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PENTAIR 1

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

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

AC Induction Motors Theory & Design PENTAIR 4

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

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

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

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

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

VFDs Why we use them Review of VFD principal PENTAIR 10

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”

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

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

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

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

Variable Speed and Wastewater Pumping
Bringing it all together PENTAIR 16

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?

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%

Wastewater Pumps & VFDs

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

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

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

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

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

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

The Take Away Technical Series 2014 PENTAIR 26

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