Quick question for the group - client in Cleveland installed VFDs on their chiller pumps last month thinking it would reduce demand charges. Instead, they're seeing 15-20 kW spikes when the VFDs ramp up quickly during hot afternoons. FirstEnergy Schedule E-19 is hitting them for these transient peaks even though average demand actually decreased. Is this normal behavior for VFDs or did the installer screw something up? The drives are supposed to have soft-start capability but these spikes suggest otherwise.
Variable frequency drives and demand spikes - FirstEnergy
Frank, this sounds like the VFD acceleration time is set too aggressively. Most installers use default settings which often prioritize fast response over demand control. You need to increase the acceleration time (ramp-up rate) to spread the inrush current over a longer period. Instead of reaching full speed in 5-10 seconds, program it for 30-45 seconds. Will reduce the instantaneous demand spike significantly. Also check if there's a current limiting parameter - many VFDs can be programmed to never exceed a specified amperage regardless of load demand.
Walt's right about the acceleration settings. Had the same issue with a TVA account in Knoxville last year. The HVAC contractor programmed the VFDs for maximum cooling response but didn't consider demand implications. Changed the ramp rate from 10 seconds to 60 seconds and the demand spikes disappeared. Building still cools properly but no more transient peaks. Sometimes you have to educate these guys that "faster" isn't always better when demand charges are involved. The extra 30-40 seconds of ramp time won't affect comfort but saves hundreds in demand charges.