Energy Efficient Operation of Drives:
In many Energy Efficient Operation of Drives applications involving constant speed operation, induction motors operate under no load or light load for prolonged periods, such as in pressing machine, conveyors, rock crushers, centrifuges, drill presses, wood saw and, some machine tools. In such applications, saving in energy can be achieved by operating the motors at low voltages while running at no load and light loads. When a motor operates at full voltages at no load, core loss has a large value. Reduction in voltage increases copper loss but reduces core loss by a larger amount. Therefore, net loss is reduced. At some voltage when core loss becomes equal to copper loss; the loss has minimum value and efficiency is maximum. Any increase or decrease of voltage from this value increase the loss. Therefore, for each loading, there is an optimum value of voltage for which the loss is minimum. Energy saving is achieved by operating the induction motor at optimum voltage values.
The energy saving depends on following factors:
Duty cycle of motor: Energy saving occurs mainly during no load and light load operation. Therefore, energy saving is larger in drives which operate at low duty cycles.
Operation with overvoltages: When motor is located in the area where overvoltages are common, larger amount of energy can be saved. At overvoltages, because of saturation of magnetic circuit of motor, the core loss becomes larger than its full voltage value. Operation at the optimum voltage gives rise to larger reduction in loss and greater saving in energy.
Quality of motor design: Badly designed motors use inferior core material and operate with saturation even at full voltage. Therefore, they have larger core loss at full voltage and over-voltages than the motors with good design. Operation of badly designed motors at the optimum voltages reduces the loss by a larger amount than in a motor with good design.
Oversizing of motor: Often a motor of power rating higher than necessary is employed. Optimum voltage control gives rise to greater saving of energy in oversized motors.
As explained above, for each loading there is an optimum voltage. The loss minimization requires that the motor voltage be continuously varied with the load. Therefore; an ac voltage controller must be incorporated between motor and the source. When an ac voltage controller is used to get variable voltage, harmonics are present in the motor terminal voltage. These harmonics produce losses, and therefore, the energy saving is reduced. The economic benefits of the net energy saved should be compared with the increase in cost due to the addition of an ac voltage controller to arrive at a decision about the desirability of using this energy saving scheme. In several applications requiring soft start, ac voltage controller is always there, and therefore, no additional cost is incurred in implementation of energy saving scheme. Energy saving scheme is particularly beneficial in single-phase motors.
Substantial energy saving can also be achieved in variable frequency induction and synchronous motor drives. The common approach to variable frequency control is to operate the motor at a constant flux up to base speed by keeping V/f ratio constant. From point of view of energy consumption, this is not correct approach. V/f ratio should be varied to change flux so that the operation takes place with minimum loss.
Let us examine the operation of a motor at a given frequency when load is varied. When V/ f ratio has rated value, flux is also at the rated value, and motor has nearly maximum efficiency and minimum loss at the rated torque. When torque is reduced, motor copper loss reduces but the core loss remains constant. When V/f ratio is reduced to make core and copper losses equal, minimum loss operation is obtained. Further reduction in torque will require additional reduction in V/f ratio to obtain minimum loss operation. What is true for one frequency is also true for other frequencies. This minimum loss operation control strategy is complex and difficult to implement but is within the reach of current technology. Amount of energy saved depends on the duty cycle of motor and quality of its design.
The variable flux operation for minimum loss has one disadvantage—slow dynamic response compared to constant flux operation. However, there are large number of applications where slow dynamic response is acceptable.