International Journal of Science, Engineering and Technology Research (IJSETR), Volume 3, Issue 3, March 2014

ISSN: 2278 – 7798All Rights Reserved © 2014 IJSETR

439

STACKER/RECLAIMER LONG TRAVEL DRIVEOPERATION WITH VFD - A PERFORMANCE STUDY

Niraj Kumar Sahu, Ram Prakash Bhatele

1Abstract— The aim of this paper to study the performance ofstacker/reclaimer long travel drives with the use of VFD andcomparison of the same with the earlier system. In a thermalpower station stacker/reclaimers (S/R) are the most needful andimportant equipments used to stack the excess coal receivedfrom mines in coal yard of coal handling plant for futurerequirements and reclamation of the stacked coal is doneduring emergency condition when coal supply from minesstopped due to various reasons. For stacking and reclamation ofthe coal from coal yard stacker/reclaimer has to travel around600m length on the track . For travelling of the S/R in the trackSix nos. of Long Travel (LT) drives are used. Initially thecontrol of the initial torque for long travel drives were donewith the use of resistance box. Now Variable Frequency Drive(VFD) is used in the long travel drives of S/R for controlling thetravel speed of the S/R. By use of VFD in LT drives of S/Rresulted in smooth starting of the LT drives and reduction ofthe running currents of LT drives at reduced speeds. By the useof VFD a considerable energy savings and smoother operationof the long travel drives is observed as compare to earliersystem.

Index Terms—Coal Handling Plant, Energy Saving, LongTravel (LT), Stacker/Reclaimer (S/R), Variable FrequencyDrive (VFD).

I. INTRODUCTION

Electricity is essential for growth and developmentof the industries. India is a developing country and thedemand of electricity in the country will be more and more,we have to put our sincere efforts to reduce the cost ofgeneration of the electricity. One of the solutions for this isby reducing the auxiliary power consumption in the powerstation.

Now a days we are facing acute problem of coalsupply to thermal power stations used for generation ofelectricity. For reduction in the cost of power generation theauxiliary power consumption & repair and maintenance costof the station should be reduced. For conservation of theconventional resources we have to take some corrective andeffective measures. As a measure of cost reduction now adays VFDs are used to run the auxiliary equipments used forpower generation in a thermal power station to reduce thepower consumption, where there is a speed variation inequipments is required and which is being done by means ofother conventional methods of control without saving theenergy.

Manuscript received Feb , 2014.Niraj Kumar Sahu, Research Scholar, SVN University Sagar(MP)

India,Ram Prakash Bhatele, Principal, SRIT Jabalpur(MP) India,

The Stacker/Reclaimer is the biggest equipment asshown in Fig.-1 used in coal handling plant of a thermalpower station for stacking of the excess coal received fromthe mines in the coal yard and the same coal have to bereclaimed during emergency conditions when the coal supplystopped by the mines. For stacking and reclamation of thecoal in the coal yard, the stacker/reclaimer have to travel inthe track for about 600mtrs length. To travel in the length 06nos. long travel drives as shown in Fig.-2 are used. Duringstacking and reclamation of coal the operators have tocontrol the speed of the travel as per requirement of the job.As S/R is a heavy machine requires high initial torque. Forobtaining high initial torque, resistance is being injected inthe rotor circuit through slip ring of LT motors from theresistance box. But there was no saving in energy becausethe long travel drives were running with full currents as pertheir ratings.

Fig.1 photograph of stacker/reclaimer

Fig.2 schematic diagram of stacker/reclaimer & LT drives

Long Travel Drives

International Journal of Science, Engineering and Technology Research (IJSETR), Volume 3, Issue 3, March 2014

ISSN: 2278 – 7798All Rights Reserved © 2014 IJSETR

440

VFD is now a days common for controlling thespeed of induction motors by variation of frequency. Toreduce the power consumption in long travel drives ofstacker/reclaimer at low speed, variable frequency drives(VFD) is now used for controlling the speed of long travel.By use of the VFD in stacker reclaimer the operation of theLT drives become smoother.

II. OPERATION AND ADVANTAGES OF VFD

Fig.3 VFD schematic diagram

The induction motors are the extensively used in thepower plant for various auxiliaries which are operating atfixed speed as per the supply frequency. Alternating currentgiven to the stator windings of an induction motor produces amagnetic field that rotates at synchronous speed. This speedmay be calculated by dividing line frequency by the numberof magnetic pole pairs in the motor winding i.e. Speed (rpm)= frequency (hertz) x 120 / no. of poles. The rotor of aninduction motor attempts to follow this rotating magneticfield, and, under load, the rotor speed slightly slips behindthe rotating field. This slip speed generates an inducedcurrent, and the resulting magnetic field in the rotor producestorque.

The torque developed[12] by the induction motorfollows the equation below:T = k1 . m . I2where: m= k2 .V1/f1

m : magnetising flux (Wb)T : torque available on the shaft (Nm)I2 : rotor current (A) à depends on the load!V1 : stator voltage (V)k1 & k2 : constants à depend on the material and on themachine design.

To have the flux constant the ratio of voltage tofrequency should be constant.

Since an induction motor rotates nearer tosynchronous speed, the most effective way to change themotor speed is to change the frequency of the applied voltage.

A variable frequency drive (VFD), as shown inFig.3, is an electrical variable speed. When the system needsto work at reduced speed for long time at reduced load itwastes energy. A VFD allows us to adjust the motor-speedcapability and match it with motor-output load. This is howit saves energy. Varying the frequency output of the VFDcontrols motor speed: Speed (rpm) = frequency (hertz) x 120/ no. of poles.

The VFD uses the IGBT[4], the IGBT can switchon and off several thousand times per second and preciselycontrol the power delivered to the motor. The IGBT uses amethod named “pulse width modulation” (PWM)[1][4] to

simulate a current sine wave at the desired frequency to themotor.

The utilisation of VFD in a system gives variousadvantages like[4][6]:-

1.Soft starting features for a longer life of electrical andmechanical equipment2.It reduces the power consumption of drives at reducedspeed.3. It improves the power factor of induction motor.4. It improves the electrical efficiency of induction motor.5. The operation of drive system is smooth.

III OPERATION OF LONG TRAVEL DRIVES OF S/RWITH RESISTANCE BOX AND VFD.

a) S/R LT Drives operation with Resistance Box:-

Fig.4 connection diagram of slip ring induction motor

LT drives are used in stacker/reclaimer fortravelling of the stacker/reclaimer in the track of S/R in coalyard area. 06 nos. of LT drives are used in thestacker/reclaimer and the power supply to the all motors arefrom common source or we can say that the motors areconnected in parallel. The motors used in the long traveldrives are slip ring induction motor ( as shown in fig.4). Byadding external resistance, in the slip ring induction motor,makes the rotor resistance high when starting, thus the rotorcurrent is low and the starting torque is maximum. As themotor reaches its base speed (full rated speed), after theremoval of external resistance and under normal runningconditions, it behaves in the same way as a squirrel cageinduction motor. Resistance box was used for the injection ofthe resistance in the rotor circuit. By operation of the longtravel drives with resistance box there was no saving in thepower consumption. The drives were taking current as pertheir rating and same time the power factor of the driveswere very poor.

There are some disadvantages in this method ofspeed control. As the rotor resistance is increased, the “I2*R” losses also increases which in turn decreases theoperating efficiency of the motor. It can be interpreted as theloss is directly proportional to reduction in speed. Since the

International Journal of Science, Engineering and Technology Research (IJSETR), Volume 3, Issue 3, March 2014

ISSN: 2278 – 7798All Rights Reserved © 2014 IJSETR

441

losses are more, this method of speed reduction is notbeneficial.

Following observation was recorded with theoperation of S/R LT drives with resistance box.:-

Table-1 ( observation for single motor with Resistance box in S/RLT)

S.N. CurrentDrawn(Amp)

PowerFactor

Frequency(in Hz)

Voltageapplied (inVolts)

1 18 0.6 50 415

b) S/R LT Drives operation with VFD:-

Fig.5 photographs of VFD used in stacker/reclaimer LT drives

By use of VFD in an induction motor there is apossibilities available to run the drive at desired speed byvariation of frequency i.e. from 0 to 50Hz. Now VFD isinstalled in stacker/reclaimer long travel drive operation fortravelling of stacker/reclaimer in the coal yard area.(asshown in Fig.5)[11], the power supply to the all motors arefrom common source through a single unit VFD or we cansay that the motors are connected in parallel As the S/R is aheavy equipment in the coal handling plant the starting of thelong travel starts with jerk. By use of VFD the starting speedis controlled by variation of the frequency and hence, thestarting of the drives become smooth. Also due toavailability of variation of the LT speed by the VFD, theposition the stacker/reclaimer is now controllable forstacking and reclamation of the coal. Power consumption ofthe drives at reduced speed drastically reduced as well as slipof the motor is also reduced. Also all the motors are takingalmost same current and running at same speed.Observations are shown in following Table-2.

Table-2 ( observation for single motor with VFD in S/R LT)

S.N. Frequency(in Hz.)

Speed(in RPM)at 2.8%slip

Measuredspeed ofmotor (inRPM)

Currentwith VFD(in Amp.)

PowerFactor(with useof VFD)

1 30 583.2 585 7 0.9

2 35 680.4 684 8.2 0.9

3 40 777.6 784 8.7 0.9

4 45 874.8 883 9.3 0.9

5 50 972 982 10.1 0.9

IV CALCULATIONS

Rating of stacker/reclaimer LT Motor - 3 ph., 7.5kw, 415V,972RPM, 50Hz. 06 nos. Long Travel Drives are used.

As per the observations of Table-1 followingcalculations are done:-

(i) While operation of the stacker/reclaimer LT Motor wasdone with the resistance box as per table-1, powerconsumption is calculated below:-

Power = 1.732xVoltage(V) x Current(I) x power factorPo = 1.732x415x18x0.6 = 7.76 kw

Total power consumption by 06 motors = 7.76x6= 46.56 kw.

(ii) As shown in Table-2, the current of the paddle feeder isless at reduced frequency and also the power factor improvedto 0.9. Hence, power drawn by the motor with differentfrequencies is as calculated below:-

Power (P) = 1.732xVoltage(V)xCurrent(I)xpower factor x 6

% saving in power = (Pr-P)*100/Prwhere, Pr- total power of motors with resistance box which is46.56kw as above, P-total power drawn by the motors withVFD.

(1). at freq. 30Hz, P1 = 1.732x415x7x0.9x6= 27.17 kw

% Saving of power = (46.56-27.17)*100/46.56= 41.64%

(2). at freq. 35Hz, P2 = 1.732x415x8.2x0.9x6= 31.82 kw

% Saving of power = (46.56-31.82)*100/46.56= 31.64%

(3). at freq. 40Hz, P3 = 1.732x415x8.7x0.9x6= 33.76 kw

% Saving of power = (46.56-33.76)*100/46.56= 27.49%

(4). at freq. 45Hz, P4 = 1.732x415x9.3x0.9x6= 36.09 kw

% Saving of power = (46.56-36.09)*100/46.56

International Journal of Science, Engineering and Technology Research (IJSETR), Volume 3, Issue 3, March 2014

ISSN: 2278 – 7798All Rights Reserved © 2014 IJSETR

442

= 22.48%

(5). at freq. 50Hz, P5 = 1.732x415x10.1x0.9x6= 39.20 kw

% Saving of power = (46.56-39.20)*100/46.56= 15.80%

The above calculations are shown in Table-3 & graph no.7.

Table-3 (power drawn & % power saving with VFD in S/R LTdrive)

S.N.

Frequency(in Hz.)

Current withVFD(in Amp.)

PowerFactor (withuse of VFD)

Powerdrawn Pin kw

% savingof Power

1 30 7 0.9 27.17 41.64

2 35 8.2 0.9 31.82 31.64

3 40 8.7 0.9 33.76 27.49

4 45 9.3 0.9 36.09 22.48

5 50 10.1 0.9 39.2 15.8

Fig.7 graph between freq. & % saving in power

(iii) The speed of an Induction motor, n = 120*f*(1-s)/p.Where n = motor speed, f = frequency, p = no. of poles,s = slip

Hence, by variation in any of the above threeparameters the speed of the motor can be changed.

As shown in Table-2, the measured speeds of theInduction motor at different frequency is higher than thespeed at 2.8% slip as given on motor nameplate.

%Slip of motor (s) = (Ns-N)*100/Ns.where Ns-synchronous speed, N-actual motor speed.The slip at different frequency and measured speed

is shown in Table-4. Fig.8 shows the graph betweenfrequency Vs % slip at measured speed.

Table-4 (measured slip with VFD in S/R LT drive)

S.N. Frequency(in Hz.)

Speed(in RPM) at2.8% slip

Measuredspeed of motor(in RPM)

Slip (s) at measuredspeed (in %)

1 30 583.2 585 2.5

2 32 680.4 684 2.28

3 35 777.6 784 2

4 38 874.8 883 1.88

5 40 972 982 1.8

Fig.8 graph between freq. & % slip at measured speed

V CONCLUSIONS

As shown in the Tables-2,3 & 4 and Fig.7 & 8 bythe use of VFD in stacker/reclaimer long travel drives thecurrent consumption at reduced speed is reduced very muchand hence, energy saving. The VFD works as soft starter forthe drives and hence, the starting of long travel drivesbecome smoother without any jerk and hence, it can be saidthat the life of associated components of the drives will bemore compare to operation of drives with resistance box.Operation of the long travel become easier and needs lessmaintenance. The slip of the Induction motor also reduced byuse of VFD as shown in the Table-2 measured value of thespeed is more compared to rated slip speed of the motor andhence, the electrical efficiency of the motor has increasedbecause most of the power is utilised to drive the motor rotor.Further studies may be needed to know the applicability ofVFDs in other equipments of coal handling plant of athermal power station.

ACKNOWLEDGMENT

Kind co-operation of the management as well asemployees of NTPC Ltd. Korba for this academic work ishighly acknowledged.

NTPC Limited (Formerly National Thermal PowerCorporation) is the largest power generation company inIndia. The total installed capacity of the company is 42,454

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International Journal of Science, Engineering and Technology Research (IJSETR), Volume 3, Issue 3, March 2014

ISSN: 2278 – 7798All Rights Reserved © 2014 IJSETR

443

MW including Coal, Gas, and Solar plants located across thecountry, NTPC Ltd. Korba being one of them.

REFERENCES

[1]. Thida Win, Nang Sabai, and Hnin Nandar Maung , World Academy ofScience, Engineering and Technology 42 2008,"Analysis of VariableFrequency Three Phase Induction Motor Drive".[2]. Neetha John, Mohandas R, Suja C Rajappan, International Journal ofEmerging Technology and Advanced Engineering Website: www.ijetae.com(ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 3,March 2013) 784 , "Energy Saving Mechanism Using Variable FrequencyDrives".[3]. "Variable Frequency Drives and Energy Savings",by: StephenPrachyl,White Paper,April 2010,Siemens Industry.[4]. "Operation And Application Of Variable Frequency Drive(VFD)Technology",Carrier Corporation Syracuse, Tech note,New YorkOctober 2005.

[5]. "Energy Savings with Variable Frequency Drives",2007 , Tech note,Rockwell Automation.[6]. "Power generation Medium voltage drives for more efficient andreliable plant operation",Power Brochure ABB 2011.[7]. "Variable Frequency Drives",Energy Efficiency Reference Guide byCeati International.[8]. "Adjustable Speed Drive", Reference Guide,Fourth Edition, August1997,Revised by:Richard Okrasa, P.Eng.Ontario Hydro.[9] Waide, Paul; Brunner, Conrad U. (2011). . International Energy Agency.Retrieved Jan. 27, 2012."Energy-Efficiency Policy Opportunities for ElectricMotor-Driven Systems".[10] Eisenbrown, Robert E. (May 18, 2008). "AC Drives, Historical andFuture Perspective of Innovation and Growth". Keynote Presentation for the25th Anniversary of The Wisconsin Electric Machines and PowerElectronicsConsortium (WEMPEC). University of Wisconsin, Madison, WI, USA:WEMPEC.[11]. Brochures, Vacon Plc Runsorintie 7, 65380 Vaasa, Finland.[12]. Technical guide – Induction motors fed by PWM frequency inverters-

by WEG.