In mining industries and ore beneficiation projects high voltage crusher motors are normally used to crush the lumped ores to sizeable ores for further usability. In the event of mains power failure during crushing stage crusher stops its operation and ores remain inside cause jamming preventing the crusher from restart with loaded materials upon resumption of mains power. The bottleneck to get the crusher ready for next cycle affects mining operation, production and revenue. The problem is quite common as power cuts and disturbance in grid power system resulting into momentary voltage dip is prevalent in mining area. The use of Uninterrupted motive power supply (UMPS) drive along with battery and charger will continue crushing for 15 minute with backup power till the crushing is over and materials are discharged from the crushing chamber and crusher is made ready for next cycle. This saves ore materials from wastage and enhances productivity, energy saving and revenue of the mining operation.
Gyratory crusher is the principal equipment in the iron ore processing plant with multistage crushing. The design of the crusher drive including motor and power system has to be robust and of high reliability. In a Gyratory Primary Crusher, ore materials are nipped, compressed and crushed between two rigid surfaces i.e. by an eccentric gyratory movement of mantle and a fixed part called concave. Open side setting & close side setting range of one 2000 tph primary crusher will be 205 mm to 130 mm respectively and the feeding material size is of 1200 mm. The quality and quantity of the material are produced by the crusher are the result of interaction between the crusher and ore material. Crusher design depends on hard ore, High grade and low grade ore material.
The crusher is driven by electric motor. Drive system transmits power from the motor to the crusher’s pinions shaft and also provides a form of overload protection. Normally in any ore beneficiation project squirrel cage induction motor is used as crusher motor. Since the rating of crusher motor is high, it is operated either with 6.6 KV or 11 KV. Under the Mains power failure condition crusher ceases its operation and as a result crushing materials remain inside cause jamming and disruption of mining activity until materials are removed and crusher is made ready for next cycle of operation. This causes wastage of ores, requirement of additional manpower and increase in idling time affecting the production milestone and revenue.
This paper intends to establish how Primary Crusher motor will continue crushing for 15 minute with backup power through uninterrupted motive power supply (UMPS) drive in case of blackout without requiring to be oversized in its rating, so that no jamming occurs in crusher for larger feeding material dumping, thus avoiding the disruption of mining production.
Proposed Method
When mains power is available the crusher works normally and continues to crush the material being dumped into the crusher. During the crushing stage, if the power fails crusher stops its operation and ores remain inside cause jamming which prevents the crusher from restart with loaded materials. When mains power resumes, crusher can be restarted after most of the material is taken out, which is a time consuming activity and also wastage of material.
In case of power failure, UMPS will serve the purpose of completing the crushing cycle and will be made crushing motor ready for the next cycle.
Complete crushing time takes mostly 2 to 3 minutes to fall out through the bottom of crushing chamber after one dumping of material in the crusher. However, UMPS battery shall be able to provide back-up for 15 minutes. In case of failure of mains power, UMPS drive should cater the full load of crusher without any interruption till the crushed materials are removed from the crushing chamber. Proper indication and audio/annunciation alarm to be sent to control room operator to warn them that crusher is running on battery backup so that operator can take proper corrective action and stop further dumping of ore material, till restoration of normal power supply. Provision of indication may also be kept for dumper operator through signal light, so that in case of failure of mains power, they would stop dumper operation. Operation & Maintenance group of mining plant will be benefitted as it will reduce wastage of materials, time and manpower for removal of materials from crusher with bolder size up to 1200mm and make the crusher ready for next operation on resumption of mains supply.
Brief Description of UMPS Drive
In this UMPS scheme, crusher motor is operated with 690V AC at 50 Hz and the point of source of 690V is 6.6/0.69 kV converter transformer(crusher motor is also rated for 690V, 50 Hz). 690V UMPS is a combination of 12 pulse variable frequency drive (VFD), battery and battery charger. When mains power is available, Crusher motor is fed from mains through VFD and float charging of 690V (2V VRLA) battery takes place through battery charger. VFD provides constant frequency or speed to the motor irrespective of input voltage or frequency variations. During mains power failure, condition battery becomes the main power source of Crusher Motor for maximum back-up of 15 min. Selected UMPS drive should be able to deliver continuously the rated current of the crusher motor even if the crushing ore material is of hard grade ore also.
Separate 3 ph 415V UPS is considered for the auxiliaries of primary crusher unit which primarily include lubrication system also. Auxiliary UPS back up time will be same as that of UMPS.
Rectifier converts 690V AC power to 1050V DC power considering peak factor and modulation index.1050V DC shall be maintained to achieve synchronous speed of 750 rpm of crusher motor. During power failure 1050V DC shall be available by Battery at DC Bus of UMPS through booster charger. 2V VRLA Battery with 420 numbers of cells generates 840V DC. Through 2 Quadrant Boost Charger 1050V DC is achieved.
Inverters which are IGBT based technology convert DC Bus voltage of 1050V to 690V AC at 50Hz frequency to feed Crusher motor terminal. If DC voltage is lower than 1050V i.e. at rated VRLA Battery voltage of 840V (420 cells each of 2V nominal) and at end cell voltage of 735 V (420 cells each of 1.75V end cell voltage) Motor Synchronous speed will come down to 626 rpm and 548 rpm respectively. So in UMPS System maintaining DC bus voltage is very important.
Figure 2: Typical Crusher Curve
Benefits of UMPS Drive
UMPS shall have following facility:
a. UMPS to ensure smooth acceleration of the crusher without any adverse impact on the power system and motor as the starting current is limited to rated current of the motor
b. There is yet another hidden benefit that UMPS solution can offer that with respect to behavior of crusher with UMPS drive during momentary voltage dip in the incoming power system during grid disturbance, Gyratory crusher with traditional drive ,i.e. direct on line control will trip in all probability causing severe breakdown of the crusher. This has happened in mines couple of times resulting in loss of production in two to three shifts. It may be noted that unloading of such big crusher is done manually which is not only time consuming but unsafe also. UMPS solution can be a rescue to this situation. It is believed that this benefit of the UMPS can help the crusher operation in long way as disturbance in grid power system resulting into momentary voltage dip is not uncommon in mining area.
c. Unloading of jammed crusher with pumping additional current into the motor for a short time. This is the most vital duty of the UMPS since it can apply pull out torque of the motor into the crusher shaft. This unique feature of the UMPS can help the crusher operator to unload the machine from stalled condition in an extra-ordinary way. This is not possible without UMPS drive system.
690V UMPS selection shall be such that it should be capable to deliver overload peak of Gyratory Crusher. Refer the Typical Crusher Curve (Fig:2). It is observed that motor current reaches its peak for uncertain period of time and the maximum peak current may be up to 2XIn (In=Crusher Rated current) depending on the lumped ore size and type (hard/fine ore etc).
Conclusion
Selection of battery and VFD rating is the main factor in this proposed scheme. The mining process is much diversified and the composition of the ore also varies even in the same mine which requires different process to adapt on case to case basis.
A novel concept based on economically sized UMPS drive with Inverter grade motor has been developed and established that UMPS based crusher drive is a superior and robust solution which can be adopted across the mining industry with immense benefits in terms of greater operation flexibility in unlimited number of starts, ease of maintenance, hence higher uptime and above all high possibility of energy saving and least requirement of reactive power. Further it reduces wastage of ores, additional manpower, and disruption of mining operation and enhances productivity and revenue to a bigger extent.
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