Variable frequency drives for pump applications
variable frequency drive (VFD) is also known as adjustable speed drive which is used to control the speed of electric motor. Speed control of motor is achieved by changing the frequency and voltage applied to the motor. There are different control methods such as V/f control, VVC+, and Flux control techniques that are available for VFDs to control the motor speed and torque of standard AC motors. AC motors are being extensively used in applications like fans, pumps, and air compressors. For example Flow and pressure are regulated through the use of a throttling device, such as a valve, damper, or bypass. VFD + Motor provide more efficient way to control varying flow rates and pressures. VFDs are best known for providing precise process control and it has been widely used in industrial applications like controlling heating, ventilation and air conditioning (HVAC) systems which results in significant energy savings.
ABy Dr. S. Paramasivam & Sujith Kumar Neelam loads that requires constant torque irrespective of the motor speed. i. e., torque requirements at very low speed and very high speed are same. Constant torque loads can be categorized applications like conveyors, extruders, positive displacement pumps, and compressors. Application of VFDs in this type of loads results in energy savings from low speed operation. The precise speed control of this load types could be achieved with the application of VFDs. One more advantage with VFDs in this type of application is soft start and stops which reduces the transients and improves the life of the motors.
Constant power loads: Constant power loads means the loads that draw constant power irrespective of motor speed. Relation between power (P), speed (N), and torque (T) is given as P proportional to product of N and T Hence, to keep the power as constant, torque here varies inversely proportional to the motor speed. An example of constant power loads includes lathes, drilling, and milling. Application of VFDs cannot result in energy savings in this type of loads, because the power is kept constant. But application of VFDs for this type of loads could result in better process control.
Variable torque loads: Variable torque loads means the loads that requires variable torque for the operation. The relation between the torque and speed for this variable torque loads is torque varies directly with the speed squared. The relation between the power and speed for this variable torque loads is power varies directly with the speed cubed. From these relations, it is observed that a reduction in speed significantly decreases motor energy use and demand. For example, decreasing motor speed by 10 reduces required power by 19 Examples of variable torque loads include centrifugal fans, blowers, and pumps. Application of VFD in this type of load could result in very high energy savings at lower speeds. Hence, VFDs are most commonly used for this load type. Additional benefits with VFDs are more precise process control and reduced stress on the system.
From the above three different loads, it is clear that energy savings are more in variable torque loads at reduced speeds. Now let us calculate the amount of energy savings with VFD in pump application at different speeds with a numerical example.