CEE Oct-Dec 2002

1 f voltage wi ll decrease the starting, torque. B11t •notors are designed in such a way that these provide sufficient stat1ing torque even when operated at 395-400 V. So some energy can be conse rved by operating the motors at a lower voltage without much affecting the starting torqu e. However thi s method may be applied only if the motvr loading is 50-60 per cent , otherwise desired results may not be obtained. v) Phase voltage unbalance -A perfect balance can never be maintained in a pl ant due to conti11ually changing loads which cause the phase voltage unbalancl.. Improper distribution of lighting, blown fuses of capaci tors banks w ill a·lso unbalance the load and cause phase voltage unbalance. Phase Unbalance Vol1age : Negative Sequeiice Volt I Posilive ,<,'equence Vol! When this is applied to three– pha se mot ors. the phase voltage unbalance causes additiona l nega ti ve sequence current to ci rcula te in the motor when it is de lt a connected. w·hi ch in turn caus~s more heat losses primarily in th e ro tor. The severest conditions occur when one phase i'i opened and the motor run on single phasing power. The first best method to avoid this type of losses is equal distribution of the single– phase loading. Sha red neutral should always be avoided. A lternati vely the motors may be connected in star instead of delta, as there will be no c irculating current then. However torque will reduce. which may be compensated b) adding more copper to the wi nding. Hence some energy can be saved by thi s method also. vi) Friclion losses - Motors are associated with friction losses as they are rotating machinery; these account for 5-8 per cent of the total motor losses. These can be minimised by providing good quality bearings and grease. Greasing should be clone as spec ified by the motor manufacturer. Greasing frequency should be increased if the ambient temperature is hi gher than ori gina lly specified. If belt drive is used, then belt tension should be checked period ically. Excessive belt tension may result in damage to bearings, shaft, whi ch, in turn, will compound the losses. Also, beltsmay be replaced with latest flat t ype belts which have hi gher efficiency than V-belts. This wi ll resu lt in some energy saving. vii) Cooling fans - Normally I-tT– motor suppliers provide forward curve, bidirectional fans for cooling of l-IT motors. These are less efficient. Aft er commiss ioning, when the direction of the l-IT motor is defined , th ese fans may be replaced with backward-curve, unid irectional fan impellers wh ich are more efficient and compact in si ze. Thi s yie lds good power sav ings. Dampers Large-rating fJns are used in cement plants to create draught and ci rcula'tion of air. Dampers usually control the volume of gas to be admitted as per process req uirement. But these are assoc iated wi th some pressure drop. A s an increasingly larger damper area comes in the way of air, the opening decreases. the pressure drop across the damper increases. resulting in energy losses instead be controlled by varying the speed of drive, keeping the damper I 00-per-cent open. The amount of energy saved in this manner can be calculated by the following formula: Pressure (P) of the gas is directly proportional to square of fl ow (Q) The flow (Q) i s directly propotiional to the fan speed (N)_ Motor output (Po) P * Q = N ' A Iso motor output ( 1-S) * P. I where S: is the slip of the motor and Pi the input power. Al so ( 1- S) = N which means P, is di rectl y proportional to square of the speed. This shows that in the case of fan applications, the motor output is directly proportional to the cube of the speed. while the input power is proportional to the square of the speed. So by controlling the speed of motor, we can save substantial amount of energy. Speed can be controll ed either by using SPR (poss ibl e only in case ofslip ring motors) or by using VYVF (variable voltage variable frequency) panels. Usuall y large HT motors are slip ring motors. It is usually advised to install SPRS for large size liT motorsand vvvr: panel s for L T drives, as VVYF panels are very expensive lo r l-IT drives. So before dec iding between SPRS and VVVF panels, the pay– ba ck period f'or bo th may be compared. T y pi cal appli cation s arc compressors, fans and blowers. In a 700 -k W mo tor used for fan applications, '' ith a damper to regulate !low or air. the effect of varying the damper posit ion from 65 to 80 per cent (open). can be achi eved by va ry ing th e mo tor speed, keeping the damper I 00 per cent op-:n. A 90-100 kW saving of across it. The vol ume of gas can energy can thus be achieved. 9