Application research on three-phase inverter with space-vector pulse modulation*

Hong-li LYU

Department of Information Engineering，Tangshan College，Tangshan 063000，China

1.Introduction

The technology of pulse width modulation has been used in the inverters widely．In recent years，voltage three-phase inverter control with voltage space vector PWM control technique has been used extensively and has a good prospect for development．The space voltage pulse width modulation(SVPWM)method is based on achieving the ideal circular flux of asynchronous motors when the power line provides symmetric sinusoidal three-phase voltage［1］．The actual flux acquiring from the different switch modes of inverter bridge arm devices is used to approach the ideal flux wave．The deviation of them determined the switch states of inverter bridge arm devices and so the PWM wave is achieved．Compared with SPWM control，the SVPWM control scheme have some advantages such as the DC voltage utilization rate is higher，the harmonics for output voltage can be reduced as the same as regulating the fundamental output voltage and the switch loss can be reduced［2］．The conventional SVPWM algorithm is complexity and needs to be realized by the high-speed microprocessor although it can be gotten a fast dynamic response．So the improved SVPWM algorithm has become a hot spot．According to the different combination and different work time of zero vectors it can have different modulation methods for SVPWM．A-mong these methods， the two-phase modulation，which the switch device of any bridge arms will maintain the certain switch state on the time 60°of within any half cycle，has a smaller switch loss［3］．

2.The principle of SVPWM

The general frequency converters is mostly used the diode bridges as rectifiers and used the semiconductor switch devices(such as IGBTs)or IPM as inverters，which is defined as AC-DC-AC inverter of voltage source．The main circuit of three-phase inverter for asynchronous motor is shown in Figure 1．

In Figure 1，Uiis the voltage of DC side．ua，ub，ucis the three-phase output voltage respectively．，，is the three-phase neutral voltage of inverter bridge arms respectively．ia，ib，icis output currents．

Figure 1．The main circuit of three-phase inverter for asynchronous motor

The three-phase sinusoidal voltages are divided six intervals each 60°in one work cycle．There are two phases voltages’sign are sameness but opposition with the third voltage’s sign，as shown in Figure 2．So it only needs to detect the three-phase voltage’s synchronous pulse signal at arbitrary time．

Figure 2．The interval division of phase voltages

Figure 3．Eight intervals in working cycle

According to the control theory of SVPWM，there are eight basic voltage space-vectors(V0～V7)in accordance with the switch different work state．V1～V6are non-zero vectors．V0and V7are zero vectors．The relationship between the space-vectors and the intervals is shown in Figure 3．ua，ub，uccan be regard as the voltage vector V．，，can be regard as the voltage vector V'*．V and V'*satisfied the following formula［3］．

Assuming the power device IGBT as the ideal switch，its conduction control can be described as the switching function as follows:

When Sm=1 means the IGBT in the upper bridge arm is on and S'm=0 means the IGBT in the lower bridge arm is off．The average voltages for node D，E and F relative to node N in Figure 1 are as follows:

In Figure 1，the follow formula is derived．

Likewise ，

Because the system is three-phase symmetrical，so

Then，

According to the definition of switching function，the space vector can be achieved as Sa=1，Sb=0，Sc=0．

The voltage space-vector V will be get by used the composition of non-zero vectors and zero vectors in one switch cycle(Ts)．In the process of composition，the zero vectors can be combined with V0and V7，and also can be selected V0or V7separately．The two-phase modulation is selected to amply in SVPWM control system，which the switch device of any bridge arms will maintain on or off during the time 60°．

Let IGBT in the lower bridge arm for phase C is on，that is S'c=1，the space vector will be four modes in interval I，as shown in the Table 1．The new voltage space-vector V in interval I can be get by used the composition of V0，V6and V4．Figure 4 shows the modulating pulse wave of two-phase modulation in intervalⅠ［4］．

Table 1．Four working modes under regenerating operation in intervalⅠ

Figure 4．Two-Phase modulation(IntervalⅠ)

Let symbol T0，T4，T6，ΔT stand for the switching time for space vector V0，V4，V6，V respectively，then

Then

The duty ratio dra，drb，drcfor the switch Sa，Sb，S'care:

The duty ratio is related to the three-phase output voltages(，，)and the voltage of DC side(Ui)as shown in Table 2．

Table 2．Duty ratio formulas in each interval

3.The implement of control method

In order to verify the SVPWM control method，the experiment was done based on TMS320F28335 DSP chip．TMS320F28335 DSP chip is high cost performance developing from TI Company．The event manager inside TMS320F28335 can provide high speedy and efficient advanced control technology for industrial applications［5］．Figure 5 is the block dia-gram of the control system．

3.1.Hardware design

Intelligent Power Module(IPM)is selected for the main circuit of control system and it can make the hardware circuit simpler．PM25RSB120 is a specific type of module and it have seven IGBT，six freewheeling diode and some protection circuits for IGBT．Figure6 is the drive circuit for IGBT．The digital PWM signals from DSP chip is as the input signal for drive circuit．The signal conversion and photoelectric coupling isolation is accomplished through the drive circuit and the analog output drive signals is as the control input signals for PM25RSB120 module［6-7］．

Figure 5．The block diagram of the control system

Figure 6．The drive circuit for IGBT

The accuracy and real-time performance of circuit detection for main circuit is vitally important for the system accuracy．The current is sampled by Hall current sensor CS025LX and the current detecting circuit is shown in Figure 7．The current detecting signal is become the voltage signal，which the voltage range is from negative 1．65 V to positive 1．65 V，by the conversion circuit．And through the bias circuit，the signal which the range is from 0 V to 3．3 V is achieved and transmitted to the analog to digital conversion module of DSP chip［6-8］．

In the three-phase inverter system，DC bus voltage is an important input and its magnitude is directly related to the safety of IGBT．The DC voltage is sampled by Hall voltage sensor LV100 and the voltage detecting circuit is shown in Figure 8．In Figure 8，a first order low pass filter circuit is made up of R7 and C13 and the input resistance of the operational amplifier OP07 is infinite which made the impedance matching．Through the voltage detecting circuit，the voltage output signal，which is proportional to the magnitude of DC voltage，is achieved and the high frequency interference is filtered．

Figure 7．The current detecting circuit for control system

Figure 8．The voltage detecting circuit for control system

3.2.Software design

On the basis of hardware，software program is written using C language and is debugged using CCS4 software．Figure 9 is the program flow chart．

Figure 9．Programming flow chart of system

4.Conclusion

Compared with the conventional algorithm，the proposed SVPWM algorithm is simple because the sin function calculation is not required and the amount of computing is reduced．The method of interval division is easy to detect and implement in practice．The control system is implied based on TMS320F28335 DSP chip with the three-phase voltage effective value is 115 V．Figure 10 is voltage and current experimental waveform．The experimental result shows that the control method is feasible and the power factor is nearly unity．The method has high value of utility．

Figure 10．Experiment result

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