1.
PENTAIR 1

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

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

4. AC Induction Motors
Theory & Design
PENTAIR 4

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
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
Two & Four Pole Three Phase Motors

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
PENTAIR 10

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
PENTAIR 16

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,
Variable Speed nets a $23, 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
PENTAIR 26

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

29.
SECTION BREAK
PENTAIR 29