Energy conservation methods of electrical motor:
1) By Motor Survey: Large industries have a massive population of LT motors. Load survey of LT motors can be taken-up methodically to identify improvement options.
i) Sampling Criteria: Towards the objective of selecting representative LT motor drives among the motor population, for analysis, the criteria considered are: − Utilization factor i.e., hours of operation with preference given to continuously operated drive motors. − Sample representative basis, where one drive motor analysis can be reasoned as representative for the population. e.g. Cooling Tower Fans, Air Washer Units, etc. − Conservation potential basis, where drive motors with inefficient capacity controls on the machine side, fluctuating load drive systems, etc., are looked into.
ii) Measurements: Studies on selected LT motors involve measurement of electrical load parameters namely volts, amperes, power factor, kW drawn. Observations on machine side parameters such as speed, load, pressure, temperature, etc., (as relevant) are also taken. Availability of online instruments for routine measurements, availability of tail-end capacitors for PF correction, energy meters for monitoring is also looked into for each case.
iii) Analysis: Analysis of observations on representative LT motors and connected drives is carried out towards following outputs: − Motor load on kW basis and estimated energy consumption. − Scope for improving monitoring systems to enable sustenance of a regular inhouse Energy Audit function. − Scope areas for energy conservation with related cost benefits and source information.
The observations are to indicate: % loading on kW, % voltage unbalance if any, voltage, current, frequency, power factor, machine side conditions like load / unload condition, pressure, flow, temperature, damper / throttle operation, whether it is a rewound motor, idle operations, metering provisions, etc. The findings / recommendations may include: Identified motors with less than 50 % loading, 50 – 75 % loading, 75 – 100 % loading, over 100 % loading. Identified motors with low voltage / power factor / voltage imbalance for needed improvement measures. Identified motors with machine side losses / inefficiencies like idle operations, throttling / damper operations for avenues like automatic controls / interlocks, variable speed drives, etc. Motor load survey is aimed not only as a measure to identify motor efficiency areas but equally importantly, as a means to check combined efficiency of the motor, driven machine and controller if any. The margins in motor efficiency may be less than 10 % of consumption often, but the load survey would help to bring out savings in driven machines / systems, which can give 30 – 40 % energy savings.
2) By matching loads with motor rating
Industrial motors frequently operate under varying load conditions due to process requirements. A common practice in cases where such variable-loads are found is to select a motor based on the highest anticipated load. In many instances, an alternative approach is typically less costly, more efficient, and provides equally satisfactory operation. With this approach, the optimum rating for the motor is selected on the basis of the load duration curve for the particular application. Thus, rather than selecting a motor of high rating that would operate at full capacity for only a short period, a motor would be selected with a rating slightly lower than the peak anticipated load and would operate at overload for a short period of time. Since operating within the thermal capacity of the motor insulation is of greatest concern in a motor operating at higher than its rated load, the motor rating is selected as that which would result in the same temperature rise under continuous full-load operation as the weighted average temperature rise over the actual operating cycle. Under extreme load changes, e.g. frequent starts / stops, or high inertial loads, this method of calculating the motor rating is unsuitable since it would underestimate the heating that would occur. Where loads vary substantially with time, in addition to proper motor sizing, the control strategy employed can have a significant impact on motor electricity use. Traditionally, mechanical means (e.g. throttle valves in piping systems) have been used when lower output is required. More efficient speed control mechanisms include multi-speed motors, eddy-current couplings, fluid couplings, and solid-state electronic variable speed drives.