Download Microcontroller based PWM Inverter for Speed Control of a Three

Microcontroller based PWM Inverter for
Speed Control of a Three Phase Induction
Motor
M. A. Latif1, M. J. Alam2, M. A. Rashid*3, A. Karim3, N. H. Ramly3, I. Daut3
1
Dhaka Water Supply Authority (WASA), Dhaka, Bangladesh.
Department of Electrical & Electronic Engineering, BUET, Dhaka, Bangladesh.
3
School of Electrical Systems Engineering, University Malaysia Perlis (UniMAP), Malaysia.
*E-mail:[email protected]
2
ABSTRACT
Three phase induction motor has proven to be an extremely reliable electromechanical energy conversion device
for over 100 years. The speed control of induction motor is a crying need for the real world industrial applications.
However, there are so many options available for the precise speed control of induction motor except by changing
the frequency. Therefore to achieve the goal of speed control of induction motor, there is no alternative of inverters.
With the availability of high speed power semiconductor devices, the three phase inverters play the key role for
variable speed ac motor drives. In addition to the speed control, the inverter can also provide some unique features,
like voltage control, torque control, power factor correction, auto breaking, built in protection system and so forth.
In this paper, a three phases PWM inverter using MC3PHAC microcontroller with computer interface is proposed
to run a squirrel case induction motor. Some results of the proposed inverter are presented.
Keywords: Induction motor, Inverter, Speed control, PWM, MC3PHAC microcontroller
1. INTRODUCTION
Industrial drives are predom inantly ac motors of induction type. An estimated 67 percents of ac motors are
induction motors, whereas dc motors occupy only 8 percent s of t he industrial drives [1]. Induction motors are
relatively cheap, simple in construction and can be used in hostile environment [2]. The widespread proliferation
of power electronics and ancillary control circuits into motor control system in the past two or three decades have
led to a situation where motor drives, which process about two third of the worlds electric power into mechanical
power, are on the threshold of processing all of t he power via power electronics. The days of driving motors
directly from the fixed ac or dc mains via mechanical adjustment are over. The ever increasing demand for greater
productivity and higher quality of most of the industrial products that we used in our everyday lives means that all
aspect of dynamic response and accuracy of motor drives have to be inc reased. Issue of energy efficiency and
harmonics proliferation into the supply grid are also increasingly affecting the choices for motor drives circuitry
[3].
Induction motor with squirrel-case rotors are the workhorses of industry because of their low c ost and
rugged construction [4-5]. Speed co ntrol of i nduction motor is a cr ying need i n industrial application. By
changing the frequency, speed of induction motor can be controlled precisely. Therefore, to achieve the goal of
speed control of induction motor there is no alternative of inverters. Moreover, inverter not only control the speed
of induction motor but it has also some unique features like voltage control, torque control, power factor
correction, auto breaking, built in protection system etc. The core of a power electronic apparatus consists of a
converter built on a matrix of power sem iconductor switching devices that works under the guidance of control
electronics [6-8].
With the availability of high speed power semiconductor devices, the three phase inverter play the key role
for variable speed ac motor drives. In this paper, a Microcontroller based three phase Pulse Width Modulation
(PWM) inverter with necessary control circuits to run a three phase squirrel-case induction motor is presented.
Some results and constructed circuits [9] are also presented.
The paper is organized as follows. The Section 2 s hows the three phase VSI topology. The Section 3
describes the algorithm of control system. The Section 4 highlights some experimental results and finally some
concluding remarks are drawn in the Section 5.
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2. THREE PHASE VSI TOPOLOGY
N
2
1
S3
S5
Phase A
a
Phase B
b
1
D5
2
D3
1
D1
2
1
+
2
S1
C+
2
+
1
Vi/2
1
1
2
The standard three phase voltage source inverters (VSI) is shown in fig. 1. It should be noted that the switches
of any leg of the inverter (S1 and S 4, S3 and S6, or S5 and S 2) can not be switched on simultaneously because
these would result in a short circuit across the dc supply voltage.
Phase C
c
2
1
2
S6
S2
2
D6
1
2
D4
D2
1
S4
C-
2
+
1
Vi/2
1
1
2
2
Vi
Fig. 1: Three-phase VSI topology
The six valid switch states that produce non-zero ac output voltage of three phase inverter is shown in table 1. In
order to generate a gi ven voltage waveform, the inverter moves from one state to another. The re sultant ac
output line voltages consist of discrete values of voltages that are vi, 0, and –vi for the topology shown in Fig. 1.
Table 1: Valid switch states for a three phase voltage source inverter.
Switch State
State
No.
Vab
Vbc
Vca
Space Vector
S1, S2 and S6 are on and S4, S5 and
S3 are off.
1
vi
0
-vi
V1=1+j0.577
S2, S3 and S1 are on and S5, S6 and
S4 are off.
2
0
vi
-vi
V2=j1.155
S3, S4 and S2 are on and S6, S1 and
S5 are off.
3
-vi
vi
0
V3=-1+j0.577
S4, S5 and S3 are on and S1, S2 and
S6 are off.
4
-vi
0
vi
V4=-1-j0.577
S5, S6 and S4 are on and S2, S3 and
S1 are off.
5
0
-vi
vi
V5=-j1.155
S6, S1 and S5 are on and S3, S4 and
S2 are off.
6
vi
-vi
0
V6=1-j0.577
The selection of states in order to generate the given waveform is done by the modulating technique that ensures
the use of only the valid states. In order to produce 1200 out of phase load voltages, three modulating signals that
are 1200 out of phase are used. Fig. 2 shows the ideal waveforms of three phase VSI SPWM.
625
Vcb
Vca
Vcc
ωt
180
0
360
0
(a)
S1
ωt
1800
S2
360
0
(b)
ωt
180
Vab
0
360
0
(c)
ωt
(d)
Fig. 2: Three phase VSI ideal waveform for the SPWM, (a) carrier and modulating signals, (b) switch S1 state, (c) switch S2 state and (d) ac
output voltage.
3. CONTROL SYSTEM
The block diagram of proposed inverter is shown in Fig. 3. The main parts of the inverter are power part, optocoupler, control part and computer interfacing.
DC
Input
PWM
Signal
Optocoupler
PWM Gate
Signal
Power
Part
Control
Signal
Control
Part
3 Phase
Induction
Motor
Computer
Interface
Fig. 3: Block diagram of proposed inverter.
626
3.1 Power Part
The main component of the power part is the power MOSFETs. Six free wheeling diodes one with each
MOSFET is used for the bypass of back e.m.f. The schematic diagram of the power part is shown in Fig. 4. The
input dc voltage of the power part is supplied from a three phase rectifier. We need four separate grounds for the
operation of inverter three for upper MOSFETs (one for each) and one for lower M OSFETs along with si x
PWM signals. We make these grounds available along with respective gate signals through opto coupler.
2
BY329X 1200
D1
1
11N80
Phase C
11N80
2
c
D1
1
Q2
BY329X 1200
2
BY329X 1200
11N80
2
D1
BY329X 1200
Q6
Q5
Phase B
b
1
2
BY329X 1200
1
11N80
D1
D1
1
Phase A
a
Q4
Q3
11N80
2
D1
1
11N80
Q1
BY329X 1200
+ ve
- ve
g
Fig. 4: Schematic of power part of inverter.
3.2 Opto-Coupler
The main objective of opto-coupler is to make an electrical isolation between high voltage power part and low
voltage control part. Opto-coupler also performs the following two important functions:
(a) Increase the amplitude of PWM signal to desired level keeping the signal width and phase same.
(b) Manage the four separate grounds with respective PWM signal for the operation of power MOSFET.
The schematic of a single channel of opto-coupler part is shown if Fig. 5.
+5V_CS
+12V_PS
R13(1.8K)
R25(4.7K)
74HC00
A_T
2
Q1
2N2222A
3
6N136
1
2
R19(100)
R7(100)
A_T - PS
3
U1A
1
R1(10K)
G (A_T - PS)
Fig. 5: Schematic of a single channel of Opto-coupler.
3.3 Control Part
The main component of the control part is the microcontroller MC3PHAC. The main objective of usi ng
microcontroller is to produce 3-phase PWM waveform. We can configure this microcontroller in two modes viz.
host mode or computer mode and stand alone mode. In stand alone mode MC3PHAC can be c ontrolled with
switches and can be configured with variable resistors. But in host mode or computer mode we can control
MC3PHAC with software and can configure it with the software. Also in computer mode we can easily measure
the various values like actual speed, f requency, modulation index, bus voltage etc. directly. In t his work we
configure MC3PHAC as host mode or computer mode. The configuration of MC3PHAC in host mode is shown
in Fig. 6.
627
4.7k
+ 5 V_A
X1
4MHz
A
C10
0.1uF
10k
+ 5 V_A
A_T
A_B
B_T
B_B
C_T
C_B
R7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
D1N4148
D7
R12
1
D8
D1N4148
9PIN CONNECTOR
U1
4N35
4.7k
R4
5.6k
+ 5 V_D
R8 10k
C9
0.1uF
Ce(9)
10uf
nc
R9
1k
LED2 Fault
R10
5
+ 5 V_D
1k
2
DTR
Ce(com)
2.2uF,35v
D9
D1N4148
A
R13
4.7k
6
TxD
RTS
RxD
4
6
GND
4
5
9
4
8
3
7
2
6
1
1k
28
27
26
25
24
23
22
21
20
19
18
17
16
15
MC3PHAC
10uf
R6 10k
Ce(8)
0.1uF,50v
R5
C8
R3
2
R11
5
1
4N35
U2
RS232
330
Fig. 6: Schematic of control part of inverter.
3.4 Computer Interface
The circuit shown in Fig. 7 is th e schematic of a h alf duplex opto-isolated RS232 interface. The isolated
terminal interface provides a margin of safety between the motor control system and a pers onal computer. To
send data from a PC to the MC3PHA C microcontroller, it is neces sary to satisfy the serial input of the
MC3PHAC. In the idle condition, the serial input of the MC3PHAC must be at logic 1. To sen d data from
MC3PHAC to PC’s serial port input, it is necessary to satisfy the PC’s receive data (RxD) input requirements.
For the satisfy of the serial inputs of the MC3PHAC and the RxD input to the PC, transistors U1 and U2 are
used.
D1N4148
D7
R12
1
D8
D1N4148
9PIN CONNECTOR
5
2
R10
1k
RXD
(To PIN 16)
DTR
Ce(com)
2.2uF,35v
D9
D1N4148
G
R13
4.7k
6
TxD
RTS
RxD
4
6
GND
4
5
9
4
8
3
7
2
6
1
1k
U1
4N35
5
RS232
(From PIN 17)
2
TXD
1
4N35
U2
+ 5 V_D
R11
330
Fig. 7: Opto-isolated RS232 interface.
3.5 Power Supply for Control Part
The schematic for power supp ly to the control part is sh own in Fig. 8. It is designed with a full wave bridge
along with a 5 volts regulator IC MC7805. The specialty of this circuit is th at we can g et two type of power
supply from this circuit. One is 5 vol ts digital power supply and a nother is 5 vol ts analog power supply.
MC3PHAC is a mixed signal IC. It has an analog portion and a digital portion. Digital power supply is required
for digital portion and analog power supply is required for analog portion, which include internal clock
generation circuit, phase-look loop (PLL) and analog to digital converter. The digital power supply is labeled as
+5V_D and analog power supply is labeled as +5 V_A.
628
D5
1N4004
1
+5V_D
D1
D2
MC7805
1
1N4004
12-15V
DC or AC
Input
1
1N4004
IN
1000uF/50V
1N4004
.1uF
D4
3
1 ohm
C16 +
C14
D3
OUT
+5V_A
R1
GND
C15
C5
C11
.1uF
.1 uF
.1 uF
C11
.1 uF
U5 2
1N4004
G
Fig. 8: Schematic for power supply circuit.
4. RESULTS AND DISCUSIONS
In experiment, a three-phase 415 volts 1 H.P. squirrel case induction motor was used as the load. For controlling
the proposed inverter in host mode or co mputer mode, special software called Free Master or P C master
software was used. For controlling the MC3PHAC IC a sp ecial demo version of Free Master o r PC master
software called “M C3PHAC Motor Control Demo” was us ed. Almost all the functions required to control a
three phase induction motor viz. start/stop, reverse/forward run, acc eleration (Hz/sec), selection of base
frequency, PWM frequency, actual frequency, dead time, modulation index etc. are available in this demo
version. Additionally, a built in oscilloscope is available in this demo version.
The PWM signal and output voltage waveform of proposed inverter is shown in Fig. 9.
Inputs
Outputs
Inputs
Outputs
Fig. 9: PWM signal at the input and output of opto-coupler.
Some experimental results of the proposed inverter for speed control of induction motor are shown in Fig.
10. This oscilloscope shows the real ti me graphical representation of v arious quantities like actual frequency,
commanded frequency, modulation index, bus voltage etc.
629
Initial
f=50H
20Hz/Sec
f=120Hz
f=60Hz
Reverse run
20Hz/Sec
f=50Hz
f=20Hz
Fig. 10: Oscilloscope view of speed response of proposed inverter.
5. CONCLUSION
The main objective of this project work is to design and construct a three phase SPWM inverter controlled by
microcontroller and necessar y control circuit to run a three phase sq uirrel case i nduction motor. From the
theoretical and practical results the following conclusions can be made:
1. A good argument between the experimental setup and t heoretical model suggested that the constructed
inverter is accurate enough.
2. Variation of frequency and thus the speed of the motor can be controlled smoothly; the frequency can set any
value between 0 Hz to 127 Hz.
3. Motor acceleration or deceleration can be cont rolled from 0.5 Hz/Se c. to 128 Hz/Sec. and can c hange the
direction of motor at any time.
4. The constructed inverter is equally applicable for 50 Hz and 60 Hz base frequency.
5. It is possible to change the modulation index and voltage boost both in online and off line therefore it is
possible to control the output voltage.
6. PWM frequency can be changed at four presentable values viz. 5.3 KHz, 10.6 KHz, 15.9 KHz and 21.1 KHz
and can be changed at any time.
7. It is possible to adjust PWM polarity and can adjust the dead time at any vale between 0 to 31875 ns.
8. It is possible to monitor the dc bus voltage of the inverter.
9. There is the facility of resistive breaking, fault monitoring etc. in the proposed circuit.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
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M. A. Choudhury, “An analy sis of Delta modulated inverters with a pplications to su bmersible motors”, Ph.D. thesis, Faculty of
engineering and applied science, Memorial University of New Foundland, Canada, December, 1988.
M. H. Rashid, Editor-in-chief, “Power Electronics Handbook”, Academic Press, 2001.
Nagarath I J, Kothari D P, “Electric Machines”, 2/E, Tata McGraw-Hill Publishing Co mpany Limited, New Delhi, ISBN 0-07463285-X, India, 1997.
P. C. Sen, “Principles of Electric Machines and Power Electronics”, 2/E, John Wiley & Sons, Inc, ISBN 0-471-02295-0, USA, 1997.
B. K. Bose (Editor), “Modern Power Electronics”, IEEE Press, Piscataway, NJ, 1992, pp. 3-39,
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