Transcript Intelligent Technologies Control “IT Control”

ISA Northern California
Section, South Bay
September 9, 2003
Craig Chidester
909 288 7990
How Do You Size an
Adjustable Frequency
Drive?
Sizing AFDs
• Key elements
– The
– The
– The
– The
– The
– The
– The
load
motor
power supply
environment
duty cycle
mounting space
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 Ftlbs
• 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)
2
– Compare motor heating …. (I r)t watt2
seconds of energy --  I t (“Figure of
Merit”)
• FV start
2
– “(650%) x 30 sec” = “1267”
• AFD start
2
– “(150%) x 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
2
2
• “FofM” for CT = I t = (150%) x 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
•
•
•
•
•
1HP -10HP -100HP -500HP --
Bare Bones
$ 325
$ 1200
$ 7500
$35000
Loaded
$ 2000 (1)
$ 3000 (1)
$30000 (2)
$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