Single Phase to Three Phase Converter

Transcription

1 Single Phase to Three Phase Converter Naung Cho Wynn, and Tun Lin naing Abstract A new single phase to three phase converter topology for small industries is presented in this paper: Phase converter, include this paper, is a new technology that supplies three phase power from a single phase source to power inductive, resistive and capacitive loads with distinct advantages over any existing converter technology. The converter consists of DC power supply, a MOSFET Hex-bridge, integrated gate drive IC, and a DSP to generate the switching signals. The switching signals generated are a unique version of selective harmonic elimination, which produces a consistent starting point for the switching functions, independent of the number of harmonics eliminated This converter covers the basis of induction motors and different types of other motors. They are ideal for farms, workshops, garages and large building etc. Keywords MOSFET Hex-bridge, DC power supply, gate drive IC, switching signal static converter, rotary converter. A I. INTRODUCTION wide variety of commercial and industrial electrical equipment requires three-phase power. Electric utilities do not install three-phase power as a matter of course because it costs significantly more than single-phase installation. As an alternative to utility installed three-phase, rotary phase converters, static phase converters and phase converting variable frequency drives (VFD)have been used for decades to generate three-phase power from a single-phase source. Construction of three-phase power lines can cost as much as $50,000 per mile and can have an undesirable environmental impact. Even when three-phase lines are nearby, the cost of installation is considerable. Based on anticipated electricity demand for the three-phase application, the utility may or may not charge the customer for the cost of installation. Continuing monthly surcharges for the service are also common. Phase converters have historically been employed where utility three-phase power was unavailable, or where the electricity demand did not justify the cost of utility three-phase installation. Reduced motor life caused by voltage and current imbalance, harmonics that pollute the power grid and damage equipment, or the inability to operate sensitive equipment or multiple loads are just some of the problems that have limited the use of phase converters. This phase converter is a new, patented technology that supplies three-phase power from a single phase source to power inductive, resistive and capacitive loads with distinct advantages over any existing converter technology. There are various phase conversion Authors are with the Mandalay Technological University, Myanmar ( awalyacho@gmail.com., tunlinthida@gmail.com). technologies. A. Static Converter II. PHASE CONVERTERS The Static Converter is made up of two small components: A voltage sensitive relay and a standard capacitor (Cs) connected to your motor application (Red Box). The capacitor delays waveforms (or shifts the phase) during the start-up of your motor application. The relay disconnects this start capacitor after the motor has started. From this point, the motor will continue turning on the single phase supply. The performance of such a motor is fairly poor and can be compared to a car motor running on only a few cylinders Motors operated on a static converter will produce about 50-60% of their name plate power. When you add another low cost run capacitor (Red Dotted Box) to the simple design, rated power goes up to around 70% of the motors name plate power. To help with understanding, the Start Capacitor (Cs) is used only to start the motor and then it is switched out completely. The Run Capacitor (Cr) is always in the circuit and is carefully sized to balance the voltages at one load rating (generally around 50% full load). Since Cr is fixed the voltage balancing at either end (0% and 100%) is quite poor. Fig. 1 One line diagram of the static converter B. Rotary Converter Add an idle running motor to a static converter; is called rotary converter. The added motor will compensate for some of the static converter weaknesses and help extend the range of motor sizes and loads. The internal motor is inactive at average load, but works hard when loads don t match the value of the chosen run capacitor (Cr). Rotary converters are clearly somewhat better than static converters. They can run several motors of different sizes. Large motors will produce up to 90% of their nameplate power, small motors (motor being much smaller than the converters idling motor or pilot motor) a bit more. If the manufacturers oversize these motors, 343

2 the output symmetry, start capability and capacity will all be increased. C. Document Modification Fig. 2 One line diagram of the rotary converter Both rotary and static converters have difficulty adjusting voltage balance to accommodate changing load conditions. Voltage regulation schemes for rotary converters are available which switch in different amounts of capacitance as the load changes. However, it is still difficult to get good control, and the high current pulses created in the system as the capacitors are switched in and out can be a problem. Digital Phase Converters are purely electronic and non-mechanical in nature. As such they are able to produce decently balanced three phase power. Another benefit of this type of phase converter is that they are very quiet. On the downside, this type of phase converter takes a sign wave, chops it up and spits out a somewhat choppy synthesized 3 phase sign waves - their output harmonic distortion tends to be greater than a few other types of phase converters. D. Design calculation for power supply circuit 220V AC MB351.0 D1 D2 D3 D4 Fig V DC Power Supply for H-bridge Inverter There are two separate power supply used in this variable speed drive. The 200V DC power supply was used for H- bridge inverter and 12V regulated power supply was used for PWM frequency control circuit and driver circuit. This 200V DC power supply circuit is shown in Fig. 3. The maximum current of secondary transformer side is P V C1 470µ C2 220µ C3 220µ Imax max (1) s + 200V DC P max = maximum power of secondary transformer side V s = secondary voltage And, the turn ration is N s = number of secondary turns N p = number of secondary turns V p = primary voltage And, N Pmax (2) (3) Ns I s = secondary current I p = primary current For specifications P max = 500VA,V s = 18V, V p = 230V; By using equation 1, I max = 28A By using equation 2, Np = 13 N s By using equation 3, I s = 13 I p The peak voltage of secondary transformer side is 2 Vrms (4) V rms = rms value of primary voltage The average voltage is 2 Vavg (5) V p = peak voltage And the ripple voltage yields 1 Vripple (6) 2CF C = filter capacitance F = operating frequency The output DC voltage is obtained VDC Vripple (7) By using above equation 4, 5, 6, and 7, the supply voltage 200V DC are calculated. E. Hex-bridge The heart of a standard three-phase inverter is the hexbridge. The hex-bridge takes a dc bus voltage and uses six switches arranged in three phase legs as shown in Fig. 4. From the middle of each phase leg comes the line which connects to the motor. The waveforms on these lines need to be a balanced three-phase sinusoidal waveform in order to drive the induction motor. This is achieved by applying carefully controlled switching waveforms to the gate of the switches. p 344

3 + V dc - A B C Fig. 4 Hex-bridge There are two kinds of switches that can be used for our fairly high power application, Insulated Gate Bipolar Transistors (IGBTs) or MOSFETs. To meet our specifications the switch needed minimum ratings of 400V and 10A. In addition we wanted a switch which minimized losses; therefore, switches with lower resistances are more desirable. The IGBT considered for this design was the HGTP12N60B3D, which has a small equivalent resistance of approximately 0.07, but IGBTs also have a voltage drop at all times due to the collector emitter saturation voltage, which in this case is equal to 1.7V. The MOSFET investigated was the, which has an on-state resistance of Average Conduction Based Power Loss HGTG12N60B3D RMS current (A) [Single Frequency Sinusoid] Fig. 5 Power losses in IGBT vs. FET The power losses of the IGBT and the FET are plotted against the current. The IGBT has a fairly linear loss curve because the loss is mostly due to the IV sat, while the MOSFET losses are due to the I 2 R DS(on). The MOSFET losses increase faster with current, but the MOSFET losses do not surpass the IGBT losses until I is approximately greater than 9A. The motor used in our project is rated for 3A so the current should never reach 9A. In addition to having smaller losses in our operating region, the is cheaper. It is $0.96 in a quantity of 1000, while the HGTP12N60B3D is $1.70 in the same quantity. The HGTP12N60 is rated for 600V which is the very low end of IGBT ratings. IGBTs are usually used in higher power applications, so this part is not extremely common. The savings in losses and price prompted us to choose the as the switch to be implemented in the hex-bridge. F. Gate Drive The gate drive circuit needs to provide an interface between the switching signals coming from the DSP or arbitrary waveform generator and the FETs in the hex-bridge. The DSP gives a 3.3V signal, while the waveform generators allow for a specified voltage level. The gate to source voltage needed for desired operation of the FETs is on a 12 to 15V level. In addition, the high side FETs in the hexbridge do not have the source connected to ground, so the actual voltage needed to drive the gate depends on the varying voltage at the source. A single chip solution was found, the IR21362, which implements all six gate drives, including the circuitry that takes into account the issues with the high side FETs. There were other options, such as the IR2121, consisting of a single gate drive on a chip, but in the interest of lowering component count and cost, the IR21362 was chosen. Since six IR2121s would be needed, it is apparent that the single chip solution (IR21362) is more cost effective. G. Switching Signal Methodology Currently small motor drive systems are expensive and implement control schemes that use relatively high switching frequencies such as sine-triangle PWM, space vector PWM, or hysteresis current control. One drawback to the high switching frequencies is the decrease in efficiency that occurs from switching loss. However, the previously mentioned control methods do have their own merits. Each control scheme has been used widely and generates little acoustic noise since the switching frequency is on the upper end of the audible acoustic range (20 khz). These control schemes also provide good dynamic performances. However, this application does not need good dynamic performance since there are no dynamic load and speed requirements. The switching signals implemented are a unique version of selective harmonic elimination, which is currently an active research area in power electronics. This technique lowers the switching frequency considerably compared to the previously mentioned methods. In addition, this switching scheme provides direct control over harmonics present in the waveform. The selective harmonic elimination method will be implemented in a switching waveform, while sine-triangle PWM will be used as a back-up. H. Switching Signal Implementation Two DSP development boards for motor control applications are available for proto-typing DSP code development. They are the ezdsp F2407 and the ezdsp F2812 boards. The F2407 has a lower cost then F2812. However, the F2812 has a faster clock at 150 MHz compared to the 30 MHz clock on the F2407. The F2812 s analog-to-digital converter has a 12 bit resolution, whereas the F2407 only has 10 bit 345

4 resolution. This makes the F2812 more desirable since it will give a better resolution for the speed commands. In addition, constructed harmonic elimination PWM implementation in the DSP requires the use of a large switching table that cannot fit into the F2407 FLASH. The ezdsp kit provides a complete development environment: DSP board, power supply, on-board JTAG compliant emulator, and an ezdsp specific Code Composer Studio TM full featured, with debugger IDE, and ANSI C and C++ compliant compiler. The DSP board has all peripheral signals needed on the board headers, making it easy to interface the board with the 3-phase inverter circuit. I. Inverter Control The control method implemented is open loop and based upon constant volts per hertz ratio. Although the inverter uses a 200V dc bus, the voltage is able to be varied by adjusting the modulation depth of the switching signals. Additionally, in order to meet the speed specifications over the given load range an efficient load compensation method will be used to determine optimal values of voltage and frequency to be applied to the machine at a given operating point. (PCB) that is mounted to the end of the motor. There will be a fan attached to the rotor which actively cools the FETs. These facts lead to the decision to forego the use of heatsinks. The IR21362 gate drive chip requires some external circuitry. The gate resistors (R11-R16), bootstrap capacitors (C4, C5, and C7), and decoupling capacitors (C3 and C6) were chosen based on information found in [2, 3]. A resistor and capacitor (R2 and C5) had to be chosen to provide a time constant which is used for the fault clearing time. A time constant of greater than 1.5s was desired so values of 33k and 47F were chosen. The FAULT pin needs to be pulled high because it is active low. A LED was added in for visual output in the case of a fault. The protection circuitry, which includes R7, R9, R19, and C1, is chosen based on the fact that a fault occurs if the voltage on the ITRIP pin is larger than 0.5V. The variable resistor, R7, can be adjusted so that a fault is given at a desired current value. The values were chosen with the intention of initiating a fault at currents larger than 10A. Finally, three phase power for induction motor is got when DC power supply connected with Hex-bridge and gate drive. III. DESIGN DETAILS A. Hex-Bridge and Gate Drive (Component Selection) The was chosen as the switch in the hex-bridge. Often, MOSFETs need external circuitry, called a snubber, in order to improve the switching trajectory during the commutation period. The switching trajectory of the was measured (see Fig. 6) and appears to be fairly rectangular, as desired for an inductive load. Fig. 6 Switching trajectory In addition, thermal issues needed to be taken into consideration. The is rated for 10.1A continuous at 100 C. Since the motor used is rated for 3A, the current should never be greater than 5A continuously. In addition, the final product is intended to be on a printed circuit board 346

5 B. Detailed Schematic ACKNOWLEDGMENT The First named author expresses sincere thanks to the university of Mandalay Technological University. REFERENCES [1] P. T. Krein, Elements of Power Electronics. New York and Oxford: Oxford University Press, International Rectifier, Appl. Note 978. International Rectifier, Appl. Note INT985. [2] H. S. Patel and R. G. Hoft, Generalized techniques of harmonic elimination and voltage control in thyristor inverters: Part I Harmonic elimination, IEEE Trans. Ind. Applicat., vol. IA-9, no.3, pp , May/June [3] Hur Namho, Jung Junhwan, Nam K wanghee. Fast Dynamic DC-link Power balancing Scheme for a PWM converter-inverter System [A]. IECON 99 Proceedings[C]. The 25th Annual conference of the IEEE, 1999, 2:767~772 [4] Jung Jinhwan, Lim Sunkyoung, Nam Kwanghee. A Feed-back Linearizing Control Scheme for a PWM Converter inverter Having a very Small DC-Link Capacitor[J].IEEE Trans. on Ind. Appl., 1999,35(5):1124~1131 [5] Li Li, D. Czarkowski, Yaguang Liu and P. Pillay, Multilevel selective harmonic elimination PWM technique in series-connected voltage inverters, IEEE Trans. Ind. Applicat., vol. 36, pp , Jan/Feb [6] J D Van Wyk..Power Quality, Power Electronics and Control in Proceedings EPE.93, 1993, pp [7] Z Yang and P C Sen. Recent Developments in High Power Factor Switchmode Converters, IEEE Proceedings CCECE.98, 1998, pp [8] H Akagi..New Trends in Active Filters for Power Conditioning.. IEEE Transactions Industry Applications, vol 32, November/December 1996, pp [9] H Endo, T Yamashita and T Sugiura.. A High Power-factor Buck Converter, Proceedings IEEE PESC.92, 1992, pp HIN1 HIN2 HIN3 33k R5 C6 1 u MUR160 D1 R8 500 MUR160 D2 C5 1 u C4 1 u R V C7 1 u R k POT R FAULT C3 1 u C2 47u 5V VCC MUR160 D3 U VCC VB HIN1 HO HIN2 VS1 25 D4 5 HIN3 NC 24 LED 6 LIN1 VB LIN2 HO LIN3 VS FAULT NC ITRIP VB EN HO RCIN VS VSS NC COM LO1 15 LO3 LO2 IR21362 C u V1 M1 M2 M3 A_TO_MOTOR B_TO_MOTOR C_TO_MOTOR M4 M5 M6 R14 R15 R11 R16 R12 R13 Fig. 7 Hex-bridge and gate drive circuitry 347