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ISA Northern California

Section, South Bay

September 9, 2003

Craig Chidester909 288 7990

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How Do You Size an

Adjustable Frequency

Drive?

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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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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

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Sizing AFDs - The Load (2)

High inertia load (centrifuge?)A centrifuge needs 40HP @ 1160RPM to run

Centrifuge weighs 6000 lbs empty, 3 feetdiameter; Wk2= 11,760 lb-ft2

Std motor starter 650% inrush until 65% speed, set time at 30 secs

120% average torque until 65% speed

Tq = (Wk2 x N) / (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

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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 N) / (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

t = 11760 x .30 x 1160 / 308 x 345 = 39 secs

Total time = 60 + 39 secs = 99 secs (76 secsusing 100HP)

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Sizing AFDs - The Load (4)

Compare motor heating . (I2

r)t watt-seconds of energy -- I

2t (Figure of

Merit)

FV start

(650%)2x 30 sec = 1267

AFD start

(150%)2x 60 sec = 135

Both FofMs ignore the last part of eachstarting cycle which are comparable

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Sizing AFDs - The motor

Sizing AFDs based on motor HP ishazardous to your pocketbook!!

AFDs are current rated - HPs are assigned

Typical example, 150HP AFD is rated 189A 150HP, 450RPM motor rated 224A

always check motor NP current rating

And, many machinery OEMs use part ofthe motors service factor to reduce cost

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Sizing AFDs - The power supply (1)

Voltages: 208V, 230V, 380V, 460V, 575V, 2300V, etc

Single and three phase

50 and 60 HertzAvailable short circuit current

65,000A ?

Higher ?

Harmonic Distortion issue?

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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