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Variable Frequency Drives (VFDs)Limi t s and Pot ent ial Consequences

November 13, 2017

J. Clark Maness, EIStaff Professional, WithersRavenel

PRESENTATION

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T O P I C SBackground

Why use a VFD ?

Typical Problems with VFDs

Examples

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• VFD: Variable Frequency Drive, also known as: Adjustable Frequency Drive, Variable Speed Drive, AC Drive, etc.

• VFDs are adjustable speed drives used in elect ro-mechanical systems to control AC motor speed and torque

• In the water/wastewater industry, commonly used for controlling pump and blower motor speed, i.e., process control

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BACKGROUNDW h at i s a V FD ?

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BACKGROUNDHow do VFDs w or k?

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BACKGROUNDHow do VFDs w or k?

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On average, the bursts of DC power create a sinusoidal form of AC current . The imitat ion AC sine wave is called pulse width modulat ion.

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• To control the speed and torque of the most common motor in industry.

3-Phase Induction Motor

W hy use a V FD?

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• VFDs are good for many applications

• However, VFDs do have a host of issues and can potentially be more harmful than helpful.

Ty p i cal Pr ob l ems

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Bear ing Damage – Induced Shaf t Vol t age • Less than perfect sinusoidal waves produce eddy currents that• Damage can be prevented with insulated bearings or shaft brush bearings

POTENTIAL VFD PROBLEMS

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I ncr eased M ot or Cool i ng Requi r ed • Below 25hz motor fan speed will not cool motor• Supplementary fan required• Added cost of drive, cable, SCA, controls, access and maintenance• Reduced reliability

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POTENTIAL VFD PROBLEMS

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Har monics• All VFDs draw harmonic currents which distort voltage supply.

• Harmonic distort ions cause excessive heating of supply equipment and/or malfunction of motor.

• Also reduces overall efficiency of motor.

• Typical Correct ive Measures (increased drive cost):

• Series Reactors • Phase shift ing transformers and six-phase rect ificat ion• Shunt Filters• Active Filters

POTENTIAL VFD PROBLEMS

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I ncr eased Noise • Due to the harmonics, the motor noise level will increase when it is operated using a VFD• Experience shows that the sound pressure level at A scale at motor rated speed is increased by

anything between 2 and 15dBA with a VFD • This “ extra ” noise level depends mainly on the inverter switching frequency and harmonic

content .• Noise mit igat ion costs increase

POTENTIAL VFD PROBLEMS

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Ot her• Need for HVAC condit ioned building or panel to house VFDs (higher capital costs)

• Larger control cabinet required for applicat ion

• Increased overall operat ions complexity

POTENTIAL VFD PROBLEMS

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• Flow Control Valve

• Bypass valves

• Holding tank

• Pump for longer periods

• Change pump impeller diameter

• Alternate pump type

• Newly sized pumps

ALTERNATIVE SOLUTIONS

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Wetwell

Forcemain

Flow = 200 gpmTDH = 58 feet Motor Speed = 1800 rpm

Ex a m p l e 1Change in Pump Purpose

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Wetwell

Headworks System (Screenings, Grit , and FOG

Removal)

Flow = 200 gpmTDH = 32 feet Motor Speed = turned down to 1,200 rpm

VFD

Change in Pump Purpose

Ex a m p l e 1

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Well Pump Stat ion and Storage Tank

Ex a m p l e 2

Well pump 20,000 gal tank

• Well pump stat ion and storage tank provided adequate storage for site

• However, needed to provide enough water at 300 gpm 60 psi to feed sprinkler systems in camp cabins

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Ex a m p l e 2

Well pump 20,000 gal tank

Booster pumps

4,000 gal hydropneumatic

tank

66 to 86 psi

Cabin sprinklers

VFD • VFDs allowed booster pumps to feed pressurized tank at varying head demands

Well Pump Stat ion and Storage Tank

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Exist ing Duplex Pump Stat ion

Exist ing 16” FM

Ex a m p l e 3

Exist ing WWTP

• Exist ing Pump Stat ion pumped 162,000 gpd on average, but sized for 725,760 gpd.

• To allow development in Town and not increase capacity of exist ing WWTP needed to redirect flow to a different WWTP

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Exist ing Duplex Pump Stat ion

New 8” FM

Ex a m p l e 3

Other WWTP

• Cost effect ive solut ion: Install a new wetwell and divert in new forcemain. • Added VFDs to keep constant level (but not a necessary component).• However, VFD created overheating of other components in building and loss of

efficiency with harmonics.• Had to install a harmonic filter to limit voltage fluctuat ion (increased cost).

Exist ing 12” GMNew 12”

GM

New Simplex Pump Stat ion

VFD

VFDs have some drawbacks. Therefore, all scenarios should be considered on a case by case basis, and all reasonable options should be considered.

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Special thanks to Charles R. Underwood

Inc. for their insight and help

VFDs

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VFDs

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Pump Curve 1: VFD use is viable

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Pump Curve 3: Pump Oversize

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Pump Curve 2: VFD use is not viable