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Power Quality Improvement in DTC Based InductionMotor Drive Using Minnesota Rectifier

Bhim Singh and G. Bhuvaneswari

Dept. of Electrical EngineeringIndian Institute of Technology Delhi

New Delhi-110016, Indiabhimsingh1956@gmail.com,bhuvan@ee.iitd.ernet.in

Sandeep Madishetti

Dept. of Electrical EngineeringIndian Institute of Technology Delhi

New Delhi-110016, Indiasandeep.madishetti@gmail.com

AbstractThis paper presents power quality improvement at the

utility interface for a Direct Torque Controlled (DTC) induction

motor drive by making use of a Minnesota rectifier. The

proposed Minnesota rectifier for a 2.2 kW drive is designed,

modeled and simulated in MATLAB/Simulink platform. The

design of the proposed Minnesota rectifier is developed along

with the necessary modifications required for making it suitable

for retrofit applications, where presently a 6-pulse diode bridgerectifier is used. The performance parameters of the DTC drive

fed by a 6-pulse uncontrolled converter is compared with the one

fed by Minnesota rectifier for variable load conditions. The

results show that the THD and power factor at the ac mains are

improved perceivably even with the magnetic rating being as low

as 40.94% of the drive rating.

Keywords-Minnesota Rectifier; Zig Zag auto-transformer;

Power Quality Improvement; Direct Torque Control; Induction

Motor; Power Factor Correction.

I. INTRODUCTIONInduction motors, the most widely used motors in industry,

have been traditionally operated in open-loop controlapplications, for reasons of cost, size, reliability, ruggedness,simplicity, efficiency, less maintenance, ease of manufactureand their capability to operate in dirty or explosive conditions.However, because the induction motor requires more complexcontrol methods, the dc motor has dominated in high

performance adjustable speed drive applications. Withdevelopments in micro-processors/digital signal processor(DSP), power electronics and control theory, the inductionmotor can now be used in high performance variable-speed andcost-sensitive applications, such as heating, ventilation, and airconditioning (HVAC) systems, waste water treatment plants,

blowers, fans, textile mills, rolling mills etc. due to their

advantages like energy conservation and reduction in inrushcurrent drawn, etc. The use of variable frequency inductionmotor drives (VFIMDs) has further increased due to theircapability to achieve good dynamic performance using vectorcontrol (or field oriented control-FOC) and direct torque andflux control (DTC). With these control techniques, inductionmotor drives can achieve similar or even better performancethan dc motor drives. It is well known that the FOC needscomplicated co-ordinate transformations to decouple theinteraction between the flux and torque components of statorcurrents. The implementation of FOC is difficult and verysensitive to parameter variations. Direct torque control (DTC)

is relatively simple in implementation and less sensitive to theparameter variations yet performs as well as FOC technique[1]. It is based on the decoupled and independent control ofstator flux and torque providing a quick and robust response.However, the conventional DTC strategy using switching tableof a six pulse voltage source inverter (VSI) presents notabletorque, flux, current and speed ripple. In DTC, stator voltage

vectors are selected according to the differences between thereference and actual torque and stator flux linkage.

The DTC based induction motor drive (IMD) [2] uses asingle-phase or three-phase uncontrolled ac-dc converter (forrectification of ac mains voltage), an energy storage element(capacitor filter for smoothening the dc link voltage), and athree-phase voltage source inverter (VSI) for feeding a three-

phase squirrel cage induction motor. Fig. 1 shows the basicblock diagram of conventional DTC based IMD with un-controlled three-phase diode bridge rectifier. Such type ofutility interface suffers from problems related to power qualitysuch as poor power factor, injection of current harmonics intothe ac mains, variation in dc link voltage with fluctuations in

the voltage of input ac supply, equipment overheating due toharmonic current absorption, voltage distortion at the point ofcommon coupling (PCC) due to the voltage drop caused byharmonics currents flowing through system impedance anddecreased rectifier efficiency. These power quality problemscan cause malfunction of sensitive electronic equipments,interference in telephone and communication lines due to highfrequency switching, failure of switching capacitors and other

power equipment and loss of data. Different internationalorganizations have given guidelines to impose strict limits onthe levels of harmonic current emissions through variousstandards such as IEEE-519, IEC 61000-3-2, IEEE-1531 etc.

Different active and passive filtering techniques [3] are

used to mitigate distortions in the input line current. Theapproach of active power factor correction can achieve unity

power factor and very low total harmonic distortion (THD) ofac mains current along with a good regulation of the dc linkvoltage. But the disadvantages are high cost (cost is nearlydoubled), increased complexity in control (particularly ifoptimum performance is desired under preexisting supplyvoltage distortions), and higher dc-link voltage due to boostoperation.

The main objective of this paper is to design an improvedpower quality AC-DC converter at the front end of a DTC

978-1-4577-1510-5/11/$26.00 2011 IEEE

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H HTe S(1) S(2) S(3) S(4) S(5) S(6)

1

1 V2(110) V3(010) V4(011) V5(001) V6(101) V1(100)

0 V7 V8 V7 V8 V7 V8

-1 V6 V1 V2 V3 V4 V5

-1

1 V3 V4 V5 V6 V1 V2

0 V8 V7 V8 V7 V8 V7

-1 V5 V6 V1 V2 V3 V4

B. DTC Based IM DriveDTC scheme consists of three main blocks: Flux and torque

hysteresis controller, voltage switching table and actual flux,torque and speed estimators [6]. In this control, the torque andstator flux of the drive are directly controlled by invertervoltage space vector selection through a lookup table as shownin Table-I. The switching vectors to the inverter are selected insuch a way as to minimize flux and torque errors.

A speed regulator with limiter is used to generate referencetorque Te

*.

* *

e(n) e(n-1) p ( ) e(n-1) i e(n)T = T +K ( - ) + K e n (7)

where, e is the speed error between the reference speed (r*)

and the sensed speed (r).

The command stator flux and s*torque Te

*magnitudes arecompared with the respective estimated values (s, Te), and theerrors are processed through hysteresis-band controllers asshown in Fig. 2. The flux hysteresis controller is as follows:

H= 1 if |s*| |s| > +HBs (8)

H= 1 if |s*| |s| < HBs (9)

The torque hysteresis controller is given by

HTe = 1 if |Te*| |Te| > +HBTe (10)

HTe= 1 if |Te*| |Te| < HBTe (11)

HTe= 0 if HBTe< (|Te*| |Te|) < +HBTe (12)

where, HBs and HBTe are the flux and torque predefinedhysteresis bands.

The actual stator flux sand electro-magnetic torque Teareestimated by using vdss, vqssand idss, iqssas,

ds= (vdss idssRs)dt + ds0 (13)qs= (vqss iqssRs)dt + qs0 (14)

2 2

s ds qs+ = (15)

Te= (3/2)(P/2)( ds iqss qsidss) (16)

where, Rs is the stator resistance, ds0and qs0 initial stator dqfluxes.

Considering d-axis is aligned with the a-axis, the three-phase voltages vabcare transformed into stationary dq referenceframe (vdssand vqss) by using Clarke transformation as,

vqss= (1/3)(2vavbvc) (17)

vdss= (1/3)(vbvc) (18)

The stator phase voltages vabc are determined by theswitching states (Sa, Sband Sc) and sensed dc link voltage Vdcusing the following equations:

va= (Vdc/3)( Sa+2SbSc) (19)

vb= (Vdc/3)(Sa+2SbSc) (20)

vc= (Vdc/3)(SaSb+2Sc) (21)

TABLEI. INVERTER VOLTAGE SWITCHING

Similarly, three-phase currents iabc also transformed intotwo-phase idssand iqssusing above equation as,

iqss= (1/3)(2iaibic) (22)

idss= (1/3)(ibic) (23)

where, iaand ibare the sensed stator currents. ic= (ia+ib).

III. MATLAB SIMULATIONDTC based IMD fed with a simple diode bridge rectifier

and Minnesota rectifier are simulated in MATLAB/ Simulinkplatform. The simulink model for DTC based IMD withMinnesota rectifier is shown in Fig. 5. The model of the

proposed three phase Minnesota rectifier is shown in Fig. 6.The rating of the induction motor considered in this simulationis 2.2 kW (3 hp), 230 V and 50 Hz.

IV. RESULTS AND DISCUSSIONSPerformance of the DTC based IMD is studied for both the

configurations namely, a six-pulse diode bridge rectifier andMinnesota rectifier at the front end. Fig. 7 shows the dynamic

performance of the drive fed from a six-pulse diode bridgerectifier for different load conditions. Waveforms consist of

source phase voltage (vas), source line current (ias), rotor speed(Nr), stator currents (iabc), electromagnetic torque (Te) and Vdcfor light-load (20% of TFL=2.8N-m) and rated load (TFL=14N-m). The ac mains current waveform and its harmonic spectra atfull load and light load are shown in Figs. 8 and 9 which showsthat THD at full load and light load are respectively 66.65%and 100.13%. The power factor also deteriorates from 0.8 atFL to 0.69 at light load. From these results it can be concludedthat it is necessary to use improved power quality converters atfront end of the DTC based IMD. As the next step, a Minnesotarectifier is employed in place of the uncontrolled 6-pulseconverter.

The waveforms of a DTC based IMD for different load

conditions fed from a Minnesota rectifier at the front end areshown in Fig. 10. The ac mains current and its harmonicspectra for full load and 20% load are shown in Figs. 11 and12. It can be noted that the THD of ac mains current at full loadis 4.88% and the power factor obtained is 0.997. At light load,THD of ac mains current is 9.28% and power factor is 0.986which shows a significant improvement as compared to thecase fed from a simple diode bridge rectifier. Table-II showsthe comparison of different power quality indices at twodifferent load conditions for DTC based IMD with a simplediode bridge rectifier and a Minnesota rectifier. Thecomparison of THD and PF with variable load for DTC based

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Figure 9. AC mains current and harmonic spectrum for DTC based IMDwith a simple diode bridge rectifier at light load (20%).

Figure 10. Dynamics of a DTC based IMD with Minnesota rectifier.

1.58 1.59 1.6 1.61 1.62 1.63

-10

0

10

FFT window: 3 of 100 cycles of selected signal

Time (s)

0 5 10 15 200

5

10

Harmonic order

Fundamental (50Hz) = 10.5 , THD= 4.87%

Mag(%ofFundamental)

Figure 11. AC mains current and harmonic spectrum for DTC based IMD

with Minnesota rectifier at rated load.

1.48 1.49 1.5 1.51 1.52 1.53-4

-2

0

2

4

FFT window: 3 of 100 cycles of selected signal

Time (s)

0 5 10 15 200

5

10

Harmonic order

Fundamental (50Hz) = 3.189 , THD= 10.83%

Mag(%ofFundamental)

Figure 12. AC mains current and harmonic spectrum for DTC based IMD

with Minnesota rectifier at light load (20%).

Figure 13. Variation of THD and PF with the load on DTC based IMD with asimple diode bridge rectifier and a Minnesota rectifier.

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