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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 load The motor The power supply The environment The duty cycle The mounting space The budget.

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Isa northern california section south bay

ISA Northern California Section, South Bay

September 9, 2003

Craig Chidester

909 288 7990



Sizing afds
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
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
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
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
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 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
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
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
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
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
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
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


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