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2. The Effects of VFDs on Centrifugal Wastewater Pumps Technical Series 2014 2 PENTAIR

3. Course Objectives • Understanding of AC Induction Motors • VFDs- Are they Magic? • Variable Speed and Wastewater Pumping

4. AC Induction Motors Theory & Design 4 PENTAIR

5. 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

6. Motors • Rotational Speed of AC induction motors depend on two things: • Poles in the stator • Frequency of power (Hz) • Poles in the motor are a constant • Speed varies directly with Frequency

7. 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

8. Poles & Speed Two & Four Pole Three Phase Motors • 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 RPM • 120 x Frequency / Number of poles • 120 is constant • 120 x 60 = 7200 • 7200/ 4 = 1800RPM

9. 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

10. VFDs Why we use them Review of VFD principal 10 PENTAIR

11. 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”

12. 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

13. 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

14. 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

15. 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

16. Variable Speed and Wastewater Pumping Bringing it all together 16 PENTAIR

17. 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?

18. 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%

19. Wastewater Pumps & VFDs

20. Cost Savings on a VFD • 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. PRESENTATION TITLE

21. 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

22. 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

23. Wastewater Pumps & VFDs • Potential Pump Issues • Service 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

24. 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

25. Wastewater Pumps & VFDs • Other Factors • More 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 PRESENTATION TITLE

26. The Take Away Technical Series 2014 26 PENTAIR

27. 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

28. HYDROMATIC HPE www.HydromaticHPE.com • Information on Pumps • Specifications • CAD/DWG/PDF Drawings • Downloads • White Papers • Distributor Locator More content added each month

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