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How do variable speed drives achieve energy saving

Variable speed drives regulate the speed of motors and in turn the speed of the fan or pump by controlling the energy that goes into the motor rather than restricting the flow of a process running constantly at full speed. A variable speed drive can save over 60% of the energy as it controls the energy at source, only using as much as is necessary to run the motor with the minimum speed and torque. Large amounts of energy can be saved on fan and pump systems, because of the affinity laws for pressure and flow rates. The Affinity laws state -Flow is directly proportional to speed Torque is directly proportional to speed squared Power required is proportional to speed cubed Therefore, this means that if 100% flow requires full power 75% flow requires 0.753= 42% of full power 50% flow requires 0.53= 12.5% of the power. Mechanical control methods such as inlet guide vanes, throttling valves, discharge dampers do not take advantage of the affinity laws. With mechanical flow control methods the motor always runs at full speed and the flow is mechanically restricted. The variable speed drive wholesale saves energy by reducing the actual speed of the motor when full flow is not required. A variable speed drive is fitted to the system and the discharge damper removed so there is no restriction to airflow. The speed of the motor is reduced to 40Hz which gives the same airflow as before when the motor was run at full speed and a discharge damper used. Now the input power is typically 50% of full load power. Therefore by using a variable speed drive, the power being consumed is reduced by typically 45%. Centrifugal Fans Massive potential energy savings using a variable speed drive compared to the two most common methods of flow control for fans: - Inlet guide vanes require about 60% power to give a flow rate of 50% - A discharge damper requires a huge 90% power to give 50% flow. The following table shows the typical input power to a motor when run at full speed with flow rate is restricted by an outlet damper compared to the typical input power when the same motor is run at reduced speed from a variable speed drive, achieving the same air flow rate as with the outlet damper. It can be seen that if an outlet damper reducing the air flow rate to 80% uses 95% input power, a variable speed drive achieving the same air flow rate uses 50% input power. The variable speed drive supplier make variable speed drives that can also stop a motor completely when it is not required as re- starting with a variable speed drive causes far less stress than starting direct on line - soft start is an inherent feature of the drive. Regulating the motor speed has the added benefit of easily accommodating capacity rises without extra investment, as speed increases of 5-20% is no problem with an AC variable speed drive as long as there is enough spare capacity in the system. If the variable speed drive has an internal PID loop, it will be possible to automatically control flow or pressure based on feedback from a sensor within the system. This can make further energy savings as the motor can slow right down if very little flow or pressure is required.