Arch. Min. Sci., Vol. 53 (2008), No 2, p

Arch. in. Sci., Vol. 53 (2008), No 2, p. 319 329 319 ZBIGNIEW KASZTELEWICZ*, JERZY SZYAŃSKI** ENERGY SAVING CONTROL ETHOD OF ELECTRICAL DRIVES IN COVEYOR BELTS WITH REGULATED SPEED IN SURFACE BROWN COAL INE ETODA ENERGOOSZCZĘDNEGO STEROWANIA NAPĘDE PRZENOŚNIKA O REGULOWANEJ PRĘDKOŚCI TAŚY W KOPALNIACH WĘGLA BRUNATNEGO In the paper energy saving control method of motors in double drum front station drive of conveyor belt with regulated belt speed is presented. The used control method assures equal load of all motors in static as well as dynamic states. Furthermore, proposed control method assures the work of the squirrel cage motors with equal slip. In the drive, high efficient low voltage squirrel cage motors fed by means of field oriented controlled frequency converters, supplied from IT mains (3 500V), were used. The proposed solution has been successfully used in coal conveying system on the new polish opencast Drzewce in the Konin brown coal mine in the year 2006. Keywords: electrical drive, frequency converter, conveyor belt, front station drive, brown coal, overburden W artykule przedstawiono energooszczędną metodę sterowania silników w dwubębnowym napędzie czołowym przenośnika o regulowanej prędkości taśmy. Zastosowana metoda sterowania zapewnia jednakowe obciążenia wszystkich silników w stanach dynamicznych i statycznych oraz pracę z jednakowym poślizgiem. W napędzie zastosowano wysokosprawne niskonapięciowe siniki klatkowe sterowane wektorowymi napięciowymi przemiennikami częstotliwości zasilanymi z sieci izolowanej IT 3 500V. Ciąg węglowy oparty na zaproponowanym rozwiązaniu został uruchomiony na nowej odkrywce Drzewce w Kopalni Węgla Brunatnego Konin w 200 roku. Słowa kluczowe: napęd elektryczny, przemiennik częstotliwości, przenośnik taśmowy, napęd czołowy, węgiel brunatny, nadkład * AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY, DEPARTENT OF OPENCAST INING, AL. ICKIEWICZA 30, 30-059 KRAKOW, POLAND ** TECHNICAL UNIVERSITY OF RADO, FACULTY OF TRANSPORT, UL. ALCZEWSKIEGO 29, 26-600 RADO, PO- LAND

320 1. Introduction Classical drive solutions don t assure proper, precise and energy saving control of electrical motors in static and dynamic states. Solution of conveyor belts based on slip ring induction motors were broadly used in 70 s in last century. These drives solution didn t give the possibility of regulation of the speed of the belt in the function of load. In the classical solution one of the main problems during the project phase and exploitation of high power conveyor belts is non-equal load sharing of the drive entities in static movement. The main reason of this phenomenon is a stiff characteristic of induction motors, longitudinal and diagonal elastic deformation of rubber belt and diagonal deformations of rubber facings of the drums fig. 1 (Gładysiewicz, 2003). Fig. 1. Double drum main drive of conveyor belt which is fed from industrail mains where the frequency and amplitude of supplied voltage are constant: a) i c) echanical characteristics of two motors parallel drive of one drum; b) Torques of driving drums with different slips of motors and with the assumption that produced mechanical characteristics of drum s motors 1 and 2 are equal. (SK ij squirrel cage motor, where i number of the drum, j number of the motor which drive the drum) Rys. 1. Dwubębnowy napęd główny przenośnika taśmowego zasilanego z sieci przemysłowej o stałej wartości napięcia i częstotliwości: a) i c) charakterystyki mechaniczne dwóch silników przy równoległym napędzie jednego bębna napędowego; b) momenty bębnów napędowych przy różnych poślizgach silników i założeniu jednakowych sumarycznych charakterystyk mechanicznych silników bębna 1 i 2. (SK ij silnik klatkowy, gdzie i nr bębna, j nr silnika napędzającego bęben)

321 The rotational speed of drums in multi-drum front station of conveyor belt is different what in turn results to the work of the drums with different slips. This kind of effect can cause increased drive moment on one of the drums and as a consequence loss of grip between the belt and drum. The difference of rotational speed is dependent on many aspects, mainly associated with conveyor s belt i.e.: resilience of the belt, strength of tightening, length of belt, load of the belt and others. In high power motors the differences in mechanical characteristic are crucial as well Taking into account contemporary development of power electronics converters classical drive s solution can be regarded as historical. (Czopek, 2001; Kasztelewicz, 2006a, 2006b). 2. Digital control of conveyor belt s drive Proposed by authors digital control method of multi-motor drives of conveyor belts assures equal load of motors in static and dynamic states. This kind of drive s solution is energy efficient, have lower exploitation costs in comparison with still used classical drives. oreover, frequency converter based control of conveyor belt gives the possibility of control of speed of the belt in broad rage fig. 2. Digital control of asynchronous motors has no reasonable alternative when control of speed and torque is needed. In the modern industrial projects digitally controlled asynchronous motors have practically displaced classical drive system (Orłowska, 2003; Jeftenic et al., 2006). Fig. 2. Frequency converters based digital control of the conveyor belt s double drum drive with regulated belt speed Rys. 2. Cyfrowe sterowanie silnikami napędu głównego przenośnika z zastosowaniem napięciowych przemienników częstotliwości do sterowania każdym silnikiem

322 3. The method of equalization of squirrel cage motors load in multidrum conveyor belt drive The main drive of conveyor belt in brown coal mine usually consists of two driving drums on the front station.. Thus, there can be installed maximum 4 drive entities two for each drum. As driving units high efficient squirrel cage motors fed by frequency converters are used. Coal conveyor belts in brown coal mines are usually horizontal or bended with 16 and length in range from 100 m to 2000 m. It is estimated that power of driving motors is not higher than 1W for 1000 m of length of conveyor belt.. These conveyor belts are self braking i.e. there are stopping themselves after switching of the power. During the stop mechanical brakes are enabled which in turn prevent the belt from moving backwards by means of gravitation force. During the preliminary conveyor belt s calculation witch precede the analysis of load of individual motors, it is assumed that distribution of circumference forces on the driving drums is proportional to the installed power. This assumption makes possible the execution of full cycle project calculations together with belt selection, diameter of drum, forces in the belt during the static movement and the arrangement of drums of the main drive. In conveyor belts where two or more driving drums are used after the preliminary calculations the load (torques) of individual motors was assessed taking into to account following aspects: (Gładysiewicz, 2003): Different stiffness motor s mechanical characteristics which result from production s deviation Different circumference speed in the places where rubber belt overlaps the drums what in turn is the result of elastic deformation of the rubber belt. Different radii of drums which results from bends of rubber lining. The stiffness of motor s mechanical characteristics fed by power electronics converter can change because of complicated control and construction of frequency converters. The utilization of methods which correct the stiffness of characteristics in multi-motor drives is crucial for assure equal load sharing of individual motors. Equation (1) describes the difference between the loads(torques) of driving motors- ΔT i when the stiffness of motor s mechanical characteristic is different fig. 1a and 1c. A ij represents he stiffness of motor s mechanical characteristic CT ij, k ij stands for coefficient of relativ motor s mechanical characteristic CT ij. Two motor drive individual drum what in turn causes that their synchronous speed ω synchr and slip s are equal. T T i where: and T i1 T nom T ij i 2 A s k and A s ( k k 2) i s ij nom i i1 and i T A s nom zn motor nominal values, k ij A ij A (1)

323 Indexes i and j represent accordingly the number of the driving drum and number of the motor which drive the drum. The indexes are equal to 1 or 2 for four-motor double drum front station drive of conveyor belt. Equation (2) expresses the difference of loads (torques) between two driving drums of conveyor belt ΔT 1-2 which results from different angle speeds of the motors which drive these drums fig. 1b. 2 1 T1 2 T1 T2 A ( s1 s2) A synchr here: T T 1 11 T 12 and T 2 T 21 T 22 (2) Complete decrease of drive s torque of main conveyor belt ΔT p which consists of two driving drums on the front station is expressed by means of (3). T p T T T (3) 1 2 12 The usage of frequency converters for multi-motors drive digital control makes possible lossless equalization of k ij coefficients which define the change of motors mechanical characteristics in static work, in relation to nominal characteristic of one of the main drive s motors of conveyor belt. Frequency converters enable the adjustment of voltage supply for individual motor [s ij = f (u ij )] in order to fulfil relationships expressed by means of (4). If equation (4) is fulfilled the difference between both motors ΔT i is negligible small. Usually the level of voltage supply can by changed by means of frequency converter in range of ±10% U n. k 11 k12 and k 21 k 22 (4) Frequency converters make possible the utilization of different synchronous speeds for motors which drive first ω synchr1 and second ω synchr2 driving drum. When different synchronous speeds are used the differences between the loads of the drum can be minimalized. In such case different speed of the drums will not influence their loads and ΔT 1-2 (2) will be equal to zero, according to equation (5). s synch 1 1 synch 2 2 1 s2 T1 2 synch 1 synch 2 0 (5) The set of different synchronous speeds ω synch1 and ω synch2 of the drums motors has to be done automatically. The used control method should has the ability of self regulation of synchronous speeds dependent on the load and linear speed of the belt which is set. The self regulation mechanism has to equalize the load torques of drums T 1 and T 2 independent from the load and the speed of the belt.

324 From the conducted analysis it results that in comparison with classical drive solution the usage of power electronics converters to control the induction motors enables the elimination of driving torque s decrease of multi-motor front station ΔT p (3). Assuming equal load of the motors in both driving drums the difference of synchronous speeds of the motors Δω synchr (6) varies and is proportional to the actual difference between driving drums speeds synchr 1 2 (6) The determination of the control method of multi-drum drive which realize the regulation of the synchronous speeds difference value Δω synchr, thereby coordination of conveyor belt s driving drums speed is the subject matter of the further part of the paper. Assurance of the proper drums speed control is crucial for optimal work of the conveyor belt. Fig. 3. Configurations of drive sets of conveyor belts where power electronics converters are used: a soft start of the conveyor belts, no regulation of the belt s speed; b soft start of the conveyor belt with regulation of the belt s speed; c soft start of the conveyor belt with regulation of the belt s speed and equalization of the driving torques on the drums; d soft start of the conveyor belt with regulation of the belt s speed and equalization of the driving torques of all motors. (C squirrel cage motor, B1, B2 driving drums of front station, Softstart soft start device, FC drive s frequency converter) Rys. 2. Konfiguracje układów napędowych przenośników taśmowych z zastosowaniem przekształtników energoelektronicznych: a) łagodny rozruch przenośnika, brak sterowania prędkością taśmy; b) łagodny rozruch przenośnika ze sterowaniem prędkości taśmy; c) łagodny rozruch przenośnika ze sterowaniem prędkości taśmy i wyrównaniem momentów napędowych na bębnach; d) łagodny rozruch przenośnika ze sterowaniem prędkości taśmy, wyrównaniem momentów napędowych wszystkich silników. (C silnik klatkowy, B1, B2 bębny napędowe stacji czołowej, Softatart urządzenie łagodnego rozruchu, FC napędowy przemiennik częstotliwości (napięciowy lub prądowy))

325 Configuration depicted in the fig. 3d has the best drive features. Frequency converters are matched to the power of individual motor. In case of breakdown of individual frequency converter further work of the drive is possible. In such case each motor is monitored by its own frequency converter. When frequency converters are used the creation of high starting torque in zero speed has to be taken into consideration. The start of fully loaded conveyor belt after emergency stop occurs often in mining practice. The authors have used frequency converters which utilized filed oriented vector control method. The nominal power of the converters was increased in respect to nominal power of induction motors in order to achieve 2.2T n of the motor Frequency converters producers still don t use the functions which makes possible automatically adjustment of frequency converters for forcing the same torque of the motors in case when one drum (shaft) is driven by means of two separate motors. Described feature can be realized by means of external controller e.g. PLC. 4. The realization of energy efficient control method of conveyor belt s duble drum drive sets In the conveyor belts drive sets with regulated belt speed and the powers of individual low voltage motors up to 1.2 W, there exists technically the possibility of usage of low voltage frequency converters. (Antoniak, 2007; Kasztelewicz et al., 2008). The drive stet with new high efficient squirrel cage motor and low voltage frequency converter is still more economical justified than the drive set with low efficient medium voltage slip ring motor and medium voltage frequency converter. The efficiency of the motors has decisive meaning when the efficiency of the whole conveyor belt system is taken into account. Because of the working conditions of the conveyor belt the motors work not fully loaded for significant part of time. Because of this reason it should be pursuit to minimize the share of self losses of the motor (mainly no load losses) in the balance of energy consumption. ining supply mains of frequency converters are isolated from ground (IT mains). In the Konin brown coal mine on the new opencast Drzewce, exploited since 2006, the supplying voltage type 3 6kV or 3 500V is used. In the drives of conveyor belts in opencast Drzewce the new high efficient squirrel cage motors 315kW/742 rot / min 489A/500V (type SXh 355 H8Es 315kW) were used. In order to assure high starting torque the frquency converters which use filed oriented control method (FOC) were used. During the test different control methods were checked. The best result were achieved when the master-slave concept of control of multi-motor drive was used were the loads of individual motors are equalized and one of the frquency converters works as a master and sets the torques for other motors (slaves). The quality of control was assess on the basis of the following criteria: the value of loaded conveyor belt s starting torque motors torque at start of conveyor when the master motor frequency increase according S (sinus) shape curve

326 the stability of the belt s velocity after the starting phase the equality of the loads of individual motors in the time of start of conveyor belt the equality of the loads of individual motors after the end of starting phase when different belt s speed was utilized. The frequency converter which works as a master controls the speed of the motor. On the other hand the power converters which serve as slaves control the driving torque in permitted range of speeds. The master sets a proper moment for slaves. In such case the regulation of load of the motors occurs what in turn leads to equalization of the load. In such case the performance of the drive depend on the drive properties of the frquency converters which were used. The control of speed can be defined as: SC (speed control) here T (torque) = variety for v (speed) = constant. The control of torque can be defined as: TC (torque control) where v (speed) = variety for T (torque) = constant. The control of speed and control of torque can be expressed by means of equation (7). SC T f ( v ), TC v f( T ), here T min T T max and v min v v max (7) where: SC means Speed Control and TC means Torque Control v speed of motor shaft T torque on shaft of motor The control scheme of multi-motor conveyor belt s drive, which fulfils the relationship (6), is depicted in the fig. 4. The drive system of conveyor belts, where the front drives of individual conveyor belts are controlled by means of described master-slave method, was successfully installed opencast Drzewce in Konin brown coal mine in 2006. Tab. 1 shows the number of drum s motors and their placement in the individual drives of conveyor belts. Tab. 1 depicts the values of the belt s forces in the start and stop phase in conveying system of the opencast. The conveyor belt s set in opencast Drzewce in Konin mine is designed for the brown coal transportation and works in no-maintenance mode with remote control. The system enables, with maximal efficiency of excavator, the load of 500 ton (10 wagons each 50 tons) in the time of 20 minutes. Theoretical during 24 h the transportation of 36 000 tons of brown coal is possible. In the system of the coal conveyor belt the individual speed of conveyor belt depends on the load.

327 Fig 4. The speed control method of double drum s drive of the front station: ω s set rotational speed value, T 1 load torque of motor 1, k i (i = 2, 3, 4) correction coefficient of set load torque T 1 Rys. 4. Zasada sterowania dwubębnowym napędem czołowym przenośnika z regulowaną prędkością taśmy: ω s zadana wartość prędkości kątowej, T 1 moment obciążenia silnika 1, k i (i = 2, 3, 4) współczynniki korekcji zadanego momentu obciążenia T 1 TABLE 1 The power of the motors and drive sets of the coal conveyor belts system of the new opencast Drzewce in Konin brown coal mine TABLICA 1 oc silników i układ napędów czołowych systemu przenośników taśmowych ciągu węglowego nowej odkrywki Drzewce w kopalni Konin Conveyor name Drive system DW 1 2 55 otors power Force in belt in stop state (10 N) Force in belt in motion state (10 N) (kw) max. min. max. min. real DW 2 3 315 14 6 12 5 9 DW 3 4 315 16 7 14 6 10 DW 4 2 315 11 6 10 5 8 DW 5 2 315 11 6 10 5 8 DW 6 2 315 12 6 11 5 8 DW 7 1 315 8 5 7 4 5 DW 8 2 315 12 6 11 5 8

328 5. Summary The equal load sharing between three motors of two drum conveyor belt drive was achieved by means of advanced functions of vector controlled frequency converters fig. 5. 1.20 1.00 : Torque : Speed S1: Torque S2: Torque 0.80 Relative values 0.60 0.40 0.20 0.00 0 2 6 10 14 18 22 26 30 34 38 42 46 50 54 2 4 6 8 10 12 14 16 18 20 22 Time [s] inrush and conveyor steadily work [s] free run to stop of conveyor [s] Fig. 5. Waveforms of driving torques of motors of DW2 driving station (tab. 1) and speed of conveyor belt with no load: A Torque tightening the belt, B coast of the motors (stop of the conveyor belt), motor fed by master frequency converter, S1, S2 motors working as slaves Rys. 5. Przebiegi czasowe momentów napędowych silników stacji napędowej DW2 (tab. 1) i prędkości taśmy nieobciążonego przenośnika: A moment napinający taśmę, B wybiego silników (zatrzymywanie przenośnika), silnik zasilany przmiennikiem wiodącym (master), S1, S2 silniki nadążne (slave) The usage of master-slave configuration of frequency converters which control the conveyor belt s motors gave the possibility of construction of energy efficient drive set of conveyor belts with regulated rubber speed. Energy consumption of conveying system which has regulated conveyors belt speed, in the new opencast Drzewce in Konin brown coal mine, is above 50% lower in respect to classical conveying systems (constant speed) in Kazimierz and Jóźwin opencasts. Seven conveyor belts with different lengths in opencast Drzewce work with speed decreased to 2.5-3 m/s. The conveying systems of Kazimierz and Jóźwin opencasts have conveyors belt which work with classical drives solution (slip ring motors 6 kv with constant belt speed 5.24 m/s).

329 REFERENCES Antoniak J., 2007. Przenośniki taśmowe w górnictwie podziemnym i odkrywkowym. Wydawnictwo Politechniki Śląskiej. Gliwice. Czopek K., 2001. Optymalizacja modernizacji ciągu technologicznego kopalni z wykorzystaniem funkcji multiplikatywnej., Archiwum Górnictwa 46/2, PAN, Kraków. G ł adysiewicz L., 2003. Przenośniki taśmowe. Teoria i obliczenia. Oficyna Wydawnicza Politechniki Wrocławskiej Wrocław. Jeftenić B., Bebic., Štatkić S., 2006. Controlled multi-motor drives. International Symposium on Power Electronics, Electrical Drives, Automation and otion SPEEDA. Kasztelwicz Z., Szymański J., 2007. Nowoczesne napędy węglowych przenośników taśmowych o regulowanej prędkości taśmy. Górnictwo Odkrywkowe 5-6/2007 Poltegor-Instytut, Instytut Górnictwa Odkrywkowego, Wrocław. Kasztelewicz Z., 2006. The universal metod of deposit management programming in multi-pit lignite opencast mines. Part I, Archiwum Górnictwa, 51/3, PAN, Kraków. Kasztelewicz Z., 2006. The universal metod of deposit management programming in multi-pit lignite opencast mines. Part II, Archiwum Górnictwa, 51/3, PAN, Kraków. Kasztelwicz Z., Szymański J., 2008. Nowa metoda sterowania napędem przenośników o regulowanej prędkości taśmy w kopalniach węgla brunatnego. Przegląd Górniczy, s. 35-42, Nr 1/2008. Orłowska T., 2003. Bezczujnikowe układy napędowe z silnikami indukcyjnymi. Oficyna Wydawnicza Politechniki Wrocławskiej. Projekt celowy nr 6T12 2004C/06507, 2005. Opracowanie i wdrożenie układu napędów systemu przenośników taśmociągowych z automatyczną regulacją prędkości taśmy. Received: 15 January 2008