30 MOTORS & DRIVES Image 1. Net present value calculations of HVAC electricity expense of a 2,500-hp VFD over a 25-year life (Images courtesy of TMEIC) Parameter Cost of electricity (cents/kWhr) Motor HP Estimated heat loss Estimated tonnage required kW used by HVAC system Drive running hours multiplier Value 6.51 2,500 75 kW (3 kW for every 100 hp) 21.53 tons (0.86 tons for every 100 hp of motor) 21.53 kW (1 kW for every ton of HVAC, typical) 0.75 = 6,570 hours per year (Assume drive runs 75% of the time in a year) $9,206 (6,570*21.53*0.0651) $9,391 edium voltage (MV) drives are commonly applied to pump systems in water/wastewater, oil and gas, and power generation industries. Two key application considerations that may not have received due attention are how to manage the heat rejected from the drive, and how to ensure the environment of the drive installation. A typical MV drive is 96 to 97 percent e cient (including isolation transformer, blower fans and auxiliaries). e rest is rejected as heat to the indoor environment, requiring air conditioning and heater (HVAC) systems to maintain speci ed temperature, relative humidity and air quality levels. HVACs come with capital expenditure (capex) cost, but the variable costs to run, maintain and service have a material impact on the total cost of ownership (TCO) of the variable frequency drive (VFD) installation. ese costs may not be understood initially, but can become signi cant when assessed over a 25-year service life span. One alternative thermal management implementation eliminates the need for an HVAC and building/E-house to ensure the drive environment. Recent experience and research in electrical enclosure temperature augmentation techniques have enabled MV drives to be moved outdoors. However, due diligence is necessary when selecting them. HVAC electricity expense Year 1 HVAC electricity expense Year 2 (assume 2% infl ation rate in electricity prices) HVAC electricity expense Year 3 $9,578 HVAC electricity expense Year 24 $14,518 HVAC electricity expense Year 25 $14,808 Net present value (NPV) of HVAC $116,682 electricity cash expense over 25 (Above calculation does not years (assume 8% return rate) include cash outfl ows due to regular HVAC maintenance or expense due to HVAC change-outs every seven to 10 years) Why Managing Heat & Ensuring Drive Installation Environment Is Critical VFDs consist of delicate semiconductors, transistors, capacitors and electronics that are susceptible to extreme high/low temperatures, moisture, humidity and air contaminants. e VFD operator is tasked to control the process, production, labor, raw materials and other costs in their plant. But managing the VFD environment is more challenging. Image 2 shows the heat loss and the associated HVAC tonnage required to keep a 1,000 horsepower (hp) or a 5,000-hp indoor VFD in operation. e conventional method of managing heat for a typical National Electrical Manufacturers Association (NEMA) Type 1 indoor VFD has been to either package the drive in an industrial control building (or E-house) or install in a motor control center (MCC) room. December 2019 | Pumps & Systems