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Energy Costing


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OK, now that this drive is performing adequately, I'm going back to trweaking the controls system to minimize energy costs.


How can I get a feel for the cost difference of running this fan between 0% and 10%, and between 50% and 60%. That is, I need to know how much power is used off schedule when it is running at it's current minimum setting (10%), and what it costs me to increase the duct pressure with a 10% change in frequency (from say 50% to 60%, eg).


single 50 HP SAF is controlled by: Altivar 58 VFD

Drive settings:

460V, 60Hz, 61A, 1760 RPM

mtr cos phi/power factor = .95


single 20 HP RAF is controlled by an ABB VFD

Drive settings:

460V, 60Hz, ??A, 1725 RPM

power factor = ??


What I'm really asking is: "Do VFD driven motors consume power linearally with the frequency?" and is there a formula to get reasonable kW/hr number from the data above?





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Hello Birchy


The power used is proportional to the fan load /speed curve and is a function of the fan, not the drive and motor.

The power consumed by the drive and the motor is very small, it is all about the efficiency of the fan at the air flow and pressure against speed.

There are some, particularly suppliers, who will quote you the numbers, with out doing the engineering, but I believe that you really need to find out the fan curves and the operating conditions.

If the air flow is restricted by mechanical means, then there will be "cavitation" losses that can be overcome by reducing the speed.


Best regards,


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