Inveter _ Project Manual

O r e g o n I n s t i t u t e o f T e c h n o l o g y

Three-Phase Inverter – Lab Manual Tai Huynh

Spring 14

08 Fall

2

REE 413

Electric Power Conversion Systems

Oregon Tech

Three-Phase Square Wave Inverter

BEFORE STARTING…

Due See Class Schedule

Prerequisite EE 419 or similar

Expected Knowledge Basic knowledge of power electronics

Basic knowledge of LTSpice

Basic knowledge of electronic test

equipment

Basic knowledge of Arduino

programming

Test Equipment Lab Computers with LTSpice

DC Power Supply

Oscilloscope

Four Voltage Probes

A622 Current Probe (in the black lab cabinet)

Digital Multi-meter

Materials (per group) Breadboard (36V, 2A Max) and Jumper Wires

Three P-Channel MOSFET (Digikey IRF9640PBF-ND)

Three N-Channel MOSFET (Digikey IRF530PBF-ND)

Six Opto-isolator (Digikey 4N37-ND)

Arduino Uno Development Board

6ft USB Cable

Various capacitors, inductors, diodes, LEDs, and resistors

References The class textbook

http://www.bristolwatch.com/ele/arduino_power_inverter.htm

An Arduino PWM Website: http://www.arcfn.com/2009/07/secrets-of-arduino-pwm.html

Datasheets from Digikey and Jameco.

http://www.bristolwatch.com/ele/arduino_power_inverter.htm

http://www.arcfn.com/2009/07/secrets-of-arduino-pwm.html

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OBJECTIVES

To design, simulate, build, and analyze a three phase, DC to AC inverter.

To gain a better understanding MOSFET gate control.

EXPERIMENT

Introduction

An inverter is used to convert DC to AC power. Inverters are used in many

applications like solar PV, wind turbines, vehicle 120 VAC power supplies, and UPSs.

Various switching schemes are used with inverters, but this lab focus on a full bridge,

active, 60 Hz six-step three-phase inverter. Figure 1 shows a generic full-bridge three-

phase inverter from pp. 374 in the textbook.

Figure 1 - Full bridge three-phase inverter schematic.

The control strategy, often called a switching scheme, needs to match the majority

of the motors and generators used in industry, thus the phases are 120 º apart from each

other. Every switch has a 50 % duty cycle and are 60 º out of phase from each other.

Active switches allow the full control, and differently from a single-phase inverter, three

switches conduct at a time. However, both configurations share the same limitations.

Note that switches S1 and S4 close and open opposite of each other, as do switch pairs

(S3, S6) along with (S3, S6). As with the single-phase inverter, these switch pairs must

coordinate so they are not closed at the same time, which would result in a short circuit

across the source. With this scheme, the instantaneous voltages vA0, vB0, and vC0 are

+Vdc or zero, and line-to-line output voltages vAB, vBC, and vCA are +Vdc, 0, or -Vdc.

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This scheme brings with it one big advantage: changing the switching frequency

can control the output frequency. However, the magnitude of the output voltage depends

on the value of the dc supply voltage. Thus to control the output voltage of a six-step

inverter, the dc input voltage must be adjusted. The expected output is shown in Figure 2.

Figure 2 - Expected output waveforms. Note the voltages are line-to-neutral.

Design, simulate, build, and analyze a three-phase inverter that will provide

12 VAC rms to a resistive and inductive load, with a 12 VDC voltage source.

Basic Calculations

See sections 8.15 and example 8-12 in the textbook for equations and examples.

1. Download the datasheets for the particular components used in this lab, and

become familiar with the MOSFETS, opto-isolators, and Arduino development

board. Pay special attention to the opto-isolator terminals. Terminal 6 (transistor

base) does not need to be connected to anything.

2. Calculate the maximum and minimum current in the load like in example 8-12

and create a similar table for the Fourier series terms.

3. Calculate the load current THD, as shown in example 8-12.

4. Calculate what the current THD would be if the inductance were 5, 50, and 500

mH. Discuss why changing the load inductance has such an effect on the output

current harmonics in the final lab report.

5. Sum the power at each harmonic level, and find the total power over the load.

6. Find the rms current in the load by back-solving the power and current

relationship:

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7. Can the components handle the current, voltage, and power levels previously

calculated? Remember to compare with the thermal resistance rating using

equation 10-16.

NOTE: You can use MATLAB or a calculator to calculate the junction to

ambient air max power dissipation for the MOSFET according to example

10.5 and 10.6 in the textbook for how to do this.

8. Why or why not a heat sink for this design is needed?

LTSpice Simulation

Students who are not familiar with LTSpice, must go through Tutorial #1 in

the “Beginner’s Guide to LTSpice,” found on Blackboard, making sure to complete

the entire “simulations” section, including DC, transient, and AC analysis of the

circuit.

1. Create an LTSpice model of the inverter, using Figure 3 below.

Figure 3 - LTSpice schematic for the circuit.

2. For the gate drivers simulation, use pulse voltage sources connected to 4N27

opto-isolators since we cannot simulate the Arduino output in LTSpice, Figure 4.

Figure 4 - Gate driver schematic for: a) P-type MOSFET, and b) N-type MOSFET

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NOTE: each gate driver needs different parameters for the pulse voltage

sources depending on whether the MOSFET is an N Channel or P Channel,

and what parts of the switching sequence it is in. Table 1 below shows a

typical voltage set up in LTSpice for the gate drivers

Table 1. LTSpice gate driver voltage source set up

Vinit Von Tdelay Trise Tfall Ton Tper

Gate 1 5 0 0 0.1u 0.1u Tper/2 16.67m

Gate 2 5 0 Tper/6+Tdead 0.1u 0.1u Tper /2-2Tdead 16.67m

Gate 3 5 0 Tper/3 0.1u 0.1u Tper/2 16.67m

Gate 4 5 0 Tper /2+Tdead 0.1u 0.1u Tper /2-2Tdead 16.67m

Gate 5 0 5 Tper /6 0.1u 0.1u Tper/2 16.67m

Gate 6 0 5 Tper /3-Tdead 0.1u 0.1u Tper /2+2Tdead 16.67m

In the table, Tdead is the desired dead time that can be reasonably chosen. See

step 7 in the Experimental Circuit section for more reference.

3. Connect the 100 resistor in series with the internal diode of the opto-isolator to

limit the current from the Arduino output to around 30 mA.

4. Connect an LED and the 2 K resistor between the gate and ground of the P-

MOSFETs, and between the gate and the voltage source for the N-MOSFETs.

5. Connect a 150 μF capacitor at the output of the opto-isolator to reduce the falling

and rising time of the output signal, so that the pulses fed into the gate of the

MOSFET will be nearly perfect square.

6. Use the actual components provided in the lab and simulate the output current

THD from the inverter.

NOTE: For this simulation, place a spice directive as follows in the

schematic: “.four 60 I(R1)” (for 60 Hz fundamental, current through the

load resistor, which may not be “R1”) then run the simulation and go to the

View menu, and Spice Error Log to see the results.

7. In LTSpice, check what the current THD would be if the inductance were 5, 50,

and 500 mH. Compare answers with the calculations section.

8. Compare the rms current and average power dissipated over the load with the

previous calculations section.

9. Check the average transistor heat dissipation and make sure it won’t need a heat

sink in the build (hold “ALT” and left click over the transistor to plot the power

dissipation, then hold “CTRL” and left click over the name to get the average

power).

10. Take plenty of screen shots for the final report.

Experimental Circuit

1. Download the Arduino Code provided on Blackboard, check the outputs with an

oscilloscope, and make sure the switching sequence is working properly.

NOTE: Set the oscilloscope channels to DC Coupling, or the readings will be

incorrect and note you can only measure four from the six different signals.

2. Use the cursor function on the scope, and decrease the time scale to see the dead

time between switch 1 and 4. It should be somewhat close to the dead time set in

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the Arduino. Make sure to record the actual dead time on the scope, and make

sure that will work with the delay, rise and fall time of the switches.

NOTE:

a. A good start point is 100 μs that can be changed later, after testing it

with the load.

b. To change the dead time in the provided Arduino script, go to

“#define DEAD_TIME” and change the value in the parenthesis. The

lowest dead time is 47 μs corresponding to the value of 1.

3. Build the circuit on a breadboard, without the load connected. Note that the

ground from the Arduino is different from the power supply ground. Make sure to

do the correct connections.

a. Use the LED and 2 K resistor in series to each output of the opto-

isolators to see it turning on and off correctly. Try using one color of

LEDs for switches 1, 3 and 5, and another color of LEDs for switches 4, 6

and 2. This way it will be easier to tell what the switching sequence is.

b. Use the “#define INTERVAL” variable in the code to make the switching

sequence viewable with the naked eye. Modify the number in the

parenthesis to larger values, 3600 is a good starting point.

4. Once the LEDs and switching scheme seems to be working properly;

a. Remove the LEDs from the circuit. The 2 KΩ resistors will be connected

between the gate and ground. If the LEDs are left in the circuit, the

isolators may not be able to provide enough power to the gates since they

can only deliver 100 mA. The MOSFETs may not be driven properly,

causing them to overheat and smoke out of the box.

5. Remember to reconfigure your switching frequency before testing the circuit.

6. Turn the power supply off and limit current output to 0.75 A before connecting

the load to the circuit.

7. Connect two channels from the oscilloscope to monitor the dead time between

switch 1 and 4. Find an appropriate dead time that will ensure one switch is not on

at the same time as the other (5 μs to 200 μs are some typical values, but 100 μs is

a good starting place).

NOTE: The dead time will act differently between loaded and unloaded

conditions, so it is always a good idea to leave some buffer room for dead

time.

8. Once the dead time is set, disconnect the two probes from the gates, then use three

probes to measure three phase voltage output at the load.

NOTE:

a. Since the load is Y-connected, it is ok to connect negative terminal of

the probes to the common node of three loads, and positive terminal

of the probes go to each phase.

b. The output might have some spikes. It is due to the characteristic of

the inductive load. To terminate those voltage spikes, just simply

remove the inductors from the load.

9. Connect a Digital Multimeter to one phase of the load for measuring the rms line-

to-neutral voltage. Depending on the dead time and because of the rms value

characteristics, the output voltage may not be close to 12 V.

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10. Gradually increase the voltage of the Power Supply until the Digital Multimeter

reads 12 V at the output. The typical range will be from 14 to 17 V.

11. Connect the A622 current probe on another channel of the scope, and monitor the

output current in the load, making sure to configure the channel for current

readings at 100mV / A (for higher resolution), and that the probe itself is set this

way. Also, make sure that it has a 9 V battery in it and that it is turned on.

NOTE: Make sure to turn this probe off, once the measuring is completed.

There is no auto-off functionality on these probes!

12. The following picture shows what the probe looks like:

13. Show the instructor the final working circuit.

14. Record the current and voltage over the load with the scope, and record the

current being drawn from the power supply.

15. Finally, check the current (not voltage) harmonic frequencies and levels on the

scope by pressing the math button, operation, and FFT to see the real time values

(on the current-probe channel).

NOTE: Please see page 61- 68 in the TPS 2014 user’s manual (on

Blackboard) for details on how to use the FFT function on the scope.

16. Turn the Vertical Scale knob to ensure that the entire waveform shows up on

the screen, before going to the FFT screen. It may display erroneous FFT

results if the entire waveform is not visible on the Time Domain screen for a

couple cycles. Try doing an “auto-set” before going to the FFT screen.

17. Once on the FFT screen, turn the horizontal knob to change the frequency range

in view. 50 Hz per division should show the current harmonics properly.

18. Convert the y-axis (shown in dB) by hand to current (in amps) with the following

process (the scope does not convert it) and discuss these findings as compared to

the theoretical and simulated harmonic levels.

a. The vertical scale is most likely in 10 dB per division increments (0 dB

equals 1 Vrms measured voltage), shown on the bottom left hand side of the

screen for the channel that the current probe is on.

b. Record the level in dB.

c. Use the following equation to solve for measured Vrms

i. dB = 20log10(A/Aref)

Where A is the measured amplitude in units of milliAmps rms and

Aref is the reference amplitude (assumed to be 1 mA rms).

ii. As an example of how to calculate the inverse log of 1.25, simply

raise 10 to that power (i.e. 10^1.25, which returns 17.7828).

iii. The following is an example equation, using 50 dB as the

fundamental frequency amplitude, shown on the scope:

, A being in units of milliAmps

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iv. See this link for more details http://www.ni.com/white-

paper/4278/en/, under the “Converting to Logarithmic Units”

section

Don’t forget to take plenty of pictures for the final lab report!

19. Develop a full lab report

a. Please include an introduction and separate sections on all phases of the

design (i.e. the calculation phase, LTSpice, and actual resulting data, with

pictures from the Oscilloscope and the final circuit) for the Experimental

Circuit of the lab, with any problems or discrepancies between the design

phases, equations used in the design, and all plots, m-files, and screen

shots of the pertinent data.

b. Also, include a table at the end comparing all the important data values for

all three phases, which should include the peak to peak output voltage, the

rms output voltage, the total output power, the peak to peak current in the

load, and the rms current.

Deliverables

A hard copy of the full lab report, including:

a. A title page with the lab title, class, due date, group names, and term

b. Items described in task 4

c. 7 to 12 pages in length (including all of the discussion points, plots, tables and

pictures), double spaced, 12 point font, Times New Roman, standard report

format (NOT IEEE)

d. REMEMBER: Professional presentation goes a long way!

http://www.ni.com/white-paper/4278/en/

http://www.ni.com/white-paper/4278/en/

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APPENDIX

Figure 5 - LTSpice final line-to-neutral voltage output.

Figure 6 - Oscilloscope final line-to-neutral voltage output

Figure 7 - Switching scheme

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Figure 8 - Sequence test

Figure 9 - Deadtime between switches

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Figure 10 - Final built circuit

Figure 11 - RMS output voltage

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Figure 12 - The circuit, three phase output and rms value

Arduino Code

//REE 413

//Electric Power Conversion Systems

//Oregon Tech

//The Three Phase Square Wave Inverter

#include <TimerOne.h>

#define ch1High (12)

#define ch1Low (9)


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#define ch2Low (8)


#define ch3Low (7)

#define MOSFET_ON LOW

#define MOSFET_OFF HIGH

// we divide 360 degree (1 sin wave cycle (~16,667us) into 360 parts)

// 16,667 / 360 = 47 us

// cycle time in us

#define CYCLE_TIME (47)

// interval = 360*47us = 16680 us ~ 60Hz

#define INTERVAL (360)

// 1/2 deadtime in multiple of 47us

#define DEAD_TIME (1)

#define DUTY_CYCLE ((INTERVAL - 4*DEAD_TIME)/2)

// define on and off point

#define TIME_POINT_HIGH_ON (DEAD_TIME)

#define TIME_POINT_HIGH_OFF (TIME_POINT_HIGH_ON + DUTY_CYCLE)

#define TIME_POINT_LOW_ON (TIME_POINT_HIGH_OFF + 2*DEAD_TIME)

#define TIME_POINT_LOW_OFF (TIME_POINT_LOW_ON + DUTY_CYCLE)

int counter_ch1 = 0,

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counter_ch2 = INTERVAL/3,

counter_ch3 = 2*INTERVAL/3;

void setup()

{

Serial.begin(9600);

pinMode(ch1High,OUTPUT);

pinMode(ch1Low,OUTPUT);





Timer1.initialize(CYCLE_TIME);

Timer1.attachInterrupt(timer1Int);

}

void increase_counter(int *cnt){

if(*cnt == INTERVAL){

*cnt = 0;

}else{

*cnt += 1;

}

}

//timer1 interupt

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void timer1Int(void) //cycleTime us

{

increase_counter(&counter_ch1);



}

void control_channel(int ch, int counter, int time_point_on, int time_point_off)

{

if((counter > time_point_on) && (counter < time_point_off)){

digitalWrite(ch, MOSFET_ON);

}else{

digitalWrite(ch, MOSFET_OFF);

}

}

void loop()

{

control_channel(ch1High, counter_ch1, TIME_POINT_HIGH_ON, TIME_POINT_HIGH_OFF);

control_channel(ch1Low, counter_ch1, TIME_POINT_LOW_ON, TIME_POINT_LOW_OFF);





}

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