Highly Integrated Inverter with Multiturn Encoderand Software-based PFC for Low Cost Applications

Kilian Notzold, Andreas UphuesRetostronik GmbH

Gevelsberg, Germanyhttp://www.retostronik.de/

Ralf Wegener Member IEEE, Stefan Soter Member IEEE

Electrical Machines and Drives GroupUniversity of Wuppertal, Germanyhttp://www.emad.uni-wuppertal.de/

Abstract—Motor control gets more and more commonin fields where it was not applicable a few years ago. Theprizes for computing power due to higher integration of thepower electronics are going down. Because of this, brush-less DC and permanent magnet synchronous machinescapture new markets and replace simple grid connectedinduction machines. The rising standards for electromag-netic interference (EMI) emissions make a power factorcorrection (PFC) in a lot of applications necessary. Inaddition a PFC offers some advantages because of the DCbus regulation. In this paper an inverter with a particularlylow prize and a high performance is presented. A softwarebased PFC is integrated into the motor control circuit anda low cost absolute position encoder has been developed.In addition some synergetic effect between motor controland PFC will be shown. The experimental results show thefeasibility of the integration of a field orientated controlwith an integrated PFC in one low cost microcontroller.

I. INTRODUCTION

Due to the dramatically fallen prizes for controller per-formance it is possible to implement high-performancemotor-control algorithms in a very cheap hardware.Inverters can be used in applications which are normallydominated by direct grid connected induction machines.This allows a wide range of improvements. For exampleit is possible to control torque, speed and velocity. Theuse of an inverter enables a smarter design of the wholeapplication. The reduction of shock and jolt can increasethe mechanical durability. Normally an induction ma-chine has to be oversized for start up and load peaksand a huge amount of reactive power is needed. Withan inverter the grid does not need to supply the drivewith reactive power. Additionally a single phase supplyfor smaller power amounts is sufficient instead of athree phase supply. To comply with the internationalregulations of system perturbation (standards of EMIemissions) an integrated active PFC decreases the ripplecurrent which is responsible for EMI emissions. The

PFC controls the power factor which minimizes thereactive power consumption. A unity power factor cannearly be achieved. Due to the regulation of an activePFC, the input voltage fluctuations have no effect onthe drive and there is no need for different designsfor 115V and 230V supply grids. The PFC allows ahigher motor voltage which increases the efficiency ofthe drive. The disadvantage of an active PFC is theprice, which limits the field of possible applications. Aconsistent reduction of components can compensate this.A modern microcontroller like the Cortex M3 can handlea high level motor control and a power factor correctioncontrol algorithm simultaneously. The PFC adds onlya few components to the inverter design and there isno need for an additional specialized PFC-controller.Furthermore the number of components is reduced byusing a field oriented control with a single DC link shuntmeasurement for controlling the motor.

II. DESCRIPTION OF THE HARDWARE SETUP

The consistent reduction of components leads to anelectrical circuit whose block diagram is shown in figure1. The frontend of the inverter consists of an EMIfilter, a rectifier and a controlled boost converter. Themicrocontroller, which is supplied by a flyback converter,controls the frontend and the backend. Due to the costefficiency of a power module the backend of the inverteris not realized in discrete techniques. To decrease thecosts for the input current measurement and for themeasurements of the motor currents some signal con-ditioning is required. They are put into practice by usingone operational amplifier in each case. The voltagesare measured by resistance networks. Additionally amovement to get the reference points after a power failwith dead stop switches is not acceptable due to safetyand usability issues. A cheap solution has to be found.Absolute position detection within a single turn can be

Fig. 1. Diagram of the hardware implementation

achieved with a normal potentiometer. Every angle iscorrelated with a fixed value of resistance. The absoluteposition within one turn can be achieved by measuringthe resistance. A gear with a transmission of less thanone can adapt this single turn sensor to several turns ofa faster rotating axis.

Fig. 2. Principle of the rotary position sensor and the necessarygear

A potentiometer has some major disadvantages. Firstof all the contact of the slider can degrade and a longtimereliability is hard to archive. Second a potentiometercannot represent the full 360◦ of a circle due to themechanical limitations. A better solution is a ”RotaryPosition Sensor IC”. These circuits are detecting theposition of a small magnet by an array of hall sensors

like shown in figure 2. It is possible to detect the fullrange of 360◦ with no limitations with a resolution of12Bit. The position is transmitted via serial peripheralinterface (SPI) to the microcontroller. The necessary gearcan be very small and cheap, because it does not haveto transfer a high torque and it is just for rotating themagnet over the sensor. For example gears like this canbe found in toy servo applications shown in figure 2 andthe costs are in mass production only a few cents.

III. IMPLEMENTATION OF THE FIELD ORIENTATED

CONTROL

The figure 3 shows the scheme of the field orientedcontrol (FOC) with the reference output to the PFC. Itis a cascaded design of a position, speed and currentcontroller followed by transformation and modulation.The permanent magnet synchronous machine is equippedwith a multiturn absolute encoder described in II. Theprovided position and the measured current are trans-formed by the well known Park and Clark transforma-tions. The resulting current which has to be separatedinto the d- and q-component is the input for the currentcontroller.

Fig. 3. Block diagram of the field orientated control

The d-current should be zero because no field weaken-ing is necessary in this application. The reference valuefor Iq, representing the torque, is provided by the speedcontroller. This controller is a standard PI controllerwhich is fed by the position controller with the referencespeed. The input of the position controller is calculatedby a motion profile generator. This module generates aspeed reference value based on a sin2 curve for smoothmovements. For better control performance, precontrolvalues for speed and current are calculated by derivatingthe position reference. The current controller actuatingvalue is summed with the precontrol value resulting fromthe EMK calculation.

IV. SYNERGETIC EFFECTS OF FOC AND PFC

Normally a PFC is build by a specialized analogcontroller. By implementing the motor control and thecontrol of the PFC into one microcontroller some syner-getic effects can be achieved. The PFC can be driven inthree different conduction modes. Only with one of them,the continuous conduction mode (CCM), it is possible tofull fill the EMI standards for electrical circuits with anoutput power higher than 500W. The disadvantage ofthe CCM is a complex control loop, which is difficultto build with analog techniques. The microcontrollerallows using the CCM by controlling the PFC-Frontendwith a software algorithm. The control algorithm of thePFC is shown in figure 4. The PFC is mainly supplyingthe B6 inverter bridge which is controlled by the FOCalgorithm. So the current which has to be transferredby the PFC is known. The FOC current controller canprovide this information to the PFC control. The controlcan benefit from this information with a more precisecurrent control.

V. IMPLEMENTATION OF THE POWER FACTOR

CORRECTION

To achieve a unity power factor a high switchingfrequency is required. The switching frequency is limitedin a digital controlled PFC because of the sampling timedelay and the necessary processing time. In commonPFC applications the microcontroller has to executemany operations like conversion of input voltage, outputvoltage and inductor current, PI regulation of voltageand current, reference current calculation and duty cyclegeneration in every switching cycle. These operationslimit the switching frequency. To be able to use a cost-efficient microcontroller, the software has to be imple-mented in a smart way. Several approaches were made

Fig. 4. Block diagram of the power factor correction control

to decrease the processing time for example by usinga predictive algorithm with the duty cycle generation

and the necessary calculations are mostly independentfrom the switching frequency. With another approachthe processing time is decreased by calculating the dutycycle only every second to tenth switching cycle. Thedisadvantages of this approach are the increasing har-monics in the line current. The characteristics of the heredescribed application which is controlling a synchronousmachine and a PFC with one microcontroller, requiresan economical source code, especially for controlling thePFC. The bulk of the processing power must be reservedfor controlling the motor. In addition the code size mustbe restricted. As shown in figure 4 the duty cycle iscalculated by the input voltage, the output voltage andthe inductor current. These three values are digitizedby the analog digital converter (ADC). To decreasethe degree of capacity utilization of the ADC only theinductor current is measured every switching cycle.

Fig. 5. Jitter of the zero crossing detection

The input voltage value is displayed by a look uptable which contains the values of a half sine wave.The virtual sine wave is synchronized with the real sinewave of the input voltage by a phase looked loop. Tosynchronize the virtual sine wave with the input voltageonly a fraction of otherwise needed ADC conversionsare required because the ADC is only used to find thezero crossing of the input voltage. Figure 5 shows thezero crossing detection which has a low jitter and theconstant delay can be easily corrected. The amplitude ofthe input voltage does not have to be measured becausea feed forward voltage to stabilize the line voltage isnot necessary. The output voltage is measured everythird sine half wave because it changes very slowly. Thereference current Iref is calculated by multiplying the PI

controller output and the normalized reference voltage.The duty cycle is calculated by the PI current controller.

VI. RESULTS AND CONCLUSION

The performance of the PFC is shown in the followingmeasurement results. The figure 6 shows the waveforms

Fig. 6. Line current and voltage waveforms

of the input voltage, the input current and the outputvoltage. The input voltage and the input current are in

Fig. 7. Power factor measurement

phase, the output voltage oscillates with 100Hz phaseshifted to the input voltage and current. The input current

is sinusoidal and nearly without ripple current. The errorsin the progression of the current near the zero crossingsare caused by the above described jitter of the zerocrossing detection. The power factor and the currentharmonics which results from the above described inputcurrent flow and input voltage are shown in the figures 8.On the left side in the subfigure the progression of input

Fig. 8. Harmonics of the Power factor measurement

voltage and input current are shown at an apparent powerof 323VA which results in a power factor of 0.97 anda cos Phi of 0.98. The associated harmonics of currentwhich comply with the limit values of the standards IEC-1000-3-2 and IEEE-519 are additionally shown in figure8. The step response of the speed controller is shown

402,5n/rpm

401 5

402

401

401,5

400

400,5

399,5

400

399

398,5

0 0,1 0,2 0,3 0,4 0,5t/s

Fig. 9. Speed variations while rotating

in figure 10. The target speed of 400rpm is reachedafter 15ms with nearly no noticeable overshooting. Thesecond plot displays the concentricity at final speed.The speed is fluctuating with a little offset around the

reference value. The performance can by improved byreducing these ripples. The detailed measurements of themotor control in the final paper. It was shown that it ispossible do integrated both a PFC and a FOC into onelow cost microcontroller.

450n/rpm

350

400

250

300

200

250

100

150

50

0

0 0,1 0,2 0,3 0,4 0,5t/s

Fig. 10. The step response of the speed controller

The total cost for the 1.5kW system including PFC,wide input-voltage range (90V-265V), and muliturn en-coder is less than 30$. The control resulting performanceis competitive with standard market solutions.

REFERENCES

[1] S. Busquets-Monge, J-C. Crebier, S. Ragon Design of a BoostPower Factor Correction Converter Using Optimization Tech-niques IEEE Transactions On Power Electronics, Vol. 10, No.6, pp. 1388-1396, November 2004

[2] W. Zhang, G. Feng, Y. Liu, B. Wu A Digital Power FactorCorrection (PFC) Control Strategy Optimized for DSP IEEETransactions On Power Electronics, Vol. 19, No. 6, pp. 1474-1485, November 2004

[3] M. Fu, Q. Chen A DSP based controller for Power FactorCorrection (PFC) in a rectifier circuit IEEE Transactions OnPower Electronics, Vol. 19, No. 6, pp. 1474-1485, November2004

[4] S. Buso, P. Mattavelli, L. Rossetto, G. Spiazzi Simple digitalcontrol improving dynamic performance of powerpreregulatorsPower Electronics Specialists Conference, Vol. 1, Issue , pp. 103- 109, June 1997

[5] On Semiconductor Power Factor Correction HandbookHBD853/D, Rev. 3, September 2007

[6] Wegener, R.; Senicar, F.; Junge, C.; Soter, S. Low Cost PositionSensor for Permanent Magnet Linear Drive, Seventh Interna-tional Conference on Power Electronics and Drive Systems –PEDS 2007, Bangkok, Thailand

[7] Wegener, R.; Gruber, S.; Notzold, K.; Soter, Optimization ofa Low-Cost Position Sensor for a Permanent Magnet LinearDrive, Power Conversion Intelligent Motion Power Quality –PCIM China 2008, Shanghai, China

[8] Pellegrino, G.; Bojoi, R.; Guglielmi, P. Performance Comparisonof Sensorless Field Oriented Control Techniques for Low CostThree-Phase Induction Motor Drives, Industry ApplicationsConference, 2007 – Pages 281 - 288

[9] Pinewski, P.J. Implementing a simple vector controller , Amer-ican Control Conference 1997 – Pages 262 - 266

[10] Seung-Ho Song; Jong-Woo Choi; Seung-Ki Sul Current mea-surement of digital field oriented control, Industry ApplicationsConference, 1996 – Pages 334 - 338