Describe Energy Efficient Motors (EEM).
Energy-efficient motors (EEM) are the ones in which, design improvements incorporated specifically to are increase operating efficiency over motors of standard design (see figure). Design improvements focus on reducing intrinsic motor losses. Improvements include the use of lower-loss silicon steel, a longer core (to increase active material), thicker wire (to reduce resistance), thinner laminations, smaller air gap between stator and rotor, copper instead of aluminum bars in the rotor, superior bearings and a smaller fan, etc.
Energy-efficient motors now available in India operate with efficiencies that are typically 3 to 4 percentage points higher than standard motors. In keeping with the situations of the BIS, energy-efficient motors are designed to operate without loss in efficiency at loads between 75% and 100% of rated capacity. This may result in major benefits in varying load applications. The power factor is about the same or may be higher than for standard motors. Furthermore, energy-efficient motors have lower operating temperatures and noise levels, greater ability to accelerate higher-inertia loads, and are less affected by supply voltage fluctuations. Measures adopted for energy efficiency address each loss specifically as under.
Stator and Rotor I Rlosses:
These losses are major losses and typically account for 55% to 60% of the total losses. PR losses are heating losses resulting from current passing through stator and rotor conductors. IR losses are the function of a conductor resistance, the square of current. Resistance of conductor is a function of conductor material, length and cross sectional area. The suitable selection of copper conductor size will reduce the resistance. Reducing the motor current is most readily accomplished by decreasing the magnetizing component of current. This involves lowering the operating flux density and possible shortening of air gap. Rotor I'R losses are a function of the rotor conductors (usually aluminium) and the rotor slip. Utilization of copper conductors will reduce the winding resistance. Motor operation closer to synchronous speed will also reduce rotor losses.
Core Losses:
Core losses are those found in the stator-rotor magnetic steel and are due to hysteresis effect and eddy current effect during 50Hz magnetization of the core material. These losses are independent of load and account for 20-25% of the total losses.
The hysteresis losses which are a function of flux density, are be reduced by utilizing low-loss grade of silicon steel lamination. The reduction of flux density is achieved by suitable increase in the core length of stator and rotor. Eddy current losses are generated by circulating current within the core steel laminations. These are reduced by using thinner laminations.
Friction and Windage Losses:
Friction and windage losses results from bearing friction, windage and circulating air through the motor and account for 8-12% of total losses. These losses are independent of load. The reduction in heat generated by stator and rotor losses permit the use of smaller fan. The windage losses also reduce with the diameter of fan lending to reduction in windage losses.
Stray Load-Losses:
These losses vary according to square of the load current and are caused by leakage flux induced by load currents in the laminations and account for 4 to 5% of total losses. These losses are reduced by careful selection of slot numbers, tooth/slot geometry and air gap. Energy efficient motors cover a wide range of ratings and the full load efficiencies are higher by 3 to 7%. The mounting dimensions are also maintained as per IS1231 to enable easy replacement.
As a result of the modifications to improve performance, the costs of energy-efficient motors are higher than those of standard motors. The higher cost will often be paid back rapidly in saved operating costs, particularly in new applications or end-of-life motor replacements. In cases where existing motors have not reached the end of their useful life, the economics will be less clearly positive.
Because the favourable economics of energy-efficient motors are based on saving in operating costs, there may be certain cases which are generally economically ill-suited to energy-efficient motors. These include highly intermittent duty or special torque applications such as hoists and cranes, traction drives, punch presses, machine tools, and centrifuges. In addition energy, efficient design of multi-speed motors are generally not available. Furthermore, energy-efficient motors are not yet available for many special applications, e.g., for flame-proof operation in oil-field or fire pumps or for very low speed applications (below 750 rpm). Also most energy-efficient motors produced today are designed only for continuous duty cycle operation.
Given the tendency of over sizing on the one hand and ground realities like: voltage, frequency variations, efficacy of rewinding in case of a burnout, on the other hand, benefits of EEM's can be achieved only by careful selection, implementation, operation and maintenance efforts of energy manager.
A summary of energy efficiency improvements in EEMs is given in the Table.
