 0
            DCbl = INT(PERIOD)
    ELSE
            DCbh = ABS(INT((Vbc(PT) * PERIOD * DtyCyc) + .5))
            DCbl = INT(PERIOD - DCbh)
    END IF

    IF Vc = 0 THEN
            DCch = 0
            DCcl = INT(PERIOD)
    ELSE
            DCch = ABS(INT((Vca(PT) * PERIOD * DtyCyc) + .5))
            DCcl = INT(PERIOD - DCch)
    END IF

GROUNDED PHASE ALGORITHM  FOR  EACH  PHASE

    SELECT CASE Vab
          CASE IS > Vbc           ' Vca & Vbc = 0 test
              IF Vbc > Vca THEN
                    Vgnd = Vca
                    Vc = 0   
                    Vab = Vab - Vgnd
                    Vbc = Vbc - Vgnd
              ELSE
                    Vgnd = Vbc
                    Vbc = 0
                    Vab = Vab - Vgnd
                    Vca = Vca - Vgnd
              END IF

        CASE IS > Vca                   ' Vbc & Vca = 0 test
              IF Vbc < Vca THEN
                    Vgnd = Vbc
                    Vbc = 0
                    Vab = Vab - Vgnd
                    Vca = Vca - Vgnd
              ELSE
                    Vgnd = Vca
                    Vca = 0
                    Vab = Vab - Vgnd
                    Vbc = Vbc - Vgnd
              END IF
        CASE ELSE               ' Vab = 0  test
                    Vgnd = Vab
                    Vab = 0
                    Vbc = Vbc - Vgnd
                    Vca = Vca - Vgnd
    END SELECT

Sine Grounded Phase
MathCAD Generated 3  Equation set
img6.jpg

Triplen Grounded Phase
MathCAD Generated 3 Equation set
after Gain and Offset are applied
img9.jpg

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Sine Grounded Phase BRIDGE DRIVE
Test Program PWM Data Table
img7.jpg

 Triplen Grounded Phase BRIDGE DRIVE
Test Program PWM Data Table
generated from the program
img10.jpg

Sine Grounded Phase BRIDGE DRIVE ON/OFF TIMES
MathCAD Generated Analysis from Data Table
img8.jpg

TRIPLEN Grounded Phase BRIDGE DRIVE ON/OFF TIMES
MathCAD Generated Analysis from Data Table
img11.jpg

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0 TO 100 amps rms Phase Current
26 Milli-ohm 40킾 Phase to Phase Load
Empirical Data Table,
Phase Current = phase x 20 amps
img5.jpg

0 TO 100 amps rms Phase Current
26 Milli-ohm 40킾 Phase to Phase Load
img4.jpg

Delta and WYE Load configuration
Phase to Phase DC Resistance in milli-ohms at 40킾
A - B             B - C            C - A
26.22            26.26            26.54
img6.jpg

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Empirical data, 0 TO 100 AMP rms  3   Current into a  Delta configuration,
  26 Milli-ohm 40킾 Phase to Phase Load,  Phases A, B & C  Balanced Output Graph
  img7.jpg

90 AMP rms 3 Current into the  Delta Load Configuration,
 26 Milli-ohm 40킾 Phase to Phase Load of Empirical data

images/ph90rms.gif

Comparison of the Sine and Sine + 3rd Harmonic additive
Phase Current vs Supply Current of Empirical data
 images/current1.gif

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THERMAL DISTRIBUTION OF THE MOUNTING MECHANISM FOR A CERAMIC SUBSTRATE,
PHASE A OF THE POWER MODULE WITH 60 AMPS RMS TO THE LOAD,
36 SECONDS ON, 180 SECONDS OFF DUTY CYCLE FOR A 50 MINUTE TEST PERIOD
img2.jpg

ONE CYCLE 30 MINUTES INTO THE RUN,
THERMAL DISTRIBUTION OF THE MOUNTING MECHANISM FOR A CERAMIC SUBSTRATE,
PHASE A OF THE POWER MODULE WITH 60 AMPS RMS TO THE LOAD,
36 SECONDS ON, 180 SECONDS OFF DUTY CYCLE FOR A 50 MINUTE TEST PERIOD
img3.jpg

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