Single Phase And Three Phase Supply for Same Variable Frequency Drive

Transcription

1 Single Phase And Three Phase Supply for Same Variable Frequency Drive Lotus S P 1 and Juna John Daniel 2 1 Lotus S P, Sree Buddha College 2 Juna John Daniel, Sree Buddha College Abstract Manufacturers are commonly producing three phase variable frequency drives but due to unavailability of three phase ac power, three phase variable frequency drive are often required to operate from a single phase supply, in rural areas.powering up three phase variable frequency drive from a single phase ac source results in many issues such as higher rms value of input diode current, higher ripple voltage across dc bus capacitor, higher peak input current, higher input current distortion, lower power factor, and poorer system efficiency. There are many techniques, both passive and active that are employed to address the issues faced when a single phase ac supply is used to power three phase variable frequency drives. Here proposes a new active solution for problems that occur when three phase variable frequency drive is powered from a single phase ac source and by using this technique no modification of the existing three phase variable frequency drive is required and hence same variable frequency drive can be used for both single phase supply and three phase supply based on its availability with an additional bidirectional switch. Keywords Variable Frequency Drive; PWM inverter; Single switchboost topology; Bidirectional switch; Rectifier I. INTRODUCTION For pumps etc we are using different motors such as induction motor,synchronous motor etc.for the purpose of controlling this motors,they are driven from motor controllers called variable frequency drives.variable frquency drives are motor controller which consist of rectifier,dc link capacitor and an inverter.rectifier will convert an ac input to dc,which is then given to a dc link capacitor and voltage across the dc link capacitor will act as voltage source for inverter and inverter will convert dc to ac and this output is used to drive an ac motor. Mainly we are available with two type of ac supply,single phase ac supply and three phase ac supply.normally manufacturers are producing three phase variable frequency drive with a three phase rectifier in the front end. In areas where three phase ac supply is available, these rectifiers can be directly supplied with three phase ac source.but in those areas where only single phase ac supply is available then it requires conversion of this single phase to three phase, different techniques are available for converting single phase to three phase [1] [3] but here shows a method by which available single phase can directly supplid to the input of three phase rectifier. II. VARIABLE FREQUENCY DRIVES Variable frequency drives are also known as variable speed drive, adjustable speed drive, and adjustable frequency drive or frequency converter. Variable frequency drive are mainly used in pump and fan applications, speed up or slowing down a machine or process etc. It mainly consists of a controller section and a motor section. Controller section is mainly composed of rectifier section, an inverter section and dc link. Mainly used inverter is voltage source inverter.dc link capacitor is used to smooth out the ripples in the rectifier output and it provides a stiff input to the inverter. Inverter used is a sinusoidal PWM inverter.pwm techniques are characterised by constant amplitude pulses.the width of these pulses is modulated to obtain inverter output voltage control and to reduce its harmonic content.in sinusoidal PWM inverter a high frequency triangular wave is compared All rights Reserved 30

2 a sinusoidal reference and the intersection of these two waves determines the switching instants.block diagram for variable frequency drive is shown in Fig. 1. Fig. 1. Block diagram III. VARIOUS METHODS There are different methods by which single phase supply is applied to three phase rectifier.some of the methods are dicussed below Single Switch Boost Converter Fig.2 shows the single switch boost converter topology in which a single phase supply is used to power up a three phase motor.its main advantages are harmonics are low in the input current,it has unity power factor.but it has disadvantage such as peak value of current flowing through switch S is high, inductor used causes losses and there exist ripples across dc bus. Fig. 2. Single Switch Boost Converter 3.2. Bridgeless Boost Topology Fig.3 shows the bridgeless boost topology in which a single phase supply is used to power three phase motor.number of conducting diodes is less in bridgeless boost topology when compared to single switch boost converter.one of the main disadvantage of bridgeless boost topology is that it is very expensive since it requires a fast recovery All rights Reserved 31

3 Fig. 3. Bridgeless Boost Topology 3.3. Voltage Doubler Active Front End Topology Fig.4 shows voltage doubler active front end circuit in which dc link is composed of two capacitors and hence it forms a midpoint to which one end of input supply is connected.one of its main advantage is it uses the existing switches and hence the circuit is simple. Main disadvantages are it requires external capacitors and large input inductors. Fig. 4. Voltage Doubler Active Front End Topology IV. THREE PHASE VFD WITH THREE PHASE SUPPLY Fig.1 shows the block diagram three phase variable frequency drive with three phase supply, this can be used in those areas where three phase supply is available.it mainly consist of an uncontrolled three phase rectifier, a dc link capacitor and an inverter. Three phase rectifier consist of three arms with six diodes.its operation is such that at a time one diode from an arm will conduct and remainind will be in the reverse biased condition and it will convert the applied three phase ac to dc. Inverter used is a Sinusoidal PWM inverter.pwm techniqes are charecterised by constant amplitude pulses.the width of these pulses is modulated to obtain inverter output voltage control and to reduce its harmonic content.in sinusoidal PWM inverter a high frequency triangular wave is compared with a sinusoidal reference wave and the intersection of these two waves determines the switching instants. MATLAB model for three phase VFD with three phase supply is shown in Fig 5. V. PROPOSED THREE PHASE VFD WITH SINGLE PHASE SUPPLY Fig. 6 shows proposed three phase VFD with single phase supply which can be made without any modification of the three phase VFD shown in Fig. 1.It mainly consist of a three phase uncontrolled bridge rectifier, a dc link capacitor, an inverter as in Fig. 1and in addition to this it consist of bidirectional switch and two inductors. Bidirectional switch is made up of two IGBTs connected in common emmiter configeration and in addition to it there exists inductors in the input side Operation of Proposed Three Phase VFD with Single Phase Supply Operation of proposed three phase VFD with single phase supply can be explained in three different modes Mode 1,Mode 2 and Mode3.During Mode1 control voltage VGE is applied such that switch S1 is turned ON and at that time current flows from supply side through inductor L1 and then through switch S1 and diode DS2 and then through capacitor C2 back to source.during Mode 2 control voltage is such that switch S1 is turned OFF and at that time current flows from supply All rights Reserved 32

4 through inductor L1, capacitor C1 and C2 back to source.mode 3 is same as that of Mode 1. MATLAB model for three phase VFD with three phase supply is shown in Fig 7. Fig. 6. Circuit diagram for three phase rectifier with single phase supply[4] VI. SIMULATION RESULTS Fig. 5. MATLAB model for three phase VFD with three phase supply Fig.8. Waveforms for three phase VFD with three phase supply Fig. 7. MATLAB model for three phase VFD with single phase All rights Reserved 33

5 Fig. 9.Waveform for three phase VFD from single phase supply VII. CONCLUSION From the above results it is clear that for the same variable frequency drive with three phase rectifier in the front end can be provided with three phase supply where three phase supply is available and it can be supplied with single phase supply whwre three phase is not availabe.hence same Variable frequency drive can be used with both single phase and three phase supply. REFERENCES [1] E.C.dos Santos,C. B. Jacobina,E. R. C. da Silva, and N. Rocha, Single phase to three phase power converters:state of art, IEEE Trans.Power Electron.,vol 27,no. 5,pp ,May [2] M. Lazic,M. Zivanov, and B. Sasic, Designing of multiphase boost converter for hybrid fuel cell/battery power sources, in Paths to Sustainable Energy,J. Nathwani and A. Ng, Eds. Rijeka,Croatia:InTech,Dec. 30,2010,ch. 19. [3] Y.Jang and M. Jovanic, A bridgeless boost rectifier with optimized magnetic utilization, IEEE Trans. Power Electron.,vol.24,no.1,pp.85-93,Jan [4] M. Swamy, An improved single-phase active front end rectifier system for use with three phase VFDs, U. S. Patent Appl.,Jan. All rights Reserved 34