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Power Electronics Laboratory Practice TE00AD57

Lab Exercise #9: 1-phase PWM inverter

Work instruction

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Introduction

This laboratory work provides a walkthrough of the commonly used pulse-width-

modulation inverter. This inverter provides a variable frequency and voltage output from

a DC source, and is a key component of a frequency converter that is used in variable

speed AC motor drives (VSD). The work is done with the LabVolt training system.

Setup and connections

Operating voltage and data connections

Connect the 24VAC operating voltage to the modules:

DACI

Chopper / Inverter Connect USB cable(s) from the module(s) to the computer Control and measurement connections (miniature banana plug leads)

To From Function

DACI analog input 1 DACI analog output 1 duty cycle control

IGBT Switching control

inputs 1 & 2

DACI Analog inputs 2 & 3 switching control

observation

IGBT ground (white) DACI analog ground (white) common ground

Main circuit Set up the circuit shown in Figure 1. Use two branches of the IGBT module, the two 8 mH inductors and the 1,5 F capacitor of the Filtering Inductors/Capacitors module. Use an external 300 resistive load (ensure max power is not exceeded during measurements). As adjustable voltage source, use the power supply DC output (adjustable 0-200VDC). Adjust to 50V DC.

Figure 1

R

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Main circuit measurements

Apply following voltage measurements:

supply DC voltage (Fluke multimeter)

IGBT output voltage. Measure this with both DACI (E1) and real oscilloscope

(Ch1).

Load resistor (filtered) voltage (real oscilloscope Ch2)

Part I: the function of a four-quadrant chopper In this part of the exercise, you will control the duty cycle of the electronic switches of a four-quadrant chopper using a dc voltage, while observing the voltage at the chopper output. Setup Start the LVDAC-EMS software.

Chopper / Inverter control settings

Parameter/ function Value clarification

function (op. mode) Four-Quadrant Chopper

Switching Frequency 2000 Hz

Command Input AI-1 (Analog Input 1) Duty cycle

control

Acceleration and deceleration

time

0.0 s

Q1, Q2, Q4 and Q5 parameters PWM

Switching control observations Switch on the DC supply (50V) and start the four-quadrant chopper. With LVDAC-EMS Oscilloscope use separate channels to display:

1. the duty-cycle control voltage (AI-1) 2. the switching control signals of electronic switches Q1, Q5, (AI-2) 3. the switching control signals of electronic switches Q2, Q4 (AI-3) 4. the voltage at the inverter output before filtering (E1).

Adjust oscilloscope settings to be able to observe the waveforms. Record oscilloscope image for your report. Setup of the duty cycle control knob For the measurements, set-up a control knob that can be used to adjust the chopper duty cycle ().

Analog Output 1 window settings

These settings will allow you to produce a voltage variable between -10 V and +10 V at Analog

Output 1 (currently set to -10 V). This voltage is used to control the duty-cycle of the electronic

switches in the four-quadrant chopper.

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Parameter/ function Value

Function Command Button

Command Name Voltage

Max Command 10

Voltage Corresponding to Max Command 10

Min Command -10

Voltage Corresponding to Min Command -10

Command Step 2

Voltage -10

Measurement: In the Analog Output 1 window, rotate the control knob so that the duty-cycle control voltage varies from -10 V to +10 V and backwards. While doing this, observe the voltage (before and after filtering) and current at the four-quadrant chopper output, as well as the waveforms of the switching control signals on the Oscilloscope display. Do the voltage and current at the four-quadrant chopper output correspond to an ac waveform, i.e., varying from a polarity to the other? Successively set the duty-cycle control voltage to -10V, -8V, -6V, , 8V, 10V. For each chopper duty cycle value ( = [0, 10%, , 100%] ), measure (with the real oscilloscope):

knob position

the duty cycle (measure with the measurement function) chopper input voltage

the average voltage at the four-quadrant chopper output

the average voltage at the load (resistor)

In your report: chopper operating principle

1.1 based on the LVDAC-EMS Oscilloscope images (IGBT control) and main circuit diagram, explain how the four-quadrant chopper operates (i.e. generates adjustable DC output x V to +x V)

1.2 explain the term duty cycle analysis of the measurements

1.3 compare the average (DC) voltage of the chopper output before and after the filter 1.4 describe the relationship between the duty-cycle control voltage and the average DC

voltage at the four-quadrant chopper output

1.5 insert oscilloscope images (set time so that 2-3 duty cycles are visible) with unfiltered (Ch1) and filtered (Ch2) output voltage with duty cycles 70%, 50% and 10%

Once you have recorded all measurements and oscilloscope images, you can stop the voltage source and the four-quadrant chopper. Same main circuit and measurement points will be used in part II.

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Part II: Single-phase PWM inverter using a four-quadrant chopper In this part of the exercise, you will control the duty cycle of the electronic switches of a four-quadrant chopper using a alternating duty-cycle control voltage while observing the voltage at the chopper output. setup

Analog Output 1 window settings

These settings will produce a sinusoidal voltage varying slowly between -10 V

and +10 V at Analog Output 1.

Parameter/ function Value

Function Function Generator

Waveform Sine

Frequency 1 Hz

Amplitude 10 V

measurement Switch on the DC supply (50V) and start the four-quadrant chopper. Observe the chopper output voltage at the load (after filtering) on the Oscilloscope display. Describe what happens. Record an oscilloscope image that illustrates the phenomenon for your report. Then, in the Analog Output 1 window, gradually increase the frequency of the duty cycle up to 10 Hz while observing the load resistor voltage at the four-quadrant chopper output on the Oscilloscope display. What you have is a PWM frequency converter! Next tune the output AC frequency to 50 Hz.

In your report: 2.1 present with oscilloscope image 50Hz unfiltered and filtered output voltage, and explain how you can make AC voltage from a DC source with help of controllable switches (how the PWM frequency converter works). Same setup will be used in part III.

Part III: switching frequency measurement With the 50Hz inverter output frequency, alter now the switching frequency (Chopper / Inverter control, initially set to 2kHz) and study the impact to output voltage level and output voltage sine wave form. What happens if you decrease the switching frequency below 2000Hz? What happens if you increase the switching frequency? Record the output waveforms with 3-4 different switching frequencies that illustrate your observations. Next change the output (modulation sine wave) frequency from 50Hz to 5Hz and use the same earlier selected 3-4 switching frequencies. Check and record the waveforms.

In your report: 3.1 explain the impact of switching frequency to the output voltage level 3.2 present with oscilloscope graphs the impact of the switching frequency to the output voltage waveform

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3.3 based on your observations and theory, what kind of considerations one should make to select optimal switching frequency in a PWM inverter Same main circuit and measurement points will be used in part IV.

Part IV: Relationship between output voltage, input voltage, and modulation index

On frequency converters, it is necessary to vary the output frequency, but also the output voltage. In this section you will study different means to alter the output voltage (filtered AC voltage RMS value). Input DC voltage adjustments: Continuing from the previous measurements using a sinusoidal PWM AC converter at 50Hz, study the the relation between the input and output voltages. measurement alter the input voltage from the adjustable DC power supply with suitable steps between 20V- 100V, and measure the filtered output voltage (Upeak and Urms).

In your report: 4.1 based on your measurements and theory explain the relation between the input (DC) and

output (AC) voltages Modulation index adjustments: The modulation index m refers to the relative value of the modulation sine wave amplitude. With the current setups sine wave amplitude 10V the duty cycle output control voltage varies (sine function) between -10V and 10V and that causes the duty cycle variation respectively from 0 to 100%. In this case the modulation index m = 1. If the modulation sive wave amplitude is decreased to 8V, the duty cycle will vary between 10%-

90% (min=10% and max=90%), and the modulation index would be =

100%=

90%10%

100%=

0,8. measurement With 50VDC input adjust the modulation index between 0 1 and measure the output load resistor AC rms voltage. Also if possible (PIT TESTATA VIEL) see what happens if you increase the modulation index over 1 (overmodulation)

In your report: 4.2 based on your measurements and theory explain the relation between the input (DC)

voltage, modulation index and output (AC) voltage 4.3 explain what happens with overmodulation. In what kind of situations overmodulation could

be used? What problems overmodulation causes? Once you have completed the measurements, stop the voltage source and the single-phase PWM inverter. Close LVDAC-EMS, turn off all equipment, and remove all leads and cables.