**ISA Northern California Section, South Bay**September 9, 2003 Craig Chidester 909 288 7990**Sizing AFDs**• Key elements • The load • The motor • The power supply • The environment • The duty cycle • The mounting space • The budget**Sizing AFDs - The Load (1)**• How much starting torque? • Centrifugal pump or fan? • 110% starting torque usually adequate (VT) • Conveyor? • 150% starting torque (CT) • Is 200% breakaway torque required? • The AFD industry conveniently has (2) ratings: 110% or 150% for 60 seconds**Sizing AFDs - The Load (2)**• High inertia load (centrifuge?) • A centrifuge needs 40HP @ 1160RPM to run • Centrifuge weighs 6000 lbs empty, 3 feet diameter; Wk2= 11,760 lb-ft2 • Std motor starter • 650% inrush until 65% speed, set time at 30 sec’s • 120% average torque until 65% speed • Tq = (Wk2 x ΔN) / (308 x t) • Tq = 11,760 x .65 x 1160 / 308 x 30 = 960 • 960 ft-lbs / 120% / 4.5 ft-lbs/HP = 178HP • Voltage drop, stiction, loaded -- 200HP min**Sizing AFDs - The Load (3)**• But with an AFD …. • Goal is to get motor to 70% speed in 60 seconds while limiting inrush to 150% • Tq = (Wk2 x ΔN) / (308 x t) • Tq = 11760 x .70 x 1160 / 308 x 60 = 517 Ft-lbs • 517 / 150% = 345 Ft-lbs continuous • 345 ft-lbs / 4.5 ft-lbs/hp = 76 HP -- use 100HP • Δt = 11760 x .30 x 1160 / 308 x 345 = 39 sec’s • Total time = 60 + 39 sec’s = 99 sec’s (76 sec’s using 100HP)**Sizing AFDs - The Load (4)**• Compare motor heating …. (I2r)t watt-seconds of energy -- I2t (“Figure of Merit”) • FV start • “(650%)2x 30 sec” = “1267” • AFD start • “(150%)2x 60 sec” = “135” • Both “FofM’s” ignore the last part of each starting cycle which are comparable**Sizing AFDs - The motor**• Sizing AFD’s based on motor HP is hazardous to your pocketbook!! • AFD’s are current rated - HP’s are assigned • Typical example, 150HP AFD is rated 189A • 150HP, 450RPM motor rated 224A • … always check motor NP current rating • And, many machinery OEM’s use part of the motor’s service factor to reduce cost**Sizing AFDs - The power supply (1)**• Voltages: • 208V, 230V, 380V, 460V, 575V, 2300V, etc • Single and three phase • 50 and 60 Hertz • Available short circuit current • 65,000A ? • Higher ? • Harmonic Distortion issue?**Sizing AFDs - The power supply (2)**• Harmonic distortion • IEEE-519 (latest edition) is the Guideline • Typically recommends 5% THD (V and I) • Point of Common Coupling (PCC) is key • With 3% choke, < 35% AFD load at PCC < 5% THD(V) at 460V side • Locate PCC for THD(I) on 4160V (MV) side • Otherwise, spend a whole bunch of money**Sizing AFDs - The environment**• AFD’s are rated for ambient and altitude • 110% (VT) usually 40C (104F) • 150% (CT) usually 50C (122F) • 20% derate to get from 40C to 50C • 3300 feet elevation maximum standard • 3% derate per 1000 feet above 3300 • 100HP VT, 50C, 6000 ft = 137HP (150HP) • Outdoors? Sun load adds 60 watts / Sq ft (based on maximum face dimension)**Sizing AFDs - The Duty Cycle**• AFD’s are rated 100% current continuously • And … 110% (VT) or 150% (CT) for 60 seconds in a 10 minute interval • Cyclical loads that require repetitive accel torque may need to be oversized • “FofM” for CT = I2t = (150%)2x 60=“135” • If same load is accel’d in 25 sec’s every 2 minutes at 135%, then FofM is “227” • “227” / “135” -- 1.69 X base HP**Sizing AFDs - The Mounting Space**• Once a rating and modifications / accessories have been established, check for space requirements! • Not all vendors are created equal • Modern AFD’s take less space • Configurations are “landscape or portrait” • Tradeoffs on wall space vs footprint**Sizing AFDs - The Cost**• Bare Bones Loaded • 1HP -- $ 325 $ 2000 (1) • 10HP -- $ 1200 $ 3000 (1) • 100HP -- $ 7500 $30000 (2) • 500HP -- $35000 $90000 (2) • Bare Bones: 460V, Nema 1 encl, VT • Loaded: (1) Ditto plus bypass, • Loaded: (2) Ditto plus bypass, MMR w / CTs and PTs, output filter and 18-pulse