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The Effects of VFDs on Centrifugal Wastewater PumpsTechnical Series 2014

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Course Objectives

•Understanding of AC Induction Motors

•VFDs- Are they Magic?

•Variable Speed and Wastewater Pumping

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AC Induction MotorsTheory & Design

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Basic Motor Vocabulary• Poles- Number of magnetic poles (or

coils) 180-degrees apart in the stator

• Service Factor- Amount of overload motor can handle typically expressed in amps

• Synchronous Speed- Speed at which magnetic field in the motor is rotating; also the “no-load” speed of the motor. Expressed in RPM’s

• Slip- Difference between Synchronous Speed (Theoretical) and Full Loaded Speed (Actual). Expressed in a percentage

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Motors

•Rotational Speed of AC induction motors depend on two things:Poles in the statorFrequency of power (Hz)

•Poles in the motor are a constant

• Speed varies directly with Frequency

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Motors

• In the US power is 60hz or 60 cycles per second

• This is just the standard in the early days the cycles varied wildly. It was Nikola Tesla the inventor of the AC induction motor who standardized on 60Hz

• Motor speed is determined by number or North & South Poles in the motor (+/-)

Simple DC Motor

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• Motor speed is determined by number or North & South Coils in the motor (+/-)

• These always occur in twos 2-pole/4-pole/6-pole

• Formula for figuring speed in RPM120 x Frequency / Number of poles 120 is constant 120 x 60 = 7200 7200/ 4 = 1800RPM

Two & Four Pole Three Phase Motors

Poles & Speed

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Motors

•What is my Speed?– Difference between

Synchronous Speed and Full Load Speed is described as Slip– Slip is typically between 3% &

5%– So;

• 1800RPM * 0.97 = 1746RPM (1750RPM)• 1800RPM *0.95 = 1710RPM

(1700RPM)

Synchronous

RPM

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VFDs Why we use themReview of VFD principal

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VFDs- How it works

•Converts AC to DC to AC

•Allows for modulation of the Frequency

•Remember frequency controls speed in AC motors

•Power is not true Sine wave but “Block-wave”

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VFDs- How it works•The block nature of the AC

power of a VFD creates harmonics in the pump causing vibration

• Lower Pulse (slower) drives have higher harmonic distortions

•Higher Pulse (faster) drives have less harmonic distortion

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High Pulse Drives •18-Pulse drives allow for smoother operation of rotating

equipment as by using insulated gate transistors to synthesize a cleaner sine wave power curve

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High Pulse Drives

•18-Pulse and higher drives have drawbacks

•Voltage unbalance/pulses cause induced AC voltage on the motor shaft

• Leads to a continuous discharge of voltage most often across the bearing

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Why Use a VFD • Pumps are often designed for

worst case scenario. (I&I or peak demand)

• These extremes do not happen often

• However pumps slide up and down curve depending on demand and system

• The ability of a VFD to modulate speed allows for pump to run at BEP across operating range

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Variable Speed and Wastewater PumpingBringing it all together

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Using a VFD with a submersible pump

•Pump COS 100GPM at 100’TDH 3500RPM 4.04HP (at design) 5.25Impeller Diameter Change System Pressure to 56’TDH

• What Happens?

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Pump & System Head Curve • System head drops from 100’ to 56’ • At 3500RPM efficiency drops below 40% suction cavitation is a

problem •With VFD at 2625RPM efficiency is constant at 62%

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Wastewater Pumps & VFDs

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Cost Savings on a VFD

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• At Constant Speed- System would use $45,485.00.• At Variable Speed- System would use $19,742.00.• Variable Speed nets a $23,433.00 energy savings.

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Wastewater Pumps & VFDs

• The ability to maintain efficiency is useful for wastewater pumping where constant flow & pressure is not a concern

• Eliminates the need for a shaver pump

• Keeps pump from either suction or recirculation cavitation

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Wastewater Pumps & VFDs

• VFDs give the operator the ability to run the pump/motor in reverse. This can be beneficial to clear a blockage in the volute

• Eliminates throttle valves in the system

• Avoids water hammer and pressure spikes by controlling speed and ramp up & ramp down

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Wastewater Pumps & VFDs

•Potential Pump IssuesService factor for many motors

is 1.15 for constant speed but drops to 1.0 on VFD

Decrease bearing life unless shaft currents are mitigated (Insulated bearing or SGR)

First Criticals can move around

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Wastewater Pumps & VFDs

•Pipeline Issues- depending on how drive is program you can see velocity issues

For example our test pump at 100GPM and 3500RPM has plenty of flow to achieve 2FPS in a 4” force main

However at 2625RPM we only produce 75GPM when 80GPM is needed for 2FPS

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Wastewater Pumps & VFDs

•Other FactorsMore complex controls usually

utilizing a transducer

Higher noise in motor due to chatter in the motor windings produced by the block wave form

Don’t be tempted to turn pump so slow it can not move solids

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The Take Away Technical Series 2014

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The Take Away•VFDs when properly applied

to collection systems can yield substantial cost savings and longer service life

• Look for equipment that is designed for use with VFDs to ensure maximum benefit

•Weigh all options as well as Total Cost of Ownership

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HYDROMATIC HPE

•Information on Pumps•Specifications•CAD/DWG/PDF Drawings •Downloads•White Papers•Distributor Locator

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www.HydromaticHPE.com

More content added each month

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SECTION BREAK

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