Cement, Energy and Environment

substantial part of the connected load to the power supply system. The 3-phase AC induction motor is designed to perform optimally in the range of 60 to 100 % efficiency, which is linked to the rated capacity and the nature of the connected load. The load can be a resistance, capacitance and inductance or a combination of the same. For example a motor running at less than 50 % of the rated capacity will unduly downplay the power factor, which in turn will increase the load on the supply system. Motor efficiency accounts for 15 to 20 % of the total savings on electrical energy; whereas the remaining 80% relate to the process load conditions. Unlike 3-phase, single- phase induction motors generally run at 20 - 30% efficiency. They are employed for household use, e.g. motors for ceiling, table , exhaust fans etc. More efficient motors of pump and blower run at 60 to 70 % efficiency. The number of such motors is very large. A small improvement in efficiency can yield large savings in energy. These motors are constant speed drive; they can not be energy efficient unless adjustable speed drive (ASD) is used as it is not easy to vary the supply voltage and the frequency so easily. But the use of ASD-motors introduces harmonics in the power system and thus pollutes the power quality. Besides the AC-variable frequency drive (VFD), DC Brushless Drive (BLOC) motors are fast replacing the constant speed induction motors in the single-phase category in other countries. All 3-phase AC induction motors run at constant speed. To meet the changing load demand, some mechanical resistance is applied. For example, motor driving a centrifugal fan and a pump use a damper and a throttle valve respectively to control the flow of air and fluid for the changing process requirements. The action of the damper and the throttle valve resists the flow. It does not reduce the speed of the motor. The resistive action wastes the power, thereby reducing the energy efficiency of the power system. Instead, a Variable Frequency Drive (VFD) motor will reduce the motor speed to suit the process load requirement, eliminating the use of any damper or throttle valve. In this way, the VFD does not resist the flow; it achieves the desired condition by simply reducing the speed, thus it saves power and enhances the energy efficiency of the system. Power saving approximates to 25 - 40 %. It is a substantial saving. The relationship between the speed and the power is that power is proportional to the cube of the speed (P oc cube of N). For example, at full flow of air/fluid, the motor speed is 1 00 % and it consumes 100 % power. At 90 % flow, the motor speed is reduced to 90 %, but the energy consumed is 73 % only. It means that a small reduction in speed makes a large reduction in power consumption. For speed reduction low voltage e.g. 415 V/690V and medium voltage - 2.3KV, 3.3KV, 6.6KV AC VFD motors are available. Current trends are for the application of power transistors for motor speed reduction. For example, HV - IGBT (Insulated Gate Bipolar Transistor) feeds the motor with Pulse Width Modulated, which is harmonic-free current and makes it an energy-efficient solution. Some tips for improving motor efficiency: o If the motor is running at a small load, then convert the supply to Star from Delta connection. This will improve the motor efficiency o Examine and if found necessary, change the existing motor winding to more energy efficient motor. Under IEEMA standard 19:2000, motor category having higher efficiency class are designated as Eff- 1, Eff-2. The normally prevailing class is Eff-3, which meets IS 8789. o If the motor is required to run at a variable load, which is higher for the normal operation of the motor, then change over to Variable Frequency Drive (VFD). It will save on energy. VFD is a microprocessor based electronic drive; it is specially programmed to regulate the speed. Advantages The solutions following. aforesaid suggested help achieve the o Promotes energy conservation o Reduces iron and copper losses o Prevents distortions harmonic o Improves motor efficiency and energy efficiency o Enhances motor life Protective System A reliable electrical distribution and utilization network depends on an effective protection system. The protection system should be such that it safeguards the equipment against an overload current to pass through the 30