Just in case you haven't been following the controversy on ControlGlobal.com, I have published three articles in which I have claimed that using a variable frequency drive (VFD) to control the speed of the electric motor driving a centrifugal pump is actually superior to using a traditional control valve with an electro-mechanical actuator and positioner. It saves energy because the pressure drop across the valve is gone. On new installations, the pump and motor may be smaller due to the reduced head requirement. Maintenance is reduced because there is no stickiness with the VFD as there is with control valves. There are no fugitive emissions through the seal on the no longer existing control valve stem. Finally, the quality of control is actually improved since the VFD responds instantly to PID output changes, and there is a perfectly linear relationship between pump rotational speed and flow rate. Still to be proven are the effects on turn-down ratio, lack of VFD support of Foundation Fieldbus, and lack of actual user feedback in real loop control situations.
Has anybody actually tried this in a real control loop?
The only variable speed system I actually worked with had a problem. The Electricians installed it and set the response far too slow.
ReplyDeleteWhere is the response speed shown in the specs? There are control loops where response time is critical. As in some level control, pressure control and other loops. It is possible to get large control valves to stroke in 1 2 seconds, this does require attention to the installation details. Valve moving parts have much less mass than the motor rotor and the pump impeller. Fast response would require high maximum current flow. A noisy control loop would probably heat the control box and the motor far more than expected.
Installation specs for VFDs usually require steel conduit to reduce radio frequency harmonics. Filtering of the consumed power may be necessary. Power and cost calculations by sales persons often overlook these details and other costs. As usual, 'there ain't no free lunch'.