STATIC POWER INVERTERS A. PREPARATION 1. Introduction 2. Variable Speed AC Drive 3. High Efficiency DC Supplies 4. Induction Heating 5. Conversion of DC Power to AC Power at the Terminus of a High Voltage DC Transmission Line 6. Characteristics of Some Switching Devices a. Bipolar Transistors b. Thyristors c. MOSFET Devices 7. Inverter Circuit Used in this Exercise 8. Bibliography B. EXPERIMENT 1. Equipment List 2. Procedure a. General Instructions b. Principles of Circuit Operation c. No Load Frequency Response d. Efficiency and Frequency Response with Load C. REPORT Static Inverters -- 1
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B. EXPERIMENT 1. Equipment List. 1 oscilloscope with 2 10 probes? assorted meters 1 inverter 1 temperature probe? any other equipment as may be required by 2. Procedure a. General Instructions. Make connections to the inverter with power off. Although the device cannot be harmed by an excessively large input signal, a short circuit, even momentary, either across the load or from one side of load to ground, can destroy the field effect output transistors. Note also that a purely inductive load is a DC short circuit, and since the inverter is capable of a DC output, care must be taken with a purely inductive load to insure that the input waveform has zero DC component. The inverter can also be damaged if DC input power is applied with reversed polarity. Notice also that the reset button must be pressed each time the unit is powered up at DC. b. Principles of Circuit Operation. Have the instructor set the DC power level to 60 V; NEITHER side of the DC supply should be grounded! With the DC wall switch off, connect the DC outlet to the inverter input: OBSERVE CORRECT POLARITY. Connect scope channels 1 and 2, respectively, to outputs Γ and Γ of the FET bridge. Measure the FET temperatures #. Connect the line cord on the unit to a 120 VAC outlet and power up. With no load and no AC input, turn on the DC power and observe the two bridge waveforms. They should be complementary square waves. At no load, observe all features of the square waves and record one square wave. Now apply an extremely low frequency AC input (0.5 Hz) from the function generator; and observe the bridge output voltages. Vary the amplitude and frequency of this input; and note the effect on the bridge square waves. Monitor the temperature occasionally and note any changes. USE NO THERMAL COMPOUND. Static Inverters -- 25
c. No Load Frequency Response. With the setup of Part 2b, reposition the oscilloscope probes across the output of the inverter; be sure that the power is TURNED OFF while you make this change. In this section you will use the differential voltage feature of the scope to measure the voltage gain at no load, both at DC and as the frequency is swept manually from 10-10000 Hz. Using a sinusoidal drive signal swept manually from 10-10000 Hz, measure the variation of the rms input while the output is held steady, first at 40 V pk-pk and then at 80 V pk-pk, balanced about zero. If it becomes necessary to alter connections to the inverter, first TURN IT OFF. BEWARE of potentially destructive conditions as you tune through resonance(s) of the output filter: carefully control the input voltage to maintain the desired output waveform d. Efficiency and Frequency Response with Load. Measure the current, voltage, and power from the DC supply into the inverter; observe that this might require you to have some notion of the waveforms of these variables. Then determine power in from DC supply versus power out to load, both at DC and at suitably spaced drive frequencies from 20 Hz to 2000 Hz ; use the internal floating load resistances of 100, 50, and 25 ohms. During the determinations at each load, the output should be set as high as is consistent with the three following constraints: (a) the output voltage waveform should never be grossly distorted; (b) the load voltage should not exceed 80 V pk-pk ; (c) the DC component of the output should (except for DC drive) be zero. Additionally, set the load resistance to 25 Ω and take data sufficient to determine the efficiency at various peak-peak load voltages for DC and for the drive frequencies from 20, 200, and 2000 Hz. # Thou shalt not use that gooey white thermal compound in this experiment! Optional for extra debit: devastate your lab grade by devastating the inverter. Static Inverters -- 26
C. REPORT. a. Present and explain your data on the drive voltages at Γ and Γ. b. On the same sheet, plot curves of no-load gain vs. frequency at half-peak output voltage and at full output voltage. Comment cogently. c. On the same sheet, plot curves of maximum voltage out vs. frequency for 100, 50, and 25-Ω loads. Comment cogently. d. On the same sheet, plot curves of efficiency vs. load voltage at DC, and selected appropriate frequencies. Ignore the control power from the 120 VAC line and the power from the sine wave driver. Be sure to describe and justify & in minute detail PRECISELY how you determined P in and P out. e. How might the design of this experiment be improved? & Lest there be any doubt in your minds, justification is an essential part of design. Reason rather than Whimsy should motivate the tradeoffs which you make. Static Inverters -- 27