Did you know that manufacturers spend over $30 Billion on electricity annually, with motors consuming around 64% of the electrical spend? Another fact - half of those motors are controlling fans & pumps. As a result, end users are turning to VFD’s as an alternative to fixed speed controllers and throttling methods, such as dampers & valves, to help negate the rising costs of energy.
Significant opportunities exist to reduce energy consumption by altering how the flow is controlled. This is where VFD’s and Premium Efficient Motors come into play. Controlling flow with speed regulation, rather than mechanical valves or dampers, provides a significant reduction in power required to run the fan or pump.
However, in many flow applications, a mechanical throttling device is used to limit flow. Although this is an effective means of control, it wastes mechanical and electrical energy. The graph below shows you the energy usage using both a damper/valve and a VFD. The damper/valve’s energy usage is shown as the upper curve in the graph below, while the lower curve demonstrates energy usage when using a VFD. Because a VFD alters frequency of an AC motor; speed, flow and energy consumption are reduced in the system. The energy saved is represented in the green shaded area.
Centrifugal loads offer the greatest potential for energy savings by using VFD’s to control speed. Energy consumption in these load types follow the Affinity Laws, which means:
• Flow is Proportional to Speed
• Pressure is Proportional to the Square of Speed
• HP is Proportional to the Cube of Speed
That means if an application only requires 80% flow, the fan or pump will run at 80% of the rated speed and only requires 50% of rated power. In other words, reducing speed by 20% requires only 50% of the Power!
Using VFD’s on centrifugal fans takes advantage of the fan curve that occurs when the speed of the fan is changed and can be quantified with the affinity laws:
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Varying the speed on a centrifugal pump takes advantage of the change in pump characteristics when the impeller speed is changed.
Thus, applying a VFD to the pump allows control of the pump’s speed electrically, while only using the energy needed to produce a given flow. Also, the pressure is reduced, which helps reduce the mechanical stresses generated by throttling devices.
The affinity laws below are used to predict the operation of the pump at any operating point based on the original pump characteristics:
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One final note is that adding a VFD to your system gives you the ability to reduce peak demand charges on your electrical bill. VFD’s affect peak-demand electricity prices that the utility charges to companies that exceed a preset limit; such as when motors started across-the-line draw large peaks of current.
VFD’s help reduce the peaks by supplying the power needed by the specific applicaition, and gradually ramping the motor up to speed to reduce the current drawn.
Here are some resources for VFD's:
Rockwell Automation Energy Saving Calculator
RA Energy Savings VFD White Paper (Centrifugal Fans)
RA Energy Savings VFD White Paper (Centrifugal Pumps)
Want more information? Contact Your Revere Account Manager or fill out our on-line Solutions Inquiry Form .