SCHOOL OF ENGINEERING (ELECTRICAL AND ELECTRONIC) EEE8075 – ELECTRICAL POWER AND CONTROL PROJECT
FLEXIBLE INVERTER DESIGN, TESTING, AND SIMULATION
Abstract: Research and experimentation work carried out on flexible inverter is presented in the following report which is briefly divided into two part, flexible inverter testing and simulation. An inverter was constructed and tested for various modes namely PWM pulse test, three-phase sine wave generator, and BLDC motor control. A simulation of the system was done on Matlab/Simulink and the results were included and discussed. Finally, a detailed flowchart of the system working, covering both software and hardware, is also included in the report.
Lecturer: Dr Mohamed Dahidah First Name: Abdul Rehman Surname: Mohammad Hafeez Student ID: 140456282 Submission Date: 08-Jan-2015
1
Introduction .............................................................................................................................. 1
2
Experimentation ........................................................................................................................ 2
3
2.1
Test 1 – PWM Pulse Test .................................................................................................... 2
2.2
Test 2 – Three Phase Sine-Wave Generator ........................................................................ 3
2.3
Test 3 – Brushless DC Motor Control .................................................................................. 4
Simulation ................................................................................................................................. 5 3.1
Variable DC Supply ............................................................................................................. 6
3.1.1
Effect of Varying Reference Signal Value on the Output Current ................................. 7
3.1.2
Effect of Varying Triangle Carrier Wave Frequency on the Output Current .................. 7
3.1.3
Effect of Varying Load Inductance on the Output Current ........................................... 8
3.2
Variable AC Supply ............................................................................................................. 8
3.2.1
Effect of Varying Reference (Sine Wave) Frequency on the Output Current ................ 9
3.2.2
Effect of Varying Triangle Carrier Wave Frequency on the Output Current .................. 9
3.2.3
Effect of Varying the Load Inductance Wave Frequency on the Output Current ........ 10
4
Conclusion ............................................................................................................................... 10
5
Appendix ................................................................................................................................. 11 5.1
Task 6 – Hardware Based Flowchart ................................................................................. 11
5.2
Task 6 – Software Based Flowchart .................................................................................. 12
5.3
Additional Results ............................................................................................................ 13
5.3.1 6
Test 1 ....................................................................................................................... 13
References............................................................................................................................... 16
i
Figure 1.1 MOSFET ............................................................................................................................ 1 Figure 1.2 Three phase inverter ......................................................................................................... 1 Figure 1.3 BLDC motor connected to an inverter ............................................................................... 1 Figure 2.1 PWM with marked gate on voltage, time on and off, dead time ........................................ 2 Figure 2.2 Sinusoidal current plotted on excel ................................................................................... 3 Figure 2.3 Frequency range of the inverter (Max-left : Min-right) ...................................................... 3 Figure 2.4 PWM Voltage and Frequency ............................................................................................ 3 Figure 2.5 DC Motor operational speed range (Max-Left : Min-Right) ................................................ 4 Figure 2.6 Phase A voltage and Duty cycle at 2000RPM ..................................................................... 4 Figure 2.7 Duty cycle at maximum speed ........................................................................................... 4 Figure 3.1 Output Current Waveform for Standard DC Model ............................................................ 6 Figure 3.2 Output Current Waveform for different duty cycles .......................................................... 7 Figure 3.3 Output Current Waveform for different carrier frequencies .............................................. 7 Figure 3.4 Output Current Waveform for different inductances ......................................................... 8 Figure 3.5 Output Current Waveform for Standard AC Model ............................................................ 8 Figure 3.6 Output Current Waveform for different reference signal frequency .................................. 9 Figure 3.7 Output Current Waveform for different carrier frequencies .............................................. 9 Figure 3.8 Output Current Waveform for different inductances ....................................................... 10
Table 1-1 Test 1 PWM Pulse Test Results ........................................................................................... 2 Table 1-2 Hall Sensor Sequence ......................................................................................................... 5 Table 2-1 Specifications of standard model........................................................................................ 5 Table 2-2 Steady state current (D=0.5)............................................................................................... 6
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An inverter is a power electronics device that converts DC (Direct Current) to AC (Alternating Current). The input of an inverter relies on a DC source which is then converter into a square wave, modified sine wave, pulsed sine wave, or sine wave depending on the circuit design. The power electronics components used in an inverter can be either IGBT or MOSFET, which requires a control signal to switch on and off. This control signal is supplied with the help of a gate driver and a controller. For constructing our inverter device, MOSFETs are used. MOSFETs are the workhorse in power electronics at low power. They are switched on and off by applying the gate (Figure 1.1) with sufficient charge. A PIC (Programmable Interrupt Controller) is used to control the MOSFETs which works on the principle of Pulse Width Modulation (PWM). PWM is the process of modifying the width of the pul ses in a pulse drain in direct proportion to a control signal. A comparator is used which Figure 1.1 MOSFET compares the carrier wave with the control signal (reference) and the results of this is used to control the MOSFETs. The PIC chip produces the control signal which is a constant and a si ne wave resulting in a variable DC output and AC output respectively. A three phase inverter (Figure 1.2) utilizes three pairs (six) of MOSFETS and each PWM pulse is used to drive one pair of MOSFET, therefore controlling one leg of the inverter. Another important aspect to consider is the inductive nature of the wirings and components used in the circuit. As the MOSFETS will be switching, the current in these devices are in fact switching between Figure 1.2 Three phase inverter zero and a finite value. The inductance in the circuit does not allow this to happen instantaneously but requires a short duration to allow the current to switch on and off. Therefore, to ensure that the both MOSFETS in a pair are not switched on at the same time, a small dead time is kept with the help of a gate driver, to allow some time for the current to rise/fall, which otherwise would cause short circuit and damage the device. Another important function of the gate drive circuit is to provide an isolation between dc-link voltage and controller voltage, therefore acting as a protection against short circuit.
Figure 1.3 BLDC motor connected to an inverter
In addition to converting DC to AC, inverter is also used to control a BLDC (Brushless DC) motor (Figure 2.2). A BLDC motor relies on electrical commutation in contrast to traditional mechanical commutation. Electronic commutation is controlled by the rotor position, which is sensed by 3 Hall Effect Sensors positioned at 0°, 120° and 240° around the air gap of the motor. These sensors provide three bit code to the controller, which controls the switching pattern of the MOSFETs. A steady state speed is achieved when the motor torque and load torque equali ze.
1
This test was carried out to ensure proper working of the system by generating three pairs of fixed duty cycle PWM pulses.
Figure 2.1 PWM with marked gate on voltage, time on and off, dead time
Methodology used for obtaining the results is summarized below. (All the points mentioned refer to figure above) 1. Gate On voltage for all pairs obtained by measuring the amplitude of each PWM wave. (Point A) 2. Time on (point B) and time off (point C) obtained by using cursors. 3. PWM frequency and duty cycle acquired by using the “meas” option on the oscilloscope. 4. Dead time obtained by using cursors (Point E). Using the methodology above, the results for all MOSFET pairs were obtained and are tabulated below. Table 2-1 Test 1 PWM Pulse Test Results1
Terminal
J1
J2
J3
J4
J5
J6
Gate on voltage
4.9V
4.8V
4.9V
4.8V
4.9V
4.8V
Ton
12 μs
86 μs
44.2 μs
53.8 μs
24.4 μs
73.8 μs
Toff
87.2 μs
12.94 μs
55.2 μs
45 μs
74.8 μs
25.2 μs
PWM frequency
10.092 kHz
10.095 kHz
10.095 kHz
10.095 kHz
10.095 kHz
10.095 kHz
PWM duty cycle
12.22%
82.96%
44.72%
54.44%
24.74%
74.45%
Dead time
408 ns
The results show that this experiment was conducted successfully as three pai rs of PWM pluses were produced with fixed duty cycle. Moreover, the presence of a dead time show that gate driver is working properly and therefore safety against short circuit is ensured.
1
All the results obtained from the oscilloscope are placed in the appendix for reference
2
During this test, the inverter was operated to produce a 3-phase variable frequency current which was then supplied to a star connected 3 phase load. The potentiometer was used to change the frequency of the sinusoidal current. The connected load had a resistance of 47Ω and an inductance of 3.3mH per phase. Using the data obtained from the oscilloscope and analyzing it on excel, the voltage drop across the resistor was calculated. As the resistance is known, ohms law was used to calculate the current across it.
Current Waveform 0.15 ) 0.1 s p m0.05 A ( 0 t n e 0 r r-0.05 u C -0.1
0.05
-0.15
0.1
0.15
0.2
0.25
Time (Seconds) Figure 2.2 Sinusoidal current plotted on excel
As can be seen from Figure 2.2, the current follows a sinusoidal wave pattern, an AC output, which indicates that the inverter is working as desired. From Figure 2.2, following can be deduced Peak Current(iPK)=0.124 A
√
RMS Current(iRMS)= = 0.0878A
Power Dissipated(P)=iRMS2R=0.362W
Subsequent to testing the inverter three phase sinusoidal generation, it was required to find the frequency range of the inverter. It was f ound by varying the potentiometer. Setting the potentiometer at minimum gave the maximum frequency and vice versa.
Figure 2.3 Frequency range of the inverter (Max-left : Min-right)
The frequency range of the inverter lies between: 1.6703Hz < Frequency < 19.59Hz. Following this, the PWM voltage and frequency was obtained using the oscilloscope.
Figure 2.4 PWM Voltage and Frequency
The figure above shows that the frequency of the PWM voltage = 10.13 kHz, which is same as the triangular carrier wave frequency set by the PIC chip. Moreover, the amplitude of the PWM is 24V which matches with the voltage input from the DC supply. Obtaining correct results for this test show that the inverter is successfully producing a variable sinusoidal output.
3
The inverter is designed to control a brushless DC motor. To ensure that it can perform this function correctly, another test was conducted. A 3 phase 8 pole Brushless DC motor was connected to the inverter as a load and a DC input was supplied. This resulted in the rotation of the DC motor which showed that the inverter was working as desired. Moreover, the potentiometer controls the duty cycle of the PWM, therefore varying it altered the average voltage and current supplied to the motor which in turn controlled the speed of the motor. The motor rotational speed was calculated using one of the Hall sensor signals, taking into account that this is an 8-pole motor. To find the motor operational speed range of the inverter, the potentiometer was kept at its extremes and the output frequency achieved was recorded using oscilloscope, as shown bel ow
Figure 2.5 DC Motor operational speed range (Max-Left : Min-Right)
Maximum motor operational speed
=
120× 120 × 171.6 = = 2574 8
Minimum motor operational speed
=
1 2 0 × 120 × 5.336 = = 80.04 8
It was then required to rotate the motor at 2000RPM and acquire the phase A voltage and duty cycle. To rotate the motor at 2000RPM requires a frequency:
=
×
=
×
= 133.33
From Figure 2.6, it can be seen that rotating the DC motor at 133.33Hz (2000RPM) resulted in phase A voltage of 25V and a duty cycle of 58%.
From Figure 2.7, it can be seen that at maximum speed the duty cycle has increased to 60%. This indicates that varying the potentiometer changes the duty cycle, which means that it not only alters the frequency but also the average voltage and current supplied to the DC motor. This in turn varies the speed of commutation in the brushless DC motor which directly affects the speed of rotation of the motor.
Figure 2.6 Phase A voltage and Duty cycle at 2000RPM
Figure 2.7 Duty cycle at maximum speed
4
Using the oscilloscope, traces of H1 and H2 and then H1 and H3 were exported to Microsoft® excel and then processed to obtain the 3-bit hal l sequence. Table 2-2 Hall Sensor Sequence
H1
H2
H3
Angle
1
0
0
0o
1
1
0
600
0
1
0
1200
0
1
1
1800
0
0
1
2400
1
0
1
3000
1
0
0
3600 = 00
From the obtained hall sensor sequence, it is clear that the sensors are working correctly as the hall they switch on and off in the correct sequence. H2 switches on after H1 and H3 switches on after H2 and this sequence repeats itself over and over again. Obtaining desired results from this test show that the inverter is able to run and control the bladeless DC motor. The signal from the hall sensor is able to control the power electronic devices which in turn switches on and off appropriately to result in a positive torque. As the potentiometer controls the average current supplied to the motor, it therefore acts as a torque controller as torque is proportional to the current.
A model of the inverter was simulated 2 for a variable DC and variable AC supply connected to a firstorder load (RL load). Different parameters of the model were changed in order to analyze their effect on the output. A standard model, with the specifications shown in Table 3-1, was created and only one parameter was varied against others at a time, so to see the effect of each parameter on the output. Table 3-1 Specifications of standard model
2
Parameter
Variable DC Supply
Variable AC Supply
Carrier Wave
Triangular wave (f=20kHz, A=0→1)
Triangular wave (f=10kHz, A= −1→1)
Reference Signal
Constant (D = 0.5) = 50% duty cycle
Sine Wave (f=50Hz, A=1)
Resistance
R = 47Ω
Inductance
L = 3.3 mH
Simulation was done on Matlab/Simulink
5
Using the standard specifications, as mentioned in Table 3-1, following current waveform was obtained
Figure 3.1 Output Current Waveform for Standard DC Model
By setting the reference signal to 0.5, a duty cycle of 50% is achieved. From Figure 3.1, it is clear that the current is constantly varying, however its average value increases from zero to a constant value of 0.2553 A. The relational operator (comparator) constantly compares the carrier wave (triangular wave) with the constant. With the reference signal set to 0.5, the comparator gives an output signal of 1 when “∆-wave < 0.5” and a zero otherwise. An output signal of 1 means that 24 volts are supplied and zero means no voltage is supplied to the load, causing the load current to rise and fall. The presence of an inductor does not allow rapid change in current, as a result the current rises and falls gradually which can be seen from the triangular nature of the waveform above. The average value of current obtained using simulation equals 0.2553A (as shown in Figure 3.1). It can be further verified mathematically as shown below: Table 3-2 Steady state current (D=0.5)
Given: Vmax = 24V, D = 0.5, T = 0.00005s (20 kHz)
Vavg = D x V max => 0.5 x 24 = 12V Using KCL, Vavg = Ri + L
Method 1
Method 2
Using basic integration => Steady state current means
iavg =
=
= 0.2553
=0
i(t)=
−
[ (0) +
]
It can be assumed that the current reaches a steady state value after 4 time constants, where
τ = . Substituting the values gives i(t) = 0.25A ≈ 0.255A
Value of variation above average obtained from the simulation equals i max – i avg = 0.3002 – 0.2553 = 0.0449A. Some applications require higher values of current and some require l ower variations above average. In order to meet the requirements, certain parameters of the system are varied. To see the effect of each parameter on the output, following simulations were done.
6
Changing the value of reference signal changes the duty cycle of the PWM, as a result the average voltage and current changes. This can be verified from the results obtained using the simulation.
Figure 3.2 Output Current Waveform for different duty cycles
From Figure 3.2, it is clear that increasing the duty cycle increases the output current. It is so because the average voltage supplied to the l oad increases as the duty cycle increases and so does the average current. At 100% duty cycle, the comparator only produces an output signal of 1 because ∆-wave < 1. As a result, voltage supplied is in form of a step input instead of a PWM. This results in a smooth rise in the current output with no ripples, as can be seen from Figure 3.2. Therefore, to increase the current in the RL circuit, the duty cycle (reference signal) can be increased.
A further analysis was carried out to see the effect of varying the carrier frequency. For this purpose, the carrier frequency was first increased from 20 kHz to 40 kHz and then decreased to 10 kH z.
Figure 3.3 Output Current Waveform for different carrier frequencies
From the figure above, it can be seen that at higher carrier frequency the variation above average is significantly lesser than at lower carrier frequency. It is so because, at higher frequency the voltage (PWM) is switching faster as the time period has decreased (T =
). This implies that the rising and
falling time for current has decreased and therefore the max and min current ripple has reduced respectively. This is opposite at lower frequency and therefore variation above average has increased when the carrier wave frequency is set at 10 kHz. Moreover, it is clear that varying the carrier wave frequency does not affect the steady state curren t and the time constant.
7
To visualize the effect of varying the load inductance on the output current, its value was first increased from 3.3mH to 33.3mH and then decreased to 0.33mH.
Figure 3.4 Output Current Waveform for different inductances
When the load inductance is larger i.e. 33.3mH, the current not only takes longer to reach a steady state value but also has significantly less variation above average. This is opposite when a lower inductance value is used. It is so because, time constant is directly proportional to the inductance ( τ
= ),
when a larger inductance is used the time constant increases and the current takes longer to
reach its steady state value. Moreover, as
=
the variation above average or the ripple
current decreases when a larger inductance value is used. Furthermore, it is clear from Figure 3.4 that the value of inductance has no effect on the steady state current.
To get an AC output, the triangular carrier wave is compared against a sine wave signal instead of a constant. This results in a voltage PWM output with constantly changing duty cycle, which when supplied to RL load produces an output current in form of a sine wave as shown below.
Figure 3.5 Output Current Waveform for Standard AC Model
Keeping the standard values for variable AC supply, shown in Table 3-1, the figure above was obtained. The PWM voltage supply has the highest duty cycle when the sine wave is maximum (positive amplitude) which then increases till the sine wave reaches its negative amplitude. The average value of the output current remains same as earlier i.e. 0.2553A.
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Figure 3.6 Output Current Waveform for different reference signal frequency
From figure above it can be seen that changing the sine wave frequency (reference signal) changes the current output frequency, however the switching frequency remai ns unchanged. It is so because, when the sine wave frequency is increased (100Hz), the fundamental frequency of the output increases and therefore, the frequency modulation factor decreases, as
=
. Therefore, the
fundamental frequency depends on the sine wave frequency and the switching frequency depends on the carrier frequency. Hence setting the sine wave ( reference) at desired frequency will result the first harmonic at the same frequency. However, it should b e noted that the sine wave frequency does not affect the average output current and the variation above average (ripple current).
Figure 3.7 Output Current Waveform for different carrier frequencies
It can be observed from Figure 3.7 that increasing the triangular carrier wave frequency results in a smoother sine wave output current. This effect is the same as for variable DC supply, as discussed earlier.
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Figure 3.8 Output Current Waveform for different inductances
The figure above shows that increasing the load inductance results in less variation above average and also slower time response i.e. current takes longer to complete its sine wave cycle. Both of these effects are same as for the variable DC supply as discussed earlier.
The inverter designed is able to produce a PWM wi th fixed duty cycle, a variable AC output, and control a BLDC motor, therefore acting as a flexibl e inverter. The experiment results clearly indicate that PIC, gate driver, and MOSFETs are working as desired in addition to all other passive and active components used. Moreover, results from the simulation matches with the experiment, e.g. using the potentiometer to vary the duty cycle gave similar effect in compari son to when done in simulation. A conclusion as follows can be drawn from the simulation results. 1. Output current/power is directly proportional to the duty cycle i.e. reference signal value. 2. Ripple or variation above average is inversely proportional to the carrier wave frequency. 3. Ripple or variation above average is inversely proportional to the load inductance. Therefore, for a higher output increase the duty cycle; for a smoother output (less variation) either increase the carrier wave frequency or increase the load inductance value. It should be noted that only an inductive load has been used through the experiment and the simulation, therefore to verify the above conclusions under different load conditions (e.g. Capacitive load), further experimentation should be carried out. Finally, it can be concluded that all the objectives of this project were fulfilled.
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Start
Power Supply (24 VDC) Regulate Power Supply for PIC, Gate driver, MOSFET
10kHz -Carrier Wave
Variable DC Output Mode
Various operating modes
BLDC Control Mode
Variable AC Output Mode Constant Reference Signal
Sinusoidal Reference Signal
Hall Sequence obtained from the motor
Comparator
Comparator
Comparator
Potentiometer varies frequency
Potentiometer varies duty cycle (Speed of the motor)
Constant duty cycle PWM pulses
Variable duty cycle PWM pulses
PWM pulses to control the phases
Gate Driver
Gate Driver
Gate Driver
MOSFET
MOSFET
MOSFET
Supply To Motor
Control the phases of the motor (stator windings)
Output with fixed duty cycle
Sinusoidal output
Motor rotates clockwise or anticlockwise Next hall sequence and speed of the motor
End
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Start Initialize all the variables
Obtain the hall sequence from the sensors
Potentiometer controls the Duty Cycle of the PWM (Speed of the motor)
Control MOSFETs based on the hall sequence (angle) received from the sensors
If angle equals
0 degrees
60 degrees
120 degrees
180 degrees
240 degrees
300 degrees
Phase A –Positive Phase B –Negative Phase C-Zero
Phase A –Positive Phase B –Zero Phase C-Negative
Phase A –Zero Phase B –Positive Phase C-Negative
Phase A –Negative Phase B –Positive Phase C –Zero
Phase A –Negative Phase B –Zero Phase C-Positive
Phase A –Zero Phase B –Negative Phase C-Positive
Apply to respective phase windings on the stator
Yes
Current Angle=0 degrees?
Yes
Motor is rotating clockwise
Yes
Previous angle=300?
No
Motor is rotating anticlockwise
No
Motor is rotating clockwise
Yes
Current Angle>Previous angle?
No
Motor is rotating anticlockwise
Maintain average RPM
Power Supply on?
No
End
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J1 and J2 Frequency, duty cycle, and gate on voltage
J1 on and off time repectively
J2 on and off time respectively
13
J3 and J4 Frequency, duty cycle, and gate on voltage
J3 on and off time repectively
J4 on and off time respectively
14
J5 and J6 Frequency and duty cycle
J5 on and off time repectively
J6 on and off time respectively
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Electric Drives - Brushless DC and Reluctance Motors - Description and Applications. 2015 [ONLINE] Available at: http://www.mpoweruk.com/motorsbrushless.htm. [Accessed 02 January 2015]. Mpoweruk.com, (2015). Electric Drives - Brushless DC and Reluctance Motors - Description and Applications. [online] Available at: http://www.mpoweruk.com/motorsbrushless.htm [Accessed 2 Jan. 2015]. Ecmweb.com, (2012). Effects of Harmonics on Power Systems | Power Quality content from Electrical Construction & Maintenance (EC&M) Magazine. [online] Available at: http://ecmweb.com/powerquality/effects-harmonics-power-systems [Accessed 30 Dec. 2014]. Ewh.ieee.org, (2015). PULSE WIDTH MODULATED INVERTER. [online] Available at: http://www.ewh.ieee.org/soc/es/Nov1998/08/PWMINV.HTM [Accessed 30 Dec. 2014].
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